E Golightly

Jabbour Hn

Norman Je
email: jane.norman@ed.ac.uk

Prof J E Norman

Endocrine immune interactions in human parturition

Keywords:

Human parturition is an inflammatory event, modulated and influenced by a host of other environmental and physiological processes, including the endocrine hormones.

Complex bidirectional communication occurs between the two systems to bring about some of the changes that are seen in labour, an event that is not yet fully understood.

Preterm birth is a major problem in obstetrics and neonatology, with dysfunctional labour or prolonged pregnancy also making increasingly significant contributions to maternal morbidity. With better understanding of normal and abnormal parturition we may be able to develop novel ways of treating these complications of pregnancy and reduce maternal and neonatal morbidity and mortality. This review discusses the crucial role that endocrine-immune interaction plays in the process of labour and in the processes of abnormal and preterm labour. We propose that amongst these complex interactions it is the immune system that is the driving force behind human parturition.

Introduction

Human pregnancy and labour are extraordinary physiological events during which the fetal allograft must first be tolerated, nourished, allowed to develop and (when it is developmentally mature enough to survive the outside world), the process of labour mounted to deliver the fetus from the mother's body. This process of human labour is an elegantly choreographed sequence of events including the remodelling and dilatation of the cervix, rupture of the fetal membranes and the onset and maintenance A c c e p t e d M a n u s c r i p t of effective uterine contractions, culminating in expulsion of the fetus and placenta. The entire process is followed by involution of the uterus. However, although some aspects of human parturition are reasonably well understood, the complexity of interactions between different systems and tissues, together with the well-documented inter-species differences that are observed, combine to ensure that a complete understanding of what initiates labour, or how it is propagated and controlled, has not yet been achieved. The importance of achieving such an understanding is, of course, so that we may be able to develop strategies to improve outcomes for women and their infants for whom the process, or its timing, becomes abnormal.

Preterm birth is the major cause of neonatal mortality and morbidity in developed countries [START_REF] Norman | The effect of changing patterns of obstetric care in Scotland (1980-2004) on rates of preterm birth and its neonatal consequences: perinatal database study[END_REF] with a rising prevalence currently of 7.6% in Scotland [START_REF] Norman | The effect of changing patterns of obstetric care in Scotland (1980-2004) on rates of preterm birth and its neonatal consequences: perinatal database study[END_REF] and 9.6% of births worldwide [START_REF]March of Dimes White paper on Preterm Birth. The Global and Regional Toll[END_REF][START_REF] Martin | Births: final data for 2004[END_REF]. Preterm labour (with or without preterm premature membrane rupture) is the cause of preterm birth in around 75% of women [START_REF] Norman | The effect of changing patterns of obstetric care in Scotland (1980-2004) on rates of preterm birth and its neonatal consequences: perinatal database study[END_REF]. Its causes are multifactorial, with infection implicated as a major cause, although the majority of preterm births are idiopathic [START_REF] Goldenberg | Epidemiology and causes of preterm birth[END_REF][START_REF] Norman | The effect of changing patterns of obstetric care in Scotland (1980-2004) on rates of preterm birth and its neonatal consequences: perinatal database study[END_REF]. In contrast, dysfunctional or delayed parturition at term may also cause significant complications including caesarean section, for which the rate is rising having increased from 9% in 1980 in England and Wales to 21% in 2001 [START_REF] Thomas | The National Sentinel Caesarean Section Audit Report[END_REF]. Thus a better understanding of the physiological processes of normal and abnormal parturition may help us to develop strategies to treat these pathologies and reduce both maternal and neonatal mortality and morbidity.

Traditional concepts of parturition, and indeed the hypothesis for which there is most evidence in animals, suggest that labour is a hormonal event, driven primarily by changes in progesterone and/or cortisol synthesis. Such a model can be applied to human parturition, but fits less well. Over the last 10 years, we and others have proposed that the primary events in human parturition are inflammatory, with hormonal changes being secondary or complementary. The purpose of this review is to highlight the evidence for each model separately, and then to consider which of these events is likely to be pre-eminent.

Labour as an inflammatory event

A c c e p t e d M a n u s c r i p t It is not a new observation that human parturition is an inflammatory event, but in recent years there have been many exciting new developments that have helped this concept to strengthen and evolve. At its most basic level, this concept is supported by the demonstration that leukocytes, (largely neutrophils and macrophages, and also T cells) invade the myometrium, cervix and fetal membranes at or immediately after the onset of labour [START_REF] Osman | Leukocyte density and pro-inflammatory cytokine expression in human fetal membranes, decidua, cervix and myometrium before and during labour at term[END_REF][START_REF] Thomson | Leukocytes infiltrate the myometrium during human parturition: further evidence that labour is an inflammatory process[END_REF], attracted by increased tissue expression of chemokines and cell adhesion molecules [START_REF] Ledingham | Cell adhesion molecule expression in the cervix and myometrium during pregnancy and parturition[END_REF][START_REF] Winkler | Cytokine concentrations and expression of adhesion molecules in the lower uterine segment during parturition at term: relation to cervical dilatation and duration of labor[END_REF]. In parallel, leukocytes circulating in the peripheral blood increase their expression of cell adhesion molecule ligands such as CD11a and CD11b [START_REF] Yuan | Leukocytes are primed in peripheral blood for activation during term and preterm labour[END_REF], facilitating leukocyte emigration into the myometrium and cervix.

The increased expression of cytokines in the myometrium and cervix in labour [START_REF] Osman | Leukocyte density and pro-inflammatory cytokine expression in human fetal membranes, decidua, cervix and myometrium before and during labour at term[END_REF] arises in part from the invading leukocytes [START_REF] Young | Immunolocalization of proinflammatory cytokines in myometrium, cervix, and fetal membranes during human parturition at term[END_REF] with recent evidence suggesting that increased capacity of leukocytes in labour to express cytokines is initiated in the peripheral circulation [START_REF] Yuan | Leukocytes are primed in peripheral blood for activation during term and preterm labour[END_REF]. These pro-inflammatory cytokines have a multitude of effects on the reproductive tissues, including stimulatory effects on myometrial contractility via a number of different mechanisms, discussed below. Cytokines are also involved in tissue remodelling [START_REF] Sennstrom | Human cervical ripening, an inflammatory process mediated by cytokines[END_REF] and attract more leukocytes in a positive feedback mechanism [START_REF] Elliott | Interleukin 8 expression in human myometrium: changes in relation to labor onset and with gestational age[END_REF], augmenting the process of parturition. The activity of nuclear factor-kappa B (NF-κB), a transcription factor regulating numerous genes involved in a multitude of cellular processes including inflammation, is also increased in labour [START_REF] Allport | Human labour is associated with nuclear factor-kappaB activity which mediates cyclo-oxygenase-2 expression and is involved with the 'functional progesterone withdrawal[END_REF]. The most clearly described increase in NF-κB in association with parturition occurs in the fetal membranes [START_REF] Allport | Human labour is associated with nuclear factor-kappaB activity which mediates cyclo-oxygenase-2 expression and is involved with the 'functional progesterone withdrawal[END_REF], although NF-κB is also expressed in the myometrium with obvious changes being described in pregnancy [START_REF] Chapman | Expression and deoxyribonucleic acid-binding activity of the nuclear factor kappaB family in the human myometrium during pregnancy and labor[END_REF]. NF-κB may provide a feed forward mechanism for inflammatory processes in labour, given that it is itself highly inducible by pro-inflammatory stimuli [START_REF] Belt | The nuclear transcription factor NF-kappaB mediates interleukin-1beta-induced expression of cyclooxygenase-2 in human myometrial cells[END_REF][START_REF] Kniss | Blockade NF-kappaB activation prohibits TNF-alpha-induced cyclooxygenase-2 gene expression in ED27 trophoblast-like cells[END_REF][START_REF] Lappas | Nuclear factor kappa B regulation of proinflammatory cytokines in human gestational tissues in vitro[END_REF].

Taken together therefore, labour is associated with leukocyte invasion, NF-κB activation and pro-inflammatory cytokine production in some if not all of the relevant reproductive tissues. These data strongly support the hypothesis that labour is an A c c e p t e d M a n u s c r i p t inflammatory event. The functional effects of these pro-inflammatory events are described below.

Myometrium

The upregulation of pro-inflammatory cytokines within labouring myometrium stimulates and potentiates uterine contractions. Tribe et al have demonstrated that interleukin (IL)-1β induces basal and store operated calcium entry in myometrial smooth muscle cells, thus directly increasing their contractile potential [START_REF] Tribe | Interleukin-1beta induces calcium transients and enhances basal and store operated calcium entry in human myometrial smooth muscle[END_REF]. Additionally, IL-1β and tumor necrosis factor (TNF)-α stimulate arachidonic acid release, and expression of cyclooxygenase (COX)-2 thus increasing prostaglandin production in myometrial cell cultures, via greater NF-κB activity [START_REF] Belt | The nuclear transcription factor NF-kappaB mediates interleukin-1beta-induced expression of cyclooxygenase-2 in human myometrial cells[END_REF], Hertelendy et al., 1993, Molnar et al., 1993[START_REF] Rauk | Interleukin-1 stimulates human uterine prostaglandin production through induction of cyclooxygenase-2 expression[END_REF].

Prostaglandins have long been known as stimulators of myometrial contractions [START_REF] Johnston | Plasma prostaglandin metabolite concentrations in normal and dysfunctional labour[END_REF] thus there is a clear potential causal link between the inflammatory events observed in the myometrium and the clinical process of parturition.

The cervix

Cervical ripening is characterised by breakdown of collagen and remodelling of the connective tissue element of the cervix. Leukocyte invasion and the increase in protein concentrations of IL-8, IL-6 and granulocyte-CSF (G-CSF) observed in the human cervical connective tissue at term and subsequently in labour are likely to be crucial to the process of cervical remodelling to facilitate opening during parturition [START_REF] Osman | Leukocyte density and pro-inflammatory cytokine expression in human fetal membranes, decidua, cervix and myometrium before and during labour at term[END_REF][START_REF] Sennstrom | Human cervical ripening, an inflammatory process mediated by cytokines[END_REF]. Functionally, IL-8 may stimulate the release of degradative enzymes from neutrophils. These enzymes include serine proteases and matrix metalloproteinases ((MMP)-8 and MMP-9)) [START_REF] Kelly | Inflammatory mediators and cervical ripening[END_REF] which then participate in degradation of the collagen and glycosaminoglycan components of the extracellular matrix thereby leading to cervical ripening [START_REF] Hertelendy | Prostaglandins and the myometrium and cervix[END_REF]. Production of MMP-1, MMP-3 and MMP-9 from resident fibroblasts [START_REF] Ito | Calmodulin differentially modulates the interleukin 1-induced biosynthesis of tissue inhibitor of metalloproteinases and matrix metalloproteinases in human uterine cervical fibroblasts[END_REF] and smooth muscle cells [START_REF] Watari | Pro-inflammatory cytokines induce expression of matrix-metabolizing enzymes in human cervical smooth muscle cells[END_REF] may also be increased by IL-1β in the cervix whilst downregulating the expression of tissue inhibitor of metalloproteinase (TIMP)-2, an endogenous inhibitor of MMP-2 [START_REF] Watari | Pro-inflammatory cytokines induce expression of matrix-metabolizing enzymes in human cervical smooth muscle cells[END_REF], with these events being further amplified by NF-κB production. NF-κB itself may be A c c e p t e d M a n u s c r i p t activated via toll-like receptor (TLR)-4 by degraded extracellular matrix proteins such as fragments of fetal fibronectin, degraded during cervical remodelling, thus reinforcing the ongoing sterile inflammation of labour [START_REF] Challis | Inflammation and pregnancy[END_REF]. Thus, at the end of pregnancy and during labour, the cervix having remained firm and closed despite increasing pressure during pregnancy, begins, under the influence of a host of inflammatory mediators, to soften and dilate.

Fetal membranes

A similar pro-inflammatory pattern to that observed in the myometrium is seen in the fetal membranes. The production of IL-8, TNF-α, IL-6 and IL-1β are all increased in the fetal membranes and amniotic fluid during labour [START_REF] Young | Immunolocalization of proinflammatory cytokines in myometrium, cervix, and fetal membranes during human parturition at term[END_REF]. There are also increased levels of MMP-9 (Vadillo-Ortega et al., 1995) and decreased levels of the TIMPs [START_REF] Riley | Secretion of matrix metalloproteinase-2, matrix metalloproteinase-9 and tissue inhibitor of metalloproteinases into the intrauterine compartments during early pregnancy[END_REF]. Functionally, an increased MMP /TIMP ratio weakens the fetal membranes, facilitating fetal membrane rupture which is often the precursor to labour.

Careful examination of the fetal membranes from pre-labour specimens obtained at caesarean section has demonstrated that a zone of weakness exists in the region overlying the cervix [START_REF] Mclaren | Structural characteristics of term human fetal membranes prior to labour: identification of an area of altered morphology overlying the cervix[END_REF]. The features of this zone include a decrease in thickness in comparison to the rest of the fetal membranes, increased MMP-9 and decreased TIMP-3 and also increased poly (ADP-ribose) polymerase -1 (PARP1) cleavage [START_REF] El Khwad | Term human fetal membranes have a weak zone overlying the lower uterine pole and cervix before onset of labor[END_REF]. PARP1 is an enzyme involved in DNA repair and apoptosis and thus its increase indicates increased apoptosis [START_REF] El Khwad | Term human fetal membranes have a weak zone overlying the lower uterine pole and cervix before onset of labor[END_REF]. It would seem therefore that towards the end of pregnancy the fetal membranes are prepared for labour by a process of remodelling to develop a weakened area that is the site of rupture. What initiates and develops this process is not known but a greater understanding of the molecular events leading to membrane rupture may lead to developing novel therapeutic strategies to halt its premature onset in preterm labour.

The fetal membranes are specifically important, as they are the interface between the fetus and the mother, and may transmit pro-labour signals from the baby to the A c c e p t e d M a n u s c r i p t mother's myometrium and cervix. Pulmonary derived surfactant proteins and phospholipids may be key to this: as the fetus matures, increased quantities are produced and so levels increase in the amniotic fluid. These surfactant proteins may well contribute to the inflammatory response that can be observed in the fetal membranes and in the underlying cervix and myometrium [START_REF] Smith | Parturition[END_REF] given that they stimulate COX-2 activity via TLR-4 in mice [START_REF] Wang | Bacterially-induced preterm labor and regulation of prostaglandin-metabolizing enzyme expression in mice: the role of toll-like receptor 4[END_REF] and subsequent prostaglandin E2 (PGE2) production in the amnion. Increased PGE2 production in the amnion could diffuse through the chorion and decidua to stimulate myometrial cell prostaglandin production and hence contractility [START_REF] Slater | Expression of cyclo-oxygenase types-1 and -2 in human myometrium throughout pregnancy[END_REF][START_REF] Smith | Parturition[END_REF]. The fetus and placenta may also jointly trigger fetal membrane rupture, since prostaglandins, TNF-α and corticotrophin releasing hormone (CRH) (which also rise in the amniotic fluid towards term [START_REF] Alvi | Corticotrophin-releasing hormone and platelet-activating factor induce transcription of the type-2 cyclo-oxygenase gene in human fetal membranes[END_REF][START_REF] Laatikainen | Corticotropin-releasing hormone in amniotic fluid during gestation and labor and in relation to fetal lung maturation[END_REF])) all stimulate the production of MMP-9, which weakens the membranes and thus facilitates membrane rupture [START_REF] Kumar | Proinflammatory cytokines found in amniotic fluid induce collagen remodeling, apoptosis, and biophysical weakening of cultured human fetal membranes[END_REF][START_REF] Young | Immunolocalization of proinflammatory cytokines in myometrium, cervix, and fetal membranes during human parturition at term[END_REF].

Preterm Birth and Inflammation

Preterm labour may be initiated by a multitude of different mechanisms, including infection, placental haemorrhage, cervical insufficiency, and uterine over-distention.

The cause is multi-factorial and is often idiopathic; however, infection is the single biggest aetiological factor [START_REF] Goldenberg | Epidemiology and causes of preterm birth[END_REF]. Micro-organisms such as Escherichia coli, Ureaplasma urealyticum and Streptococcus agalatae are receognised by the TLRs, which stimulate increased production of inflammatory cytokines at least in part by NF-κB and initiate the cascade of inflammatory events described above, resulting in preterm parturition [START_REF] Peltier | Immunology of term and preterm labor[END_REF].

Several animal models of parturition involve stimulation with pro-inflammatory substances such as lipopolysaccharide (LPS) to mimic the onset of preterm labour [START_REF] Elovitz | A new model for inflammation-induced preterm birth: the role of platelet-activating factor and Toll-like receptor-4[END_REF][START_REF] Fidel | Systemic and local cytokine profiles in endotoxin-induced preterm parturition in mice[END_REF]. Such animal models have helped to develop potential new strategies for treating preterm birth. The importance of inflammation is demonstrated by experiments showing that the anti-inflammatory agent IL-10 not only prevents preterm birth, but also prevents neonatal brain damage in rats [START_REF] Rodts-Palenik | Maternal infection-induced white matter injury is reduced by treatment with interleukin-10[END_REF][START_REF] Terrone | Interleukin-10 administration and bacterial endotoxin-induced preterm birth in a rat model[END_REF]. More recently, Bennett's group have shown inhibited both LPS induced contractions and preterm delivery and the upregulation of NF-κB in the fetal brain [START_REF] Pirianov | The cyclopentenone 15-deoxy-delta 12,14-prostaglandin J(2) delays lipopolysaccharide-induced preterm delivery and reduces mortality in the newborn mouse[END_REF]. These results encourage the possibility of the future development of agents that could be used not only to prevent preterm labour but also to protect the developing fetus from the damaging effects of surrounding inflammation.

Labour as an endocrine event

Progesterone

Progesterone sustains the state of pregnancy and maintains uterine quiescence throughout gestation. It has long been observed that in most mammalian species a fall in plasma levels of progesterone is the initiating step towards parturition. However, contrary to what is observed in other mammalian species, most human studies have demonstrated that progesterone levels in pregnancy remain constant in plasma in the late third trimester of pregnancy and during labour [START_REF] Boroditsky | Maternal serum estrogen and progesterone concentrations preceding normal labor[END_REF][START_REF] Mathur | Progesterone, 17hydroxyprogesterone, estradiol, and estriol in late pregnancy and labor[END_REF] The fact that the progesterone receptor antagonist mifepristone is an effective agent for induction of labour to soften the cervix and increase uterine sensitivity to uterotonic agents [START_REF] Hapangama | Complex interactions between sex steroids and cytokines in the human pregnant myometrium: evidence for an autocrine signaling system at term[END_REF] does suggest that a role exists for some form of progesterone withdrawal being involved in the initiation of human parturition. An alternative concept of 'functional progesterone withdrawal' has therefore been proposed for humans, in which there is loss of the quiescent effect of progesterone on myometrium, despite continuing high systemic and local levels.

There have been several different mechanisms proposed for this, including local metabolism of progesterone [START_REF] Mitchell | Changes in 17 beta,20 alpha-hydroxysteroid dehydrogenase activity supporting an increase in the estrogen/progesterone ratio of human fetal membranes at parturition[END_REF], changes in ratios of the progesterone receptor isoforms [START_REF] Mesiano | Progesterone withdrawal and estrogen activation in human parturition are coordinated by progesterone receptor A expression in the myometrium[END_REF] and changes in levels of cofactors affecting progesterone receptor (PR) function [START_REF] Condon | A decline in the levels of progesterone receptor coactivators in the pregnant uterus at term may antagonize progesterone receptor function and contribute to the initiation of parturition[END_REF].

Progesterone can be metabolised to a less active metabolite, 20α-dihydroprogesterone and it has been shown that production of this less active form increases after the initiation of labour [START_REF] Mitchell | Changes in 17 beta,20 alpha-hydroxysteroid dehydrogenase activity supporting an increase in the estrogen/progesterone ratio of human fetal membranes at parturition[END_REF], presenting a potential mechanism for functional progesterone withdrawal. Functional progesterone withdrawal also likely occurs at the receptor leveleither via changes in progesterone receptor isoforms or [START_REF] Mesiano | Progesterone withdrawal and estrogen activation in human parturition are coordinated by progesterone receptor A expression in the myometrium[END_REF], with evidence to support this hypothesis at the level both of mRNA and protein expression [START_REF] Merlino | Nuclear progesterone receptors in the human pregnancy myometrium: evidence that parturition involves functional progesterone withdrawal mediated by increased expression of progesterone receptor-A[END_REF]. A multitude of cofactor proteins exist that interact with the progesterone receptor family to enhance or repress their action.

Several groups have postulated that changes in the level of some of these cofactors may account for the functional progesterone withdrawal. including Condon et al who demonstrated a decline in the levels of cyclic adenosine monophosphate (cAMP)response element-binding protein (CREB)-binding protein and steroid receptor coactivators 2 and 3 in human fundal myometrium thus reducing PR transcription [START_REF] Condon | A decline in the levels of progesterone receptor coactivators in the pregnant uterus at term may antagonize progesterone receptor function and contribute to the initiation of parturition[END_REF].

Estrogen

Estrogen has a stimulatory effect on contractions of human myometrium, by increasing the expression of uterine contraction associated protein (CAP)-genes such as connexin-43 and oxytocin receptors. Here again, the contrast exists between the human and other mammalian species in that in most mammals parturition is associated with an increase in circulating estrogens, but in humans, serum estrogen levels are high throughout pregnancy, gradually increasing to term [START_REF] Tulchinsky | Plasma estrone, estradiol, estriol, progesterone, and 17-hydroxyprogesterone in human pregnancy. I. Normal pregnancy[END_REF]. This increase is accelerated in the final few weeks of pregnancy but there are no significant difference in estrogen levels during labour compared to just before labour [START_REF] Boroditsky | Maternal serum estrogen and progesterone concentrations preceding normal labor[END_REF][START_REF] Turnbull | Significant fall in progesterone and rise in oestradiol levels in human peripheral plasma before onset of labour[END_REF]. mRNA expression of the estrogen receptor (ER)-α is increased in labouring myometrium, suggesting that a functional estrogen activation may also occur, which has been linked to the functional progesterone withdrawal [START_REF] Mesiano | Progesterone withdrawal and estrogen activation in human parturition are coordinated by progesterone receptor A expression in the myometrium[END_REF]. Recent data from Smith and colleagues suggests that a change in estradiol/estriol ratios and progesterone to estriol ratio might be important, and that hormone trajectories in the individual may have greater impact than absolute levels. [START_REF] Smith | Patterns of plasma corticotropin-releasing hormone, progesterone, estradiol, and estriol change and the onset of human labor[END_REF].

A c c e p t e d M a n u s c r i p t

Corticotropin-releasing hormone and the "placental clock"

Corticotropin-releasing hormone is a 41-amino acid peptide released from the hypothalamus and it regulates the hypothalamic-pituitary-adrenal (HPA) axis and plays a central role in the response to stress [START_REF] Wadhwa | Stress and preterm birth: neuroendocrine, immune/inflammatory, and vascular mechanisms[END_REF]. In human pregnancy, the placenta releases CRH and maternal plasma CRH levels increase exponentially, peaking at delivery, and returning to pre-pregnancy levels within 24 hours of birth [START_REF] Campbell | Plasma corticotropin-releasing hormone concentrations during pregnancy and parturition[END_REF][START_REF] Kalantaridou | Reproductive functions of corticotropin-releasing hormone. Research and potential clinical utility of antalarmins (CRH receptor type 1 antagonists)[END_REF]. Interestingly, placental production of CRH seems to occur only in primate species, again reinforcing the uniqueness of human parturition [START_REF] Robinson | Speciesspecific placental corticotropin releasing hormone messenger RNA and peptide expression[END_REF]. CRH binding protein (CRH-BP) produced by the liver and placenta, limits the bioavailability of CRH but its levels fall during the last six weeks of pregnancy leaving increased levels of free CRH [START_REF] Linton | Corticotropin releasing hormonebinding protein (CRH-BP): plasma levels decrease during the third trimester of normal human pregnancy[END_REF]. Women in preterm labour have significantly elevated levels of CRH compared to non-labouring women of the same gestation, a finding that can also be seen several weeks in advance of clinical signs of preterm labour [START_REF] Mclean | A placental clock controlling the length of human pregnancy[END_REF][START_REF] Wadhwa | Maternal corticotropin-releasing hormone levels in the early third trimester predict length of gestation in human pregnancy[END_REF]. Taken together, these observations form the hypothesis that CRH may function as a placental clock in humans, determining the length of gestation and onset of labour [START_REF] Mclean | A placental clock controlling the length of human pregnancy[END_REF]. A positive feedback loop has been proposed whereby fetal glucocorticoids, produced by the maturing fetal adrenal glands, promote placental CRH gene expression and CRH production, which in turn stimulates further fetal cortisol production via the HPA axis [START_REF] Robinson | Glucocorticoid stimulates expression of corticotropin-releasing hormone gene in human placenta[END_REF]. This system may also be modulated by maternal or fetal stress, which has been suggested as a possible mechanism for the increased rates of preterm delivery observed in women suffering from increased levels of stress [START_REF] Wadhwa | Stress and preterm birth: neuroendocrine, immune/inflammatory, and vascular mechanisms[END_REF].

The contribution that these increasing levels of CRH may have on parturition itself is not yet certain. CRH signals through G protein-coupled receptors which exist in several different forms and use different second messenger systems to exert their effects [START_REF] Grammatopoulos | The human myometrium expresses multiple isoforms of the corticotropin-releasing hormone receptor[END_REF]. During pregnancy the predominant CRH receptor in the myometrium is CRH receptor (R)1α, binding of which by CRH promotes myometrial relaxation via activation of cAMP [START_REF] Grammatopoulos | The human myometrial CRH receptor: G proteins and second messengers[END_REF]. However, this action is inconsistent with the concept of CRH functioning as a 'placental clock' and promoting labour. At term, the affinity of CRH for its receptor is A c c e p t e d M a n u s c r i p t reduced, and oxytocin has been implicated in promoting this reduction by inhibiting the activity of adenylate cyclase (Grammatopoulos and Hillhouse, 1999b). Thus it has been hypothesised that CRH may be involved in promoting contractile activity via its interaction with oxytocin (Grammatopoulos and Hillhouse, 1999a).

Oxytocin

Oxytocin and its receptor (OT and OTR) play an important role in parturition, although their precise role is as yet incompletely understood (Blanks and Thornton, 2003). Plasma concentrations of OT rise with labour [START_REF] Thornton | Plasma oxytocin during the first and second stages of spontaneous human labour[END_REF], as do concentrations of OT in gestational tissues (Blanks et al., 2003). Clinically, synthetic oxytocin is used to augment contractions during dystocic labour and OTR antagonists are licensed as tocolytic agents for the treatment of preterm labour [START_REF] Romero | An oxytocin receptor antagonist (atosiban) in the treatment of preterm labor: a randomized, double-blind, placebo-controlled trial with tocolytic rescue[END_REF]. However, OT appears not to be an essential participant in labour as oxytocin (-/-) null mice can deliver normally [START_REF] Nishimori | Oxytocin is required for nursing but is not essential for parturition or reproductive behavior[END_REF], Young et al., 1996). The oxytocin receptor increases at term in myometrial tissue [START_REF] Fuchs | Oxytocin receptors in the human uterus during pregnancy and parturition[END_REF] and its increased expression leads to direct contractile effects in myometrium (Blanks and Thornton, 2003).

Preterm birth and the Endocrine System

Progesterone supplementation has been used for many decades now to try to prevent preterm labour and reduce the severity of its neonatal sequelae [START_REF] Fuchs | Treatment of threatened premature labor with large doses of progesterone[END_REF]. Three large randomised controlled trials have suggested that administration of progesterone reduces the risk of preterm delivery in high-risk singleton pregnancies (da [START_REF] Da Fonseca | Prophylactic administration of progesterone by vaginal suppository to reduce the incidence of spontaneous preterm birth in women at increased risk: a randomized placebocontrolled double-blind study[END_REF][START_REF] Fonseca | Progesterone and the risk of preterm birth among women with a short cervix[END_REF][START_REF] Meis | Prevention of A c c e p t e d M a n u s c r i p t recurrent preterm delivery by 17 alpha-hydroxyprogesterone caproate[END_REF]. Importantly, this effect appears to be confined to singleton pregnancies for reasons that are as yet unclear (Norman et al., 2009a, Simhan and[START_REF] Simhan | Prevention of preterm delivery[END_REF].

Currently, the reduction in preterm delivery achieved by progesterone has not been shown to be accompanied by an improvement in perinatal mortality [START_REF] Dodd | Progesterone for the prevention of preterm birth: a systematic review[END_REF] and further studies are ongoing to explore long-term outcomes in infants whose mothers are treated antenatally with progesterone to prevent preterm delivery.

Human labour is an immune not an endocrine event

A c c e p t e d M a n u s c r i p t So if the onset of human labour has features of inflammation and endocrine changes, which is the predominant? There is little compelling evidence to suggest that in humans a hormonal switch exists that signals the end of pregnancy and begins the process of parturition. Unlike most animal species studied, humans do not appear to be dependent on the withdrawal of progesterone, nor on an increase in estrogen for labour to commence. Rather, it seems that a multitude of complex and interlinked processes combine to overwhelm the quiescent phase of pregnancy and initiate labour.

Mitchell and Taggart have suggested a model of 'modular accumulation of physiological systems' in which multiple interlinking physiological processes develop in parallel until a critical mass is achieved which results in parturition [START_REF] Mitchell | Are animal models relevant to key aspects of human parturition?[END_REF]. Their model also describes how the premature and inappropriate activation of some of the 'modules' could result in preterm labour thus such a concept could explain the onset of both term and preterm human labour. We propose that the linkages between such processes are dominated by the inflammatory system and that whilst hormones play an undoubtedly vital part in human parturition, it is the immune system that is the more important driving force.

Progesterone has long been described as an 'anti-inflammatory steroid' and this is supported by a wealth of data demonstrating its interactions with the immune system.

Progesterone itself inhibits IL-6 production in fetoplacental arteries [START_REF] Gotkin | Progesterone reduces lipopolysaccharide induced interleukin-6 secretion in fetoplacental chorionic arteries, fractionated cord blood, and maternal mononuclear cells[END_REF], and progestogens such as medroxyprogesterone acetate inhibit a range of proinflammatory cytokines and chemokines in human myometrium [START_REF] Shynlova | Monocyte chemoattractant protein-1 (CCL-2) integrates mechanical and endocrine signals that mediate term and preterm labor[END_REF][START_REF] Youssef | The Role of Toll-Like Receptors (TLR-2 and -4) and Triggering Receptor Expressed on Myeloid Cells 1 (TREM-1) in Human Term and Preterm Labor[END_REF]. Of particular interest are the interactions of progesterone with NF-κB, a transcription factor which has key functions within human parturition [START_REF] Allport | Human labour is associated with nuclear factor-kappaB activity which mediates cyclo-oxygenase-2 expression and is involved with the 'functional progesterone withdrawal[END_REF][START_REF] Elliott | Nuclear factor-kappa B is essential for up-regulation of interleukin-8 expression in human amnion and cervical epithelial cells[END_REF]. NF-κB activation increases in fetal membranes in labour [START_REF] Allport | Human labour is associated with nuclear factor-kappaB activity which mediates cyclo-oxygenase-2 expression and is involved with the 'functional progesterone withdrawal[END_REF] but is negatively repressed by activation of the progesterone receptor [START_REF] Allport | Human labour is associated with nuclear factor-kappaB activity which mediates cyclo-oxygenase-2 expression and is involved with the 'functional progesterone withdrawal[END_REF]. In contrast, NF-κB itself represses progesterone receptor activity; work from Carole Mendelson's group has demonstrated that activation of NF-κB in human myometrium upregulates inhibitory PR isoforms [START_REF] Condon | Up-regulation of the progesterone receptor (PR)-C isoform in laboring myometrium by activation of nuclear factor-kappaB may contribute to the onset of labor through inhibition of PR function[END_REF]. It may be that the removal of the immunosuppressive, quiescent influences of progesterone by the functional progesterone withdrawal may in fact itself be initiated by inflammation and activation of NF-κB (see Fig1).

A c c e p t e d M a n u s c r i p t

Estrogens can induce either proinflammatory or anti-inflammatory effects, depending on the immune stimuli, the cell types involved, the target organ, estrogen concentration and receptor expression [START_REF] Straub | The complex role of estrogens in inflammation[END_REF]. With respect to cytokine expression, estrogen in the high concentrations seen in pregnancy can inhibit proinflammatory pathways which include those activated by IL-1β [START_REF] Polan | Progesterone and estradiol modulate interleukin-1 beta messenger ribonucleic acid levels in cultured human peripheral monocytes[END_REF], IL-6 [START_REF] Keck | Effects of 17betaestradiol and progesterone on interleukin-6 production and proliferation of human umbilical vein endothelial cells[END_REF][START_REF] Kikuchi | Atheroprotective effect of estriol and estrone sulfate on human vascular smooth muscle cells[END_REF], IL-8 [START_REF] Rodriguez | 17Betaestradiol inhibits the adhesion of leukocytes in TNF-alpha stimulated human endothelial cells by blocking IL-8 and MCP-1 secretion, but not its transcription[END_REF] and TNF-α [START_REF] Rogers | The effect of 17beta-estradiol on production of cytokines in cultures of peripheral blood[END_REF] as well as inhibiting the activity of natural killer cells [START_REF] Seaman | Effect of estrogen on natural killer cells[END_REF]. In contrast, the secretion of 'anti-inflammatory interleukins' IL-4 [START_REF] Kamada | Transient increase in the levels of T-helper 1 cytokines in postmenopausal women and the effects of hormone replacement therapy[END_REF], IL-10 (Kanda and Tamaki, 1999) and transforming growth factor (TGF)-β (Hatthachote and Gillespie, 1999) are stimulated by estrogen in these high levels. Notwithstanding these clear anti-inflammatory effects of estrogen, in vivo estradiol can augment the production of pro-inflammatory cytokines by CD4+ T cells [START_REF] Maret | Estradiol enhances primary antigen-specific CD4 T cell responses and Th1 development in vivo. Essential role of estrogen receptor alpha expression in hematopoietic cells[END_REF] likely via a direct effect as circulating leukocytes express ER-α [START_REF] Straub | The complex role of estrogens in inflammation[END_REF]. Thus, although estrogen has a stimulatory effect on myometrium, it also has highly complex interactions with the immune system that may contribute to the quiescence maintained throughout pregnancy.

CRH and the HPA axis are intimately linked with the immune system and exert complex effects on the human female reproductive system [START_REF] Kalantaridou | Reproductive functions of corticotropin-releasing hormone. Research and potential clinical utility of antalarmins (CRH receptor type 1 antagonists)[END_REF].

CRH has been shown to regulate cytokine production, and in return inflammatory cytokines influence CRH production. For example both TNF-α and IL-1β stimulate production of CRH [START_REF] Dudley | Immunoendocrinology of preterm labor: the link between corticotropin-releasing hormone and inflammation[END_REF] and conversely CRH inhibits the production of IL-1 and IL-6 by peripheral blood mononuclear cells [START_REF] Salas | Effect of the corticotrophin releasing hormone precursor on interleukin-6 release by human mononuclear cells[END_REF]. CRH may reduce the cytotoxicity of natural killer cells [START_REF] Irwin | Sympathetic nervous system mediates central corticotropin-releasing factor induced suppression of natural killer cytotoxicity[END_REF] and also inhibits T cell proliferation [START_REF] Jain | Corticotropin-releasing factor modulates the immune response to stress in the rat[END_REF]. Increasing fetal cortisol production, perpetuated by increasing CRH production as described above, is also associated with fetal lung maturation and production of surfactant proteins and phospholipids which are present in the amniotic fluid. Such compounds are proinflammatory and it has been proposed [START_REF] Smith | Parturition[END_REF]) that they might stimulate production of prostaglandins and initiation of inflammation in the fetal membranes.

The effect of inflammatory cytokines on the OTR is somewhat conflicting as described in the literature. Schmid et al reported a downregulation of OTR mRNA by IL-1β in a myometrial cell line [START_REF] Schmid | Transcriptional regulation of oxytocin receptor by interleukin-1beta and interleukin-6[END_REF], and Rauk et al also reported A c c e p t e d M a n u s c r i p t down-regulation of OTR mRNA following treatment with IL-1β and up-regulation after treatment with IL-6 in primary human smooth muscle cells (Rauk andFriebe-Hoffmann, 2000, Rauk et al., 2001). However, Terzidou et al demonstrated rapid transient upregulation of OTR mRNA with treatment of primary human myocytes with IL-1β [START_REF] Terzidou | Regulation of the human oxytocin receptor by nuclear factor-kappaB and CCAAT/enhancer-binding protein-beta[END_REF] which is more consistent with the actual mileu present in labour i.e. increased levels of IL-1β, COX-2 and IL-8. The OTR promoter contains putative transcription binding sites for CCAAT/enhancer binding protein (C/EBP) and NF-κB, which are transcription factors activated by cytokines such as IL-1β and IL-6 [START_REF] Terzidou | Regulation of the human oxytocin receptor by nuclear factor-kappaB and CCAAT/enhancer-binding protein-beta[END_REF]. IL-1β and IL-6 increase the secretion of oxytocin in uterine smooth muscle cells [START_REF] Friebe-Hoffmann | Effect of IL-1beta and IL-6 on oxytocin secretion in human uterine smooth muscle cells[END_REF] and IL-1β has been observed to augment the oxytocin-stimulated contractility of myometrial strips in culture [START_REF] Molnar | Interleukin-1 and tumor necrosis factor stimulate arachidonic acid release and phospholipid metabolism in human myometrial cells[END_REF]. Thus, it seems likely that the result of the infiltration of inflammatory cells and increase in pro-inflammatory cytokines is to activate the myometrial tissue (at least in part via an increase in OTR expression) and to promote contractility.

Strategies to treat preterm birth

As knowledge accumulates about the mechanisms of preterm and term human labour, we can hope to come closer to an effective therapy for preventing preterm or dysfunctional labour. As described earlier, investigators have seen promising results in animal models with anti-inflammatory agents, although neither IL-10 nor 15d-PGJ( 2) are yet available for clinical use. Data from other animal models suggesting that progestogens may protect against traumatic and ischaemic induced brain injury (at least partly due to their inhibitory effect on pro-inflammatory cytokine production is also highly encouraging [START_REF] Sayeed | Progesterone as a neuroprotective factor in traumatic and ischemic brain injury[END_REF]. Ultimately, there is still much work to be done before the mysteries of human parturition are revealed. Although we believe that the inflammatory reaction that accompanies labour is essentially the orchestrating force that drives it (Fig 2), many fascinating questions remain to be explored before this theory can be proven. Inflammation in response to intrauterine infection is an important cause of many, but not all, cases of preterm labour. How do the inflammatory signatures of infected and non-infected term and pre-term labour compare and how may this clarify potential initiators of labour? Recently published A c c e p t e d M a n u s c r i p t gene-profiling work by Weiner et al has suggested that indeed there may be differences in the initiating pathways between term and preterm labour, which will no doubt provoke further exploration of these differences.

Conclusions

The biochemical mechanisms of human parturition in which the membranes weaken and rupture, the cervix softens and effaces and the uterus is activated to produce contractions, are both highly complicated and incompletely understood. It is clear that an intricate web of interactions exists between components of the endocrine and immune systems in order to regulate this process, and that disorders within it may lead to significant complications for both mother and infant. We propose that the farreaching influences of the inflammation of labour may prove to be the key to unlocking the secrets behind human parturition and that further elucidation of these interactions between the immune and endocrine systems will help to develop strategies to treat the complications of labour that women still face.

  A c c e p t e d M a n u s c r i p t that the anti-inflammatory prostaglandin 15-deoxy-delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) may have similar dual effects: in their mouse model 15d-PGJ(2)

  s c r i p t their cofactors. The nuclear progesterone receptor exists in more than one isoform, the two major forms being progesterone receptors A and B (PR-A and PR-B). PR-B functions mainly as an activator of progesterone-responsive genes, whereas PR-A has an inhibitory action thereby suppressing PR-B function. Mesiano et al describe a model in which a change in the ratio of the receptor isoforms from a PR-B dominant mileu to a PR-A dominant one may be seen around the time of parturition

  Figure one

Figure two

  Figure two


Erik Louw
email: e.louw@tudelft.nl

Erwin Van Der Krabben
email: e.vanderkrabben@fm.ru.nl

Hans Van Amsterdam

Hans Van Amsterdam
email: hans.vanamsterdam@pbl.nl

The Spatial Productivity of Industrial Land

Keywords: spatial productivity, spatial efficiency, industrial land, Industrial land, Spatial efficiency, Land policy, Industrial estates JEL classifications: O18, R12, R14, R52

Introduction

Land plays only a limited role in current economic theory. In classical economics land was the principal source of wealth, but from neoclassical times, economic theory has paid less and less attention to land use, generally regarding it as a production factor of relatively little importance. Having once been the main factor, land faded from the production function. The shift from land to capital and labour inputs was complete by the second half of the 20 th century. According to [START_REF] Hubacek | The Role of Land in Economic Theory[END_REF], however, growing interest in sustainable development has triggered a renewed discussion on land use and reintroduced land into the economic discourse.

Concerns about environmental pollution and the depletion of natural resources have provided incentives for new perspectives on, and the conceptualisation of, land in economic analyses, especially in regional and urban economics. In this paper we offer an example of one such 'new' perspective by developing a method that measures just how efficiently industrial land is used. The efficiency of land use is an important issue in spatial planning policies for, amongst others, combating urban sprawl. These policies often refer to the concept of the compact city, which usually implies a combination of increasing mixed-land use and increasing densities. In countries such as the United Kingdom and the Netherlands the supply of land for industrial development has become a matter of increasing concern to many local authorities [START_REF] Van Amsterdam | Land for Industrial Development[END_REF][START_REF] Louw | The production of business sites in the Netherland[END_REF]. It is assumed in those countries that the conditions for economic growth and restructuring can be improved by supplying industrial land, but there is only limited understanding of how the process of land supply and development contributes to meeting the requirements of industrial production at local and regional level. Theoretical and empirical analyses on this subject are scarce [START_REF] Van Amsterdam | Land for Industrial Development[END_REF].

In many countries, particularly in densely populated parts of the world such as Singapore, China, Japan and the Netherlands, the promotion of land-use efficiency is a major issue in sustainable spatial planning. Behind the planning rhetoric which connects land-use efficiency, development, there are more pragmatic arguments such as limited land supply (Singapore), high urbanisation rates (China) and increasing urban sprawl (the Netherlands). Whatever the reason for the increasing interest in land-use efficiency, there is relatively little knowledge on how to measure, analyse and compare it. In regional science and economic geography most of the research on spatial efficiency focuses on the location and land-use patterns of economic activities, rather than on the amount of land that is actually used, or as Bapat writes, 'the maximization of spatial activity' (Bapat, 2006: 128). Only the efficiency of agricultural landuse or the productivity of rural land is analysed regularly.

The few studies which measure and analyse the efficiency of non-agricultural land-use come mainly from China [START_REF] Meng | Industrial landuse efficiency and planning in Shunyi, Beijning[END_REF][START_REF] Liu | Evaluation of Land-Use Efficiency Based on Regional Scale[END_REF][START_REF] Maosheng | A Study on Theoretical Model and Applied Evaluation of Land Use Performance of High-tech Zone in Urban Fringe: A Case Study in Haidian District, Beijing[END_REF] and Singapore [START_REF] Zhu | The third-party logistics services and globalization of manufacturing[END_REF] and were carried out at local or regional level. They offer several indices to measure land-use efficiencies on economic, social, ecological and environmental dimensions.

Within the economic dimension, employment density is always one of the variables. In the regional science literature on agglomeration and knowledge spillovers there is clear evidence that employment density is positively related to economic performance [START_REF] Ciccone | Productivity and the density of economic activity[END_REF][START_REF] Cervero | Efficient urbanizations: Economic performance and the shape of the metropolis[END_REF][START_REF] Ciccone | Agglomeration effect in Europe[END_REF]. However, these studies consider all space as equivalent.

Or, as Ciccone (2002: 224) states: '…it was assumed that the density of production is the same throughout each Nuts 3 region'. It is not possible under such circumstances to treat land as a distinctive factor of production because it is unknown how much land is used by the various types of use, including those which do not involve production.

There is therefore a methodological difficulty when it comes to measuring the output created on land which belongs to different land-use categories. This type of methodological problem is not unknown in planning [START_REF] Burton | Measuring urban compactness in UK towns and cities[END_REF]. In this paper we shall deal with land as a separate factor of production by introducing a method to calculate the productivity of land that is used exclusively for economic activities. We define land productivity as the output (added value) Our research into industrial land in the Netherlands is also embedded in recent planning debates on industrial estates in the Netherlands. The dissatisfaction with the spatial outcome of the public planning and development of industrial land is shared by environmental pressure groups and various government agencies [START_REF] Vromraad | Werklandschappen. Een Regionale Strategie voor Bedrijventerreinen[END_REF][START_REF] Segeren | De markt doorgrond: een institutionele analyse van de grondmarkten in Nederland[END_REF]. The assumption is that spatial productivity on industrial estates is low and that this is a direct result of the common strategy by municipalities to supply generous amounts of low-priced industrial land in order to promote local economic development [START_REF] Needham | Institutional Economics and Policies for Changing Land Markets: the case of industrial estates in the Netherlands[END_REF][START_REF] Louw | Planning of industrial land in the Netherlands: its rationales and consequences[END_REF][START_REF] Van Der Krabben | Stimulating private development of industrial estates in the Netherlands: a property rights approach[END_REF]. The land is then 'spoilt' as it cannot be used anymore for other purposes. This practice also leads to negative external effects, such as landscape pollution [START_REF] Hamers | Bloeiende bermen. Vestedelijking langs de snelweg[END_REF] and the increased physical obsolescence of existing industrial estates [START_REF] Janssen-Jansen | Spatial planning, the allocation of land and undesired outcomes of property development: looking for the right planning approach[END_REF]. It is assumed that a reduction in the development of industrial estates will increase spatial productivity. So far, however, no empirical evidence has been produced to demonstrate a relationship between the supply of industrial land and spatial productivity in the Netherlands.

In an international context we are aware only of a study by [START_REF] Zhu | The third-party logistics services and globalization of manufacturing[END_REF] in Singapore, who linked the increase in spatial productivity on industrial estates to the economic policy to develop new distriparks (which facilitated the restructuring of the Singapore manufacturing industry and altered the internal structure of firms in their utilisation of space).

In this paper we present data on regional differences in the spatial productivity of industrial land and we relate spatial productivity to local spatial planning strategies (municipal policies on industrial land development). Does a generous supply of land reduce spatial productivity?

We measure the land-use efficiency of industrial land in terms of spatial productivity and analyse regional differences in spatial productivity. In section two we describe how we 6 calculate spatial productivity. In section three we present our calculated dataset and analyse it with regression analysis. We conclude with a discussion of our research results and the usefulness of the concept of spatial productivity.

The calculation of land productivity: methodology Land -or spatial -productivity is defined here as the monetary added value per unit of industrial land (usually one net hectare). Industrial land is defined as land that is used exclusively for economic activities. To calculate productivity of industrial land it is necessary to separate the added value created on industrial land from the added value created on 'other' land. We dealt with this by looking at the relative level of employment on industrial land.

Our method to calculate land productivity consisted of four steps (see also Figure 1):

1. Determine the added value (GDP) per region (40 NUTS-3 regions) and per sector (24 sectors); 2. Determine the location of the employment (industrial estate or elsewhere); 3. Allocate the added value per sector and region to industrial estates; 4. Determine the land productivity for industrial estates.

(Figure 1 around here)

The added value in a given sector was allocated to the industrial estates in a given region (AVI prs ) as:

prs prs prs AV E AVI * =
(1) and to other land (AVO prs ) as: (5)

prs prs prs AV E AVO * ) 1 ( - = (2)
The variables were defined as follows:

AV prs the total added value of a given sector in a given region. The use of regional and sector data enabled us to take account of industrial and geographical differences in labour productivity. The data on industrial land were obtained from the Dutch Integral Industrial Estates Information System (IBIS). For each of the more than 3,500 existing industrial estates IBIS annually registers, among other things, size, type, area in use, disposal of serviced land and location. Not all industrial land is listed in IBIS. For example, small plots and sites used by only one company as well as fragmented industrial land within residential areas or Central Business Districts and land use by office parks are excluded.

Employment data were available from the LISA database, which covers each establishment in the Netherlands. Such a detailed level of geographical aggregation ensured that this dataset was well-suited for a GIS analysis with the spatial data from IBIS in order to determine the volume and composition of the employment on industrial estates. The output of the GIS analysis was the level of employment on industrial estates in 24 economic sectors for each of the 40 NUTS-3 regions in the Netherlands. Land productivity was calculated for 1995, 1999 and 2003.

Three important points should be noted concerning the interpretation of the results:

1. In our calculation we used the average output per job and not the output over a year's working time, as is common in calculations for labour productivity. The advantage of this approach was that the self-employed (for whom there are no data on working times) could be included in the calculations. One disadvantage was, however, that we had to assume that part-time jobs were equally distributed across industrial estates and beyond.

2. In our calculations we implicitly assumed that the labour productivity in each sector on industrial estates was similar to the labour productivity in the same sector outside industrial estates. We are not aware of research that assumes otherwise. This also 9 meant that we could not include the heterogeneity of labour in our analysis: we could not know if and how the characteristics of the labour force on industrial estates were different from other locations. The same was true for capital (or technology) inputs.

There were no data available that distinguished between labour or capital inputs on industrial estates and other locations. That said, one should remember that the regional differences for these inputs at sector level were implicitly included in our calculation (or estimation) of land productivity.

3. In 2001 the calculations for the national accounts were revised. This influenced the added value for the service sectors in particular.

One obvious alternative indicator of spatial productivity is land rents. But one of the problems with this indicator is the shortage of data on prices paid. Secondly, as very little land has been traded recently (compared to the total stock), there are no prices to adequately cover all the land in our analysis. The latter aspect is tied in with the sunk costs of property (including land) investments and the fact that most firms in the Netherlands own their property [START_REF] Groep | Bedrijfsruimtegebruikers in beeld. Resultaten Database Bedrijfsruimtegebruikers Nederland[END_REF]. However, there are also theoretical objections to the use of land rents in the analysis. First of all, rents can only be used as an alternative indicator if land markets are perfect and efficient. In reality, this is hardly ever the case and certainly not in the Netherlands where municipalities have almost a monopoly on the land market and where land values are determined by both political and economic policy considerations. As a consequence, land prices barely reflect the value of land as a production factor [START_REF] Segeren | De markt doorgrond: een institutionele analyse van de grondmarkten in Nederland[END_REF][START_REF] Groep | Bedrijfsruimtegebruikers in beeld. Resultaten Database Bedrijfsruimtegebruikers Nederland[END_REF][START_REF] Rebelo | Land economic rent computation for urban planning and fiscal purposes[END_REF]. Or, as Evans (2004:67) puts it: 'The property market is inherently imperfect, so the market only determines a range within which the actual sale price will lie'.

Secondly, there is evidence of very low elasticity in the price of industrial land in the Netherlands [START_REF] Zuidema | Bouwprijzen en de vraag naar bouwproductie[END_REF], which is partly explained by the fact that investments in land [START_REF] Louw | Planning van bedrijventerreinen[END_REF]. This also puts the question of endogeneity into perspective.

The productivity of industrial land in the Netherlands Despite mounting concern about their impact on the national landscape, industrial estates occupy only a small part of the total surface of the Netherlands. In 2003 they accounted for 2.3%, but this figure has risen rapidly, from only 2.0% in 1995. Employment on industrial estates increased between 1995 and 1999 from 29.1% to 31.9%. In 2003 it fell to 31.7%, a modest decrease, possibly explainable by the economic recession. A similar trend is discernible in the percentage of added value produced on industrial estates, which is slightly higher than the percentage of employment (also Table 1). The net and gross spatial productivity of industrial estates have both increased over the years. These figures show beyond doubt that industrial estates are important to the Dutch economy.

(Table 1 around here)

The rest of the paper is devoted entirely to an analysis of the net spatial productivity, because the net area is the amount of land that is designated as industrial land. Figure 2 shows the regional differences in the productivity of industrial land. In 2003 s-Gravenhage (The Hague) with €8.9 million per hectare, topped the list for spatial productivity while Oost Groningen (East Groningen) came bottom with €0.8 million. Although land productivity has increased over the years in most regions, the order from top-to-bottom has remained almost unchanged (see Figure 2), thus indicating stable regional patterns. Figure 3 shows a map of the regional How can we explain the regional differences in spatial productivity on industrial estates? We drew a distinction between variables relating to spatial planning strategies (our main objective) and two general variables which we used as control variables. The first of the policy-related variables was the supply of industrial land. [START_REF] Needham | Institutional Economics and Policies for Changing Land Markets: the case of industrial estates in the Netherlands[END_REF] suggest that a high supply of industrial land in the Netherlands results in low spatial productivity. In regions with a high demand for land and low levels of supply one would expect the price of industrial land, and hence, the land productivity to be high. Unfortunately, we were not able to test this relationship empirically, because of the absence of reliable data on the prices of serviced industrial land on industrial estates. The only price data in IBIS, the national database on industrial estates, are the 'asking prices' set by the suppliers which, in most cases, probably exceed the selling prices (the prices companies actually pay for their land).

However, we could still measure the scarcity of land by calculating regional differences in the supply-demand ratio. We defined supply as the area of building land on industrial estates that was immediately available to companies in a particular year. The demand was the 12 accumulated area of land on industrial estates that was issued to companies up to that same year. Because [START_REF] Needham | Institutional Economics and Policies for Changing Land Markets: the case of industrial estates in the Netherlands[END_REF] assume that past policies influence current landuse, we used a supply-demand ratio from the past (1988) and the three years for which we calculated the spatial productivity (1995, 1999 and 2003).

The second policy-related variable was the percentage of jobs in a region located on industrial estates. A high percentage of jobs on industrial estates in a region would indicate that in the past there had been (and probably still was) a spatial policy that favoured the development of industrial estates. If the assumption in the introduction is right, a negative relationship should exist between the percentage of employment on industrial estates and spatial productivity.

The third policy-related variable was the presence of physically obsolete industrial estates. In Dutch policy debates it is argued that due to the development of many new industrial estates, companies find it more efficient to relocate than to invest in their current properties. This implies that older estates are characterised by low employment densities due to many vacant premises and the presence of marginal operating companies which cannot afford to relocate. It was therefore expected that a region with a high share of obsolete industrial land would have a low level of spatial productivity. These data were only available for 2003.

Besides these policy-related variables there were other variables that we wanted to control for.

Louw and Bontekoning (2007) suggest that regional variations in sector composition on industrial estates could be an explanation. 13 expected to increase spatial productivity, whereas a high share of manufacturing sector employment was expected to decrease spatial productivity. Because these variables were significantly correlated we only applied the share of manufacturing employment.

A second plausible explanation for the regional differences was the difference in regional employment densities. [START_REF] Ciccone | Productivity and the density of economic activity[END_REF] showed that, due to agglomeration effects, an increase in employment densities has a positive effect on the average labour productivity.

Intriguing in this respect is that regional differences in labour productivity in the Netherlands (see [START_REF] Broersma | Regional Differences in Labour Productivity in the Netherlands[END_REF] are relatively small compared with regional differences in land productivity. This suggests that regional differences in labour productivity as such have no significant influence on land productivity, and that differences in land use are therefore more important. Since industrial estates are part of the regional economy and cannot be seen as isolated sites, we applied the regional urbanisation rate (average number of addresses per sq kilometre in a region) instead of the employment densities on industrial estates (1).

Other variables such as proximity to markets or a general measure of access were not included because of the limited number of regions we could use in our analysis (see below). Moreover, we did not compare the spatial productivity of individual estates, but we did compare the average spatial productivity of estates by region. It was likely that access to markets would have an influence mainly at the level of individual estates in relation to local markets, rather than in relation to regional or national markets.

(Table 2 around here).

Table 2 provides descriptive statistics for the variables we used in our analysis. The fact that the mean, range and standard deviation are much higher for the supply-demand ratio for 1988 than for the supply-demand ratios for 1995, 1999 and 2003 stems from differences in the IBIS In 1988 only estates with a minimum size of five hectares were included in the database. In the other years the minimum size was one hectare. This has scarcely any influence on the size of the supply as most of the industrial land supply is on estates larger than five hectares. However, the accumulated amount of land issued to companies in 1995 was 2.6 times higher than in 1988. So the mean, range and standard deviation for 1988 are higher than for the other three years. The IBIS database for 1988 reflects its original purpose, namely, to monitor the supply of industrial land, whereas in the 1990s the purpose was to monitor the stock of industrial land.

To find empirical evidence for the relationship between net land productivity and the independent variables described above, we estimated ordinary least squares (OLS) models for 1995, 1999 and 2003. We analysed only net land productivity because this was the amount of land used by firms. Multi-colinearity was checked by analysing correlations between the independent variables. In 1995 and 1999 a significant correlation was found at the 1% level between the urbanisation rate and the share of manufacturing employment. In 2003 a significant correlation was found between these variables at the 5% level. The same levels of significance applied for the correlation between the supply-demand ratios for 1988 and 1995/1999 and 2003. In the 2003 dataset a significant correlation at the 1% level was found between the supply-demand ratio for 2003 and the share of obsolete estates. These variables were, however, included in the analyses because the variance inflation factor (VIF) for these variables in all of the models was below 2.5. According to [START_REF] Field | Discovering Statistics using SPSS for Windows[END_REF] multi-collinearity becomes a problem if VIF > 10.

There is also the issue of spatial autocorrelation. Although this is highly relevant in regional science when regional differences are analysed in the form of an OLS model, we argue that it was not appropriate to include spatial autocorrelation in our analysis. The reason for this is that we were not dealing with continuous space but with many discontinuous spaces in the interaction, and spatial randomness, which form the theoretical foundation of spatial autocorrelation and which assume continuous space, did not apply to our data. On these theoretical grounds the inclusion of spatial autocorrelation could be justified only if the analysis was executed on the scale of individual industrial estates. Our aggregated data were not, however, suitable for calculating land productivity on this spatial scale. Moreover, given the relatively small size of individual industrial estates, it would be unrealistic to assume that they would have a significant influence on other estates in the region, let alone on estates in a neighbouring region (2).

Because we were primarily interested in the relative importance of the independent variables and the direction of the associations, only standardised parameters are presented here (Tables 3, 4 and5). In all of the models the control variables have the expected signs. Regional urbanisation rates are positively related to spatial productivity and the level of manufacturing employment is negatively related to spatial productivity. Also, both control variables are highly significant. Only the share of manufacturing in the 2003 models is significant at the 5% level instead of the 1% level in the 1995 and 1999 models. This slight drop in significance is probably tied in with the general decline in the level of manufacturing employment on industrial estates (see Table 2). It is clear from the standardised betas that in all of the models the urbanisation rate has the largest effect on the spatial productivity of the two control variables. This finding is consistent with the findings of [START_REF] Ciccone | Productivity and the density of economic activity[END_REF], [START_REF] Ciccone | Agglomeration effect in Europe[END_REF] and more general knowledge about urban economics: high densities go hand-in-hand with intensified land use.

Among the policy variables only the supply-demand ratio for 1988 is significant and has the expected sign in all of the models for every year (Equations 1 in Tables 3, 4 and5). That 3, 4 and5) have a less uniform effect on spatial productivity. Only in the 1995 and 1999 models does this variable show the expected negative sign and then it is only significant in 1999. In the 2003 model the supply-demand ratio is positively related to spatial productivity but is not significant (Equation 2 in Table 5). These findings imply that past supply-demand ratios have more effect on spatial productivity than recent supply-demand ratios. This may be due to the high level of supply-demand ratios in 1988 compared with 1995, 1999 and 2003 (see Table 2), which is mainly due to differences in the IBIS database.

(Tables 3-5 around here)

The share of regional employment on industrial estates is, as expected, negatively related to spatial productivity (Equations 3 in Tables 3, 4 and5). This effect is not significant, as shown in Equation 3 models of Tables 3, 4 and 5. However, in the overall model for 1999 (Equation 4in Table 4) the share of employment on industrial estates is significant while both supplydemand ratios are not.

In the 2003 model the share of obsolete industrial land does not, as expected, have a negative effect on spatial productivity. The correlations between this variable and the urbanisation rate and supply-demand ratios for 1988 and 2003, which are negative, seem to suggest a relationship other than was expected. A large number of physically obsolete industrial estates does not necessarily imply low spatial productivity at regional level. The above-mentioned rationale behind the expected negative sign may not even be valid since there is a significant positive correlation between the share of obsolete industrial land and the urbanisation rate and a significant negative correlation with supply-demand ratios for both 1988 and 2003, suggesting that the degree of obsolescence may have been caused by an under-supply rather than an over-supply.

By comparing the regression coefficients in the various models we can see how 'stable' the influence of that variable is over time. This is particularly relevant for the supply-demand ratio for 1988 given that it is insignificant in the overall model for 1999 (Equation 4), but significant in the 1995 and 2003 models. We compared the non-standardised coefficients in the models with each other -thus checking for stability over time -using the Confidence Interval for Difference (Cohen et al., 2003: 46-47). We applied this technique to models containing four variables: both control variables, the supply-demand ratio 1988 and the share of employment on industrial estates. The analysis revealed no significant difference between the regression coefficients of these variables between the 1995, 1999 and 2003 models.

Accordingly, our analysis of the regional differences in spatial productivity on industrial estates provides empirical evidence for the hypothesised relationship between the historical supply-demand ratio and spatial productivity. This is confirmed by Equation 4 in Table 3 and Equation 5 in Table 5. Using all policy variables simultaneously results in significant estimates for this policy variable in 1995 and 2003. In the 1999 model only the share of employment on industrial estates was significant, but the analyses with the Confidence Interval for Difference between the regression coefficient indicated that the coefficients themselves showed no significant difference between the models. Both control variables have a significant effect on spatial productivity. Taking the magnitude of the standardised coefficients of these variables into consideration, their effect on the spatial productivity is greater than the supply-demand ratio. There is a small but growing scientific interest in land-use efficiency. However, as neither regional science nor economic geography has examined this subject in great detail, economic arguments cannot be confidently applied in spatial planning debates. It is for this reason that we developed a method to calculate the spatial productivity for industrial land. The results revealed significant regional differences in the spatial productivity of land on industrial estates in the Netherlands. It is high in urbanised regions and low in peripheral regionswhich could be expected from studies about employment densities. The regression analysis showed that spatial productivity correlates negatively and significantly with the proportion of manufacturing jobs on industrial estates and the supply-demand ratio for 1988. The negative correlation between this historical supply-demand ratio and spatial productivity indicates that the way in which industrial estates are planned and developed in the Netherlands does influence spatial productivity: an abundant supply of industrial land reduces spatial productivity on industrial estates. Moreover, it suggests -within the constraints of our dataset -that a reduction in the supply of industrial land would increase productivity on that land.

The political consequences of this outcome are significant in the Dutch context. It appears that government intervention to reduce the development of industrial estates is unlikely to impede regional economic growth, but will rather result in increased spatial productivity on industrial estates. More recent lower supply-demand ratios do have a negative effect on spatial productivity but this is not significant for all the models. It may therefore be assumed that recent supply-demand ratios do not have a substantial effect on spatial productivity anymore.

Our research findings seem to confirm the existence of agglomeration economies. It cannot, however, be concluded that agglomeration effects occur on individual industrial estates, since our calculations are based on aggregated data about labour productivity on industrial estates.

A study by [START_REF] De Vor | Agglomeration externalities and localized employment growth: the performance of industrial sites in Amsterdam[END_REF] shows that there is hardly any sign of agglomeration In policy terms, the concept of spatial productivity is relevant to debates about the efficient use of scarce land. Information on (regional differences in) industrial land productivity can be particularly valuable in countries and regions where there is strong pressure to develop available land. The regression analysis in this paper shows that differences in land productivity can be partly explained by policy-related variables, which suggests that the level of spatial productivity of industrial land in a given region can be influenced.

The usefulness of the concept of spatial productivity must be considered theoretically as well as in policy terms. By using spatial productivity we treated land as if it were an independent production factor like labour. As in the calculations for labour productivity, we allocate all the added value to one production factor, in our case, land. However, the trend in economics over the last century has been to combine land with capital, effectively condensing the production factors to only labour and capital. The underlying logic for this practice is that land does not actually participate in industrial production, as it does in agriculture. Land is a necessity for industrial production but, contrary to other production factors, it is not worn out or transformed in the process.

As a production factor land has two attributes: location and the size of the production site [START_REF] Alonso | Location and Land Use[END_REF]. According to [START_REF] Hubacek | The Economics of Land Use Change[END_REF], producers usually take proportionate decisions between land and other factor inputs, when deciding on long-term investments. In the short term producers cannot substitute land for other factor inputs, because land is a quasi-fixed factor of production. Therefore, in the short-term resource allocation takes place on land, without considering land as a factor in the production process itself. (pearson correlation 1995: 0.958; 1999: 0.968; 2003: 0.966) because these densities are implicitly used in the calculation of spatial productivity. Using the employment densities on industrial estates to explain regional differences in spatial productivity yields high R²:

1995: 0.807; 1999: 0.891; 2004: 0.891). The unexplained variance is then due to regional differences in labour productivity.

2. This can also be derived from research by [START_REF] Dunse | The spatial pattern of industrial rents and the role of distance[END_REF]

  use density and multiple land-use, with sustainable land-use and sustainable

  land. In contrast to the classical economists we omit agricultural land and analyse only industrial and commercial land which is not mixed with other land-use types. For this purpose we use data on industrial estates in the Netherlands.

  use of labour productivity expressed as monetary added value per job to determine the added value on industrial estates. Next, the added value on industrial estates in a given region (AVI pr ) was determined as: value at other locations (AVO pr ) as: a given region (LP pr ) was then calculated by dividing the total added value on industrial land in that region by the area occupied by industrial estates in the same region:

  employment in a given sector located on industrial estates in a given region.pr Sthe total area occupied by industrial estates in a given region.Indices: s the index for economic sector, s = 1 to S, the total number of economic sectors.The subscripts: pr per region.prs per region and sector.

  added value were available from the national accounts of Statistics Netherlands.

  spatial productivity in 2003, which clearly indicates that the highest spatial productivity occurs in the most urbanised regions in the western part of the Netherlands (the Randstad). The Rotterdam region (Groot Rijnmond) appears to be the exception to this general picture, with a spatial productivity of € 2.0 million per hectare, which is modest compared with other regions in the Randstad (see also Figure2). The map in Figure3also shows that the regions with the lowest spatial productivity lie in the peripheral regions of the Netherlands, with the regions in between showing modest spatial productivity.(Figure2around here) (Figure3around here)

  estates within a region. These approximately 3,500 estates accounted for only 2.3% (2003) of the total surface area of the Netherlands and were scattered all around the country in both urban and rural areas. Spatial concepts such as distance-decay, spatial

  high supply-demand ratio in 1988 led to lower spatial productivity in1995, 1999[START_REF] Cohen | Applied multiple regression: correlation analysis for the behavioral sciences[END_REF]. The supply-demand ratios for 1995, 1999 and 2003 (Equations 2 in Tables

  scale of industrial estates. Agglomeration effects most probably exceed this local geographical scale.

  Different branches of industry require different amounts of land. Service sector companies, for example, require only a limited amount of space for each job and are accommodated in multi-storey office buildings, whereas manufacturing companies use single-storey buildings. Some manufacturing sectors such as

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refineries, chemical plants and transport require large areas of land. The variations in sector composition on industrial estates should therefore influence employment densities which should, in turn, influence spatial productivity. A high share of service sector employment was

  Therefore, land use in manufacturing and services is not a production factor in the strict sense of an input, as in agriculture. However, land, in terms of land use, is a necessary condition for production. Analysing spatial productivity therefore helps to improve our knowledge of the allocation process of production factors on land and the efficiency of this process and thus contributes to discussions about sustainable land use.

	F o r r o particular amount of land. F Notes 1. Employment densities on industrial estates correlates highly with spatial productivity
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By

measuring spatial productivity we determine how much added value is created by different decisions on resource allocation (including capital, labour, knowledge and location) on a

Table 2

 2 The gross area includes roads, harbours, green space, etc. The net area is the number of ha used by firms and the supply. ** This does not include the net area which has not yet been issued to firms. Variable means, range and standard deviations for 1995, 1999 and 2003 in the 40 Net land productivity on industrial estates in regions in 2003 (x € mln / ha).

	, who found that rent

* http://mc.manuscriptcentral.com/cres Email: regional.studies@fm.ru.nl http://mc.manuscriptcentral.com/cres Email: regional.studies@fm.ru.nl

Table 3

 3 Regression analysis for net land productivity in 1995 in the 40 regions (standardised betas).

		(1)	(2)	(3)	(4)
	Urbanisation rate	0.50***	0.48***	0.48***	0.48***
		(4.21)	(3.85)	(3.63)	(4.01)
	Share manufacturing employment	-0.36***	-0.34**	-0.29***	-0.37***
		(-3.05)	(-2.64)	(-3.03)	(-3.02)
	Supply-demand ratio 1988	-0.26***			-0.35**
		(-2.80)			(-2.51)
	Supply-demand ratio 1995		-0.16		0.07
			(-1.54)		(0.52)
	Share employment on estates			-.04	-0.12
				(-0.40)	(-1.24)
	F	26.37***	21.60***	19.64***	16.18***
	Adjusted R²	0.66	0.61	0.59	0.66
	t-values in parentheses.
	*** significant at the 1% level.
	** significant at the 5% level.

Acknowledgements

The authors wish to acknowledge the assistance of the Dutch Government through the Habiforum Innovative Land Use Programme. We also wish to thank two anonymous referees and Barrie Needham for their comments on earlier versions of this paper. The usual disclaimer applies.

R12 -Size and Spatial Distributions of Regional Economic Activity < R1 -General Regional Economics < R -Urban, Rural, and Regional Economics, R14 -Land Use Patterns < R1 -General Regional Economics < R -Urban, Rural, and Regional Economics, R52 -Land Use and Other Regulations < R5 -Regional Government Analysis < R -Urban, Rural, and Regional Economics