Maternal growth restriction and stress exposure in rats differentially alters expression of components of the placental glucocorticoid barrier and nutrient transporters

The placenta plays a major role in the development of fetal growth restriction, which affects 10% of pregnancies and contributes to chronic adult disease risk. We have reported that female rats born small develop cardiometabolic dysfunction only during pregnancy. The physiological tests performed during pregnancy induced a maternal stress response as indicated by increased maternal corticosterone concentrations. This stress effected placental growth compared to females who were unhandled during pregnancy. Maternal stress and growth restriction independently program F2 offspring metabolic dysfunction. This study investigated the effects of maternal stress and growth restriction on placental and fetal metabolic parameters that may contribute to F2 offspring metabolic disease. Maternal growth restriction reduced F2 fetal weight whilst maternal stress reduced placental weight. Stressed mothers had reduced insulin and increased glucose concentrations, changes that were reflected in the fetus. Fetal β-cell number was reduced by maternal growth restriction, but was increased by stress exposure. Maternal growth restriction reduced placental Slc2a1, Igf2, Slc38a2 and Nr3c1 gene expression. Maternal stress decreased the expression of Slc2a1, Igf2, Slc38a2, Nr3c1, Slc2a3, Slc2a4, Nr3c2, Hsd11b2, Crhr1 and Ogt. Maternal birth weight effects on fetal weight were likely due to changes in placental nutrient transporter and Igf2 expression. On the contrary, maternal stress induced a systemic effect by altering maternal metabolic parameters, placental gene expression and fetal glucose and insulin concentrations. This study highlights the importance of informing pregnant women on effective ways to cope with stress during pregnancy to prevent adverse long-term disease outcomes in their children.     

Review: Adaptation in placental nutrient supply to meet fetal growth demand: Implications for programming

This review considers the hypothesis that adaptations in blood flow, exchange surface area and transporter activity enable placental supply capacity to meet fetal demand and cause alterations in fetal composition which result in life-long programming of homeostatic set points. We consider the components of placental supply capacity and describe the predominant changes each of these could impose on solute and water exchange across the placenta. We next consider the evidence that adaptations in placental nutrient supply to meet the demands of fetal growth and development do occur. Evidence from human and mouse studies suggests that adaptations occur in regulation of blood flow through the fetoplacental circulation, in exchange barrier surface area and in transporter-mediated processes for amino acids and calcium. Crucially there appear to be differences in the gestational timing of these adaptations. Finally we suggest that each of these adaptations could have separate effects on the composition of the fetus. These could affect physiological set points in different ways and so programme the lifetime responses of the individual.     

The Role of the Placenta in Fetal Programming—A Review

The fetal origins hypothesis proposes that adult cardiovascular and metabolic disease originate through developmental plasticity and fetal adaptations arising from failure of the materno-placental supply of nutrients to match fetal requirements. The hypothesis is supported by experimental data in animals indicating that maternal nutrition can programme long term effects on the offspring without necessarily affecting size at birth. There is now evidence linking body composition in pregnant women and the balance of nutrient intake during pregnancy with raised levels of cardiovascular risk factors in the offspring. Maternal body composition and diet are thought to affect fetal development and programming as a result of both direct effects on substrate availability to the fetus and indirectly through changes in placental function and structure. Alterations in placental growth and vascular resistance, altered nutrient and hormone metabolism in the placenta, and changes in nutrient transfer and partitioning between mother, placenta and fetus all have important effects on the fetal adaptations thought to be central to programming. Future interventions to improve placental function are likely to have lifelong health benefits for the offspring.     

Developmental Plasticity of the Microscopic Placental Architecture in Relation to Litter Size Variation in the Common Marmoset Monkey (Callithrix jacchus)

Fetal demand, shaped by factors such as number of fetuses, may alter placental regulation of exchange, even when maternal nutrition restriction is not overt. The marmoset is an interesting model in which to examine this aspect of placental function due to unique placentation that leads to multiple fetuses sharing a unified placental mass. We demonstrated previously that the triplet marmoset placenta exhibits significantly higher efficiency than does the twin placenta. Here, we test the hypothesis that this increased efficiency is due to increases in changes in the microscopic morphology of the placenta. Stereology was employed to analyze the microscopic architecture of placentas from twin and triplet pregnancies. Compartments of interest were the trabeculae, intertrabecular space, fetal capillaries, and the surface area of the maternal–fetal interface. Placentas from the two litters did not differ significantly in overall volume or individual volumetric compartments, but triplet placentas exhibited significant expansion of the trabecular surface area in comparison to twins (p = 0.039). Further, the two groups differed in the isomorphy coefficient, with triplet placentas having a significantly higher coefficient (p = 0.001) and potentially a more complex microscopic topography. Differences in the maternal–fetal interface may be due to developmental constraints on gross placental growth that occur earlier in gestation, such that the only option for maintaining sufficient access to maternal resources or signaling pathways late in gestation is via an expansion of the interface. Despite the significant increase in overall surface area, individual triplet fetuses are associated with much less surface area than are individual twins, suggestive of alterations in metabolic efficiency, perhaps via differential amino acid transport regulation.     

Placental adaptations to the maternal–fetal environment: implications for fetal growth and developmental programming

The placenta is a transient organ found in eutherian mammals that evolved primarily to provide nutrients for the developing fetus. The placenta exchanges a wide array of nutrients, endocrine signals, cytokines and growth factors with the mother and the fetus, thereby regulating intrauterine development. Recent studies show that the placenta is not just a passive organ mediating maternal–fetal exchange. It can adapt its capacity to supply nutrients in response to intrinsic and extrinsic variations in the maternal–fetal environment. These dynamic adaptations are thought to occur to maximize fetal growth and viability at birth in the prevailing conditions in utero. However, some of these adaptations may also affect the development of individual fetal tissues, with patho-physiological consequences long after birth. Here, this review summarizes current knowledge on the causes, possible mechanisms and consequences of placental adaptive responses, with a focus on the regulation of transporter-mediated processes for nutrients. This review also highlights the emerging roles that imprinted genes and epigenetic mechanisms of gene regulation may play in placental adaptations to the maternal–fetal environment.The placenta regulates intrauterine development in mammals by performing the function of several adult organs for the growing fetus. These tasks are achieved by a multitude of specialized cell types. Recent work suggests the placenta has the ability to ‘sense’ the maternal and fetal environment and respond to changes in a dynamic fashion. These placental adaptations are particularly important when dealing with suboptimal conditions for growth and development. In this paper, we review the current knowledge on what triggers these adaptations, how the placenta responds to maternal and fetal signals, what these signals might be and what the long-term consequences are for the function of adult organs if the placenta fails to fully adapt. We argue that a special class of genes, so-called imprinted genes, are key regulators of placental adaptive responses to physiological stressors. These genes are different from the rest because they retain information about their parental origin and because they have evolved to be ‘selfish’. They play special roles in the placenta, as we will describe, which include controlling the amounts of maternal nutrients that go across to the fetus and the manipulation of maternal physiology, sometimes in conflict with the mother’s own interests.     

Placental dysfunction and fetal programming: the importance of placental size, shape, histopathology, and molecular composition

Normal function of the placenta is pivotal for optimal fetal growth and development. Fetal programming commonly is associated with placental dysfunction that predisposes to obstetric complications and suboptimal fetal outcomes. We consider several clinical phenotypes for placental dysfunction that likely predispose to fetal programming. Some of these reflect abnormal development of the chorioallantoic placenta in size, shape, or histopathology. Others result when exogenous stressors in the maternal environment combine with maladaptation of the placental response to yield small placentas with limited reserve, as typical of early-onset intrauterine growth restriction and preeclampsia. Still others reflect epigenetic changes, including altered expression of imprinted genes, altered enzymatic activity, or altered efficiencies in nutrient transport. Although the human placenta is a transient organ that persists only 9 months, the effects of this organ on the offspring remain for a lifetime.     

Abnormal labyrinthine zone in the Hectd1-null placenta

IntroductionThe labyrinthine zone of the placenta is where exchange of nutrients and waste occurs between maternal and fetal circulations. Proper development of the placental labyrinth is essential for successful growth of the developing fetus and abnormalities in placental development are associated with intrauterine growth restriction (IUGR), preeclampsia and fetal demise. Our previous studies demonstrate that Hectd1 is essential for development of the junctional and labyrinthine zones of the placenta. Here we further characterize labyrinthine zone defects in the Hectd1 mutant placenta.MethodsThe structure of the mutant placenta was compared to wildtype littermates using histological methods. The expression of cell type specific markers was examined by immunohistochemistry and in situ hybridization.ResultsHectd1 is expressed in the labyrinthine zone throughout development and the protein is enriched in syncytiotrophoblast layer type I cells (SynT-I) and Sinusoidal Trophoblast Giant cells (S-TGCs) in the mature placenta. Mutation of Hectd1 results in pale placentas with frequent hemorrhages along with gross abnormalities in the structure of the labyrinthine zone including a smaller overall volume and a poorly elaborated fetal vasculature that contain fewer fetal blood cells. Examination of molecular markers of labyrinthine trophoblast cell types reveals increased Dlx3 positive cells and Syna positive SynT-I cells, along with decreased Hand1 and Ctsq positive sinusoidal trophoblast giant cells (S-TGCs).DiscussionTogether these defects indicate that Hectd1 is required for development of the labyrinthine zonethe mouse placenta.     

Review: Nutrient sulfate supply from mother to fetus: Placental adaptive responses during human and animal gestation

Nutrient sulfate has numerous roles in mammalian physiology and is essential for healthy fetal growth and development. The fetus has limited capacity to generate sulfate and relies on sulfate supplied from the maternal circulation via placental sulfate transporters. The placenta also has a high sulfate requirement for numerous molecular and cellular functions, including sulfate conjugation (sulfonation) to estrogen and thyroid hormone which leads to their inactivation. Accordingly, the ratio of sulfonated (inactive) to unconjugated (active) hormones modulates endocrine function in fetal, placental and maternal tissues. During pregnancy, there is a marked increase in the expression of genes involved in transport and generation of sulfate in the mouse placenta, in line with increasing fetal and placental demands for sulfate. The maternal circulation also provides a vital reservoir of sulfate for the placenta and fetus, with maternal circulating sulfate levels increasing by 2-fold from mid-gestation. However, despite evidence from animal studies showing the requirement of maternal sulfate supply for placental and fetal physiology, there are no routine clinical measurements of sulfate or consideration of dietary sulfate intake in pregnant women. This is also relevant to certain xenobiotics or pharmacological drugs which when taken by the mother use significant quantities of circulating sulfate for detoxification and clearance, and thereby have the potential to decrease sulfonation capacity in the placenta and fetus. This article will review the physiological adaptations of the placenta for maintaining sulfate homeostasis in the fetus and placenta, with a focus on pathophysiological outcomes in animal models of disturbed sulfate homeostasis.     

Review: The blood-brain barrier; protecting the developing fetal brain

While placental function is fundamental to normal fetal development, the blood-brain barrier provides a second checkpoint critical to protecting the fetal brain and ensuring healthy brain development. The placenta is considered the key barrier between the mother and fetus, regulating delivery of essential nutrients, removing waste as well as protecting the fetus from potentially noxious substances. However, disturbances to the maternal environment and subsequent adaptations to placental function may render the placenta ineffective for providing a suitable environment for the developing fetus and to providing sufficient protection from harmful substances. The developing brain is particularly vulnerable to changes in the maternal/fetal environment. Development of the blood-brain barrier and maturation of barrier transporter systems work to protect the fetal brain from exposure to drugs, excluding them from the fetal CNS. This review will focus on the role of the ‘other’ key barrier during gestation – the blood-brain barrier – which has been shown to be functional as early as 8 weeks' gestation.     

Regulation of placental nutrient transport--a review

Fetal growth is primarily determined by nutrient availability, which is intimately related to placental nutrient transport. Detailed information on the regulation of placental nutrient transporters is therefore critical in order to understand the mechanisms underlying altered fetal growth and fetal programming. After briefly summarizing the cellular mechanisms for placental transport of glucose, amino acids and free fatty acids, we will discuss factors shown to regulate placental nutrient transporters and review the data describing how these factors are altered in pregnancy complications associated with abnormal fetal growth. We propose an integrated model of regulation of placental nutrient transport by maternal and placental factors in IUGR.     

Current topic: comparative physiology of placental oxygen transport.

Development of knowledge about placental O2 transport (PO2) is discussed by focusing attention on the factors that determine umbilical venous PO2. In near-term pregnant sheep umbilical venous PO2 is much lower than maternal arterial PO2 and is about 20 torr lower than uterine venous PO2 in ewes who are the homozygous carriers of low O2 affinity ovine hemoglobin. Experimental evidence points to two main reasons for the low umbilical venous PO2 of sheep: (a) the uterine and umbilical circulations form an ineffective venous equilibration exchanger, and (b) a large uterine-umbilical venous PO2 gradient is required to draw O2 across a placental barrier which has a small O2 diffusing capacity relative to placental and fetal O2 demand and relative to the ineffective perfusion pattern. The latter explanation contradicts theoretical models which represent placental O2 transport as virtually 100 per cent blood flow limited. In near-term rabbits and guinea-pigs umbilical venous PO2 is also quite low, but for different reasons. In these species, the uterine and umbilical circulations form a countercurrent exchanger which allows the mother to perfuse the uterus at a very low rate. The effectiveness of countercurrent exchange is exploited to decrease the demand of pregnancy on the maternal circulation, rather than to increase the level of fetal oxygenation. There is suggestive, as yet inconclusive, evidence suggesting that in some species, notably the domestic cat, placental countercurrent exchange is combined with a low O2 affinity maternal hemoglobin and a sufficiently high uterine blood flow to produce a high level of umbilical venous PO2. The striking diversity and complexity of data about placental O2 transport demands great caution in applying comparative knowledge to the human placenta. Experimental evidence seems to indicate that the near-term human placenta is a venous equilibration exchanger, but the information which is presently available is inadequate for a firm conclusion.     

Maternal obesity induced by a ‘cafeteria’ diet in the rat does not increase inflammation in maternal,placental or fetal tissues in late gestation

IntroductionObesity during pregnancy can cause serious complications for maternal and infant health. While this has often been attributed to increased inflammation during obese pregnancy, human and animal studies exhibit variable results with respect to the inflammatory status of the mother, placenta and fetus. Cafeteria (CAF) feeding induces more inflammation than standard high-fat feeding in non-pregnant animal models. This study investigated whether maternal obesity induced by a CAF diet increases maternal, fetal or placental inflammation.MethodsMaternal obesity was established in rats by 8 weeks of pre-pregnancy CAF feeding. Maternal plasma inflammatory markers (IL-1β, IL-6, IL-10, IL-12p40, MCP1, GRO/KC, MIP-2 and TNFα) and expression of inflammatory genes (Tnfα, Il-6, Il-1β, Tlr2, Tlr4, Cox2 and Emr1) in maternal, placental and fetal tissues were measured at day 21 of gestation.ResultsDespite CAF animals having 63% more central body fat than controls at day 21 of gestation, plasma inflammatory markers were not increased; indeed, levels of IL-6, IL-12p40 and MIP2 were reduced slightly. Similarly, inflammatory gene expression remained largely unaffected by CAF feeding, except for slight reductions to Tlr4 and Emr1 expression in CAF maternal adipose tissue, and reduced Tlr4 expression in male labyrinth zone (LZ). The junctional zone (JZ) displayed increased Il-6 expression in CAF animals when fetal sexes were combined, but no inflammatory genes were affected by the CAF diet in fetal liver.ConclusionsMaternal obesity induced by a CAF diet before and during pregnancy does not increase the inflammatory status of the mother, placenta or fetus in late gestation.     

Localization of the placental BCRP/ABCG2 transporter to lipid rafts: Role for cholesterol in mediating efflux activity

IntroductionThe breast cancer resistance protein (BCRP/ABCG2) is an efflux transporter in the placental barrier. By transporting chemicals from the fetal to the maternal circulation, BCRP limits fetal exposure to a range of drugs, toxicants, and endobiotics such as bile acids and hormones. The purpose of the present studies was to 1) determine whether BCRP localizes to highly-ordered, cholesterol-rich lipid raft microdomains in placenta microvillous membranes, and 2) determine the impact of cholesterol on BCRP-mediated placental transport in vitro.MethodsBCRP expression was analyzed in lipid rafts isolated from placentas from healthy, term pregnancies and BeWo trophoblasts by density gradient ultracentrifugation. BeWo cells were also tested for their ability to efflux BCRP substrates after treatment with the cholesterol sequestrant methyl-β-cyclodextrin (MβCD, 5 mM, 1 h) or the cholesterol synthesis inhibitor pravastatin (200 μM, 48 h).Results and discussionBCRP was found to co-localize with lipid raft proteins in detergent-resistant, lipid raft-containing fractions from placental microvillous membranes and BeWo cells. Treatment of BeWo cells with MβCD redistributed BCRP protein into higher density non-lipid raft fractions. Repletion of the cells with cholesterol restored BCRP localization to lipid raft-containing fractions. Treatment of BeWo cells with MβCD or pravastatin increased cellular retention of two BCRP substrates, the fluorescent dye Hoechst 33342 and the mycotoxin zearalenone. Repletion with cholesterol restored BCRP transporter activity. Taken together, these data demonstrate that cholesterol may play a critical role in the post-translational regulation of BCRP in placental lipid rafts.     

Sex specific changes in placental growth and MAPK following short term maternal dexamethasone exposure in the mouse

Objectives

Maternal glucocorticoid (GC) exposure during pregnancy can alter fetal development and program the onset of disease in adult offspring. The placenta helps protect the fetus from excess GC exposure but is itself susceptible to maternal insults and may be involved in sex dependant regulation of fetal programming. This study aimed to investigate the effects of maternal GC exposure on the developing placenta.

Study design and main outcome measures

Pregnant mice were treated with dexamethasone (DEX-1 μg/kg/h) or saline (SAL) for 60 h via minipump beginning at E12.5. Placentas were collected at E14.5 and E17.5 and the expression of growth factors and placental transporters examined by real-time PCR and/or Western blot. Histological analysis was performed to assess for morphological changes.

Results

At E14.5, DEX exposed male and female fetuses had a lower weight compared to SAL animals but placental weight was lower in females only. Hsd11b2 and Vegfa gene expression was increased and MAPK1 protein expression decreased in the placentas of females only. At E17.5 placental and fetal body weights were similar and differences in MAPK were no longer present although HSD11B2 protein was elevated in placentas of DEX females. Levels of glucose or amino acid transporters were unaffected.

Conclusions

Results suggest sex specific responses to maternal GCs within the placenta. Decreased levels of MAPK protein in placentas of female fetuses suggest alterations in the MAPK pathway may contribute to the lower placental weights in this sex. This may contribute towards sex specific fetal programming of adult disease.     

Placental glycogen metabolism changes during walker tumour growth

The placenta provides all energy and nutrient requirements for healthy fetal development. The placenta in rats is capable of storing glycogen, although the placenta cells must therefore mobilize stored glycogen to its own glucose supply. Moreover, maternal glucose and/or placental lactate furnished the fetal growth. Adult female Wistar rats were divided into three groups: Control-C, tumour bearing-W; injected ascitic fluid-A. The rats were sacrificed on the 16th, 19th or 21st day of gestation, analysing the placenta and fetus weights and placental tissue samples was aliquoted for biochemical assays of glycogen and protein content and alkaline phosphatase activity. Placental sections were morphometrically analysed and glycogen positive cells were counted. The placental and fetal weight were significantly reduced in both W and A rats from 16th up to 21st day of gestation, which showed high levels of fetal reabsorption sites. Significant reduction in labyrinth zone at day 21 in both tumour bearing and ascitic fluid injected groups was shown, suggesting less substrate exchange at the maternal/fetal surface. The alkaline phosphatase activity as well total protein content were found to be reduced in W and A group. The total placental glycogen and glycogen cells decreased during tumour bearing and ascitic fluid injection, suggesting reduction in its own stored energy. Ascitic fluid injected group, representing an indirect tumour effect, presented similar reduction changes in the placenta to the tumour-bearing group. In conclusion, the tumour growth and, especially, ascitic fluid injection promoted irreversible placental tissue damage altering homeostasis and compromising fetal development.     

In Vivo Characteristics of Placental Amino Acid Transport and Metabolism in Ovine Pregnancy—A Review

The placental transport of amino acids which is nutritionally important is the net entry rate into the fetal circulation (the umbilical uptake). This entry rate is a function of transport across cell membranes, the effect of competition among amino acids for transport, particularly across the fetal surface of the trophoblast, and their metabolism and interconversion within the placenta. The result of these different interactive fluxes is that the relationship between maternal concentration and fetal supply of an amino acid differs for each amino acid. For some amino acids there are relatively large bidirectional fluxes at both the fetal and maternal surfaces of the placenta. These fluxes can be measured in vivo utilizing stable isotope methodology. There is an important interorgan exchange of amino acids between the placenta and fetal liver. This exchange is, at least in part, a function of the absence of gluconeogenesis in the fetal liver. Both glutamate and serine, which are released from the fetal liver, are taken up by the placenta from the fetal circulation and metabolized within the placenta.     

Cellular inhibitors of apoptosis (cIAP) 1 and 2 are increased in placenta from obese pregnant women

Introduction

Independent of their role in apoptosis, cellular inhibitors of apoptosis (cIAP) 1 and 2, have emerged as regulators of inflammation. Obesity in pregnancy is characterised by maternal and placental inflammation. Thus, the aim of this study was to determine the effect of maternal obesity and pro-inflammatory mediators on cIAP expression in human placenta.

Methods

The expression of cIAP was assessed in human placenta from lean (n = 15) and obese (n = 14) patients by qRT-PCR and Western blotting. Primary trophoblast cells were used to determine the effect of pro-inflammatory cytokines on cIAP expression, and the effect of cIAP siRNA on pro-inflammatory cytokines.

Results

cIAP1 and cIAP2, gene and protein expression were significantly higher in placenta from women with pre-existing maternal obesity compared to placenta form lean women. Additionally, bacterial endotoxin LPS and the pro-inflammatory cytokines tumour necrosis factor (TNF)-α and interleukin (IL)-1β significantly increased the expression of both cIAP1 and cIAP2 in primary trophoblast cells isolated from human term placenta. Knockdown of cIAP1 or cIAP2 in human primary trophoblast cells significantly decreased TNF-α induced expression and secretion of pro-inflammatory cytokines IL-6 and IL-8 and of matrix metalloproteinase (MMP)-9.

Discussion

cIAP1 and cIAP2 expression is increased in placenta from women with pre-existing maternal obesity and in response to treatment with pro-inflammatory cytokines. Functional studies in placental trophoblast cells revealed that cIAPs are involved in TNF-α induced-expression of pro-inflammatory cytokines. Given the central role of pro-inflammatory cytokines in placental nutrient transport, this data suggest that cIAP1 and cIAP2 may play a role in fetal growth and development.     

Undernutrition during the first half of gestation increases the predominance of fetal tissue in late-gestation ovine placentomes

OBJECTIVE: To investigate, in sheep, the effects of maternal undernutrition during the first half of pregnancy on placental growth and development and fetal growth. STUDY DESIGN: Six ewes (R) were subjected to a 15% reduction in nutrient intake for the first 70 days of gestation and thereafter received the recommended daily intake. Another group of six ewes (C) received the recommended daily intake throughout pregnancy. At 130 days gestation the ewes were killed and morphological and morphometrical measurements were carried out on the placenta and fetus. RESULTS: Undernutrition resulted in a significant alteration in placental morphology, which was seen as increased growth of the fetal side of the placenta in R animals. However, fetal size in late gestation was not affected by the undernutrition, suggesting that placental adaptation was successful in maintaining fetal growth. CONCLUSION: Placental adaptations, including changes in gross morphology, may preserve fetal growth if maternal undernutrition is not severe. The mechanisms remain to be elucidated.     

Comparison of leucine, serine and glycine transport across the ovine placenta

To estimate the transport rate of maternal glycine across the placenta [1-¹³C]glycine and L-[1-¹³]serine were infused intravenously in pregnant sheep using both continuous and bolus infusions. Each tracer was infused together with L-[1-¹³C]leucine, to enable a comparison with the placental transport of an essential amino acid. At steady state, fetal plasma leucine enrichment was 40 per cent of maternal enrichment, indicating that approximately 60 per cent of the entry rate of leucine into fetal plasma is derived from protein breakdown in the placenta and fetus. Fetal plasma glycine enrichment was 11 per cent of maternal and there was no detectable fetal serine enrichment. The direct flux of maternal leucine into the fetal circulation was approximately 3.0 (bolus experiments) to 3.6 (continuous infusion experiments) pmol/min (kg fetus) and greater than the estimated 1.4 μmol/min (kg fetus) direct flux of maternal glycine, despite the fact that the net umbilical uptake of glycine exceeds that of leucine. This supports the conclusion that placental glycine production is a quantitatively important contribution to fetal glycine uptake via the umbilical circulation. The fetal glycine supply from the placenta is provided by a relatively small direct maternal glycine transplacental flux and a larger contribution derived from serine utilization within the placenta for glycine production.