Factors affecting gas transfer across the placenta and the oxygen supply to the fetus

The factors that affect placental gas exchange are reviewed, with particular reference to recent measurements of the effect of changes in one or more of these factors on O2 delivery to the fetus and on fetal O2 uptake. Fetal or maternal placental blood flows and blood O2 capacities can be altered by 50% without any major change occurring in fetal O2 uptake: umbilical venous O2 content and fetal O2 delivery fall, but the O2 consumption of the fetus is maintained by increasing the fractional extraction of O2 from the blood. There is evidence that the fetus can also cope with a reduction in blood O2 affinity resulting from replacement of fetal with maternal blood. The critical level of O2 delivery is about 0.6 mmol.min-1.kg-1 in the fetal sheep. When O2 delivery is reduced below this level, by decreasing maternal placental blood flow, raising or lowering fetal haematocrit, decreasing maternal O2 capacity, or decreasing fetal O2 affinity, fetal O2 uptake tends to fall. The resultant tissue hypoxia and inability to maintain oxidative metabolism is reflected in a lowering of arterial blood pH and base excess. Whilst the results of short-term experiments suggest that there exists a large reserve for placental O2 transfer and fetal O2 supply, there is evidence that fetal O2 uptake is more tightly linked to O2 delivery when the latter is reduced for a period of days or weeks. In the long term, restriction of the supply of O2 and nutrients leads to a reduced rate of fetal growth and a reprogramming of tissue development.     

Human placental taurine transporter in uncomplicated and IUGR pregnancies: cellular localization, protein expression, and regulation

Transplacental transfer is the fetus' primary source of taurine, an essential amino acid during fetal life. In intrauterine growth restriction (IUGR), placental transport capacity of taurine is reduced and fetal taurine levels are decreased. We characterized the protein expression of the taurine transporter (TAUT) in human placenta using immunocytochemistry and Western blotting, tested the hypothesis that placental protein expression of TAUT is reduced in IUGR, and investigated TAUT regulation by measuring the Na(+)-dependent taurine uptake in primary villous fragments after 1 h of incubation with different effectors. TAUT was primarily localized in the syncytiotrophoblast microvillous plasma membrane (MVM). TAUT was detected as a single 70-kDa band, and MVM TAUT expression was unaltered in IUGR. The PKC activator PMA and the nitric oxide (NO) donor 3-morpholinosydnonimine decreased TAUT activity (P < 0.05, n = 7-15). However, none of the tested hormones, e.g., leptin and growth hormone, altered TAUT activity significantly. PKC activity measured in MVM from control and IUGR placentas was not different. In conclusion, syncytiotrophoblast TAUT is strongly polarized to the maternal-facing plasma membrane. MVM TAUT expression is unaltered in IUGR, suggesting that the reduced MVM taurine transport in IUGR is due to changes in transporter activity. NO release downregulates placental TAUT activity, and it has previously been shown that IUGR is associated with increased fetoplacental NO levels. NO may therefore play an important role in downregulating MVM TAUT activity in IUGR.     

Developmental dynamics of the definitive mouse placenta assessed by stereology

The mouse is an excellent model for studying the genetic basis of placental development, but analyses are restricted by the lack of quantitative data describing normal murine placental structure. This study establishes a technique for generating such data, applies stereological techniques on systematic uniform random sections of placentas between E12.5-E18.5 of gestation (E1.0 = day of the vaginal plug), and considers the results in the context of development of the labyrinth zone. Half of each placenta was wax embedded and exhaustively sectioned to determine absolute volumes of the labyrinth zone (Lz), junctional zone (Jz), and decidua using the Cavalieri principle. The other half was resin embedded and 1-microm sections were used to generate all volume, surface, and length densities within the Lz. Maximum placental volume is reached by E16.5, whereas the Lz volume fraction increases until E18.5 at the expense of the Jz and decidua. Within the Lz, the absolute volume and surface area of maternal blood spaces (MBS) expand rapidly between E14.5 and E16.5, with no increase thereafter. In contrast, fetal capillary development is linear and continues for longer than that of the MBS. The interhemal membrane separating maternal and fetal circulations undergoes thinning prior to expansion of maternal and fetal surface areas, achieving a harmonic mean thickness of 4.39 microm by E18.5. The specific diffusion capacity for oxygen of the interhemal membrane is maximal by E16.5, which may be necessary to support rapid fetal growth until the end of gestation.     

Early treatment of the pregnant guinea pig with IGFs promotes placental transport and nutrient partitioning near term

Appropriate partitioning of nutrients between the mother and conceptus is a major determinant of pregnancy success, with placental transfer playing a key role. Insulin-like growth factors (IGFs) increase in the maternal circulation during early pregnancy and are predictive of fetal and placental growth. We have previously shown in the guinea pig that increasing maternal IGF abundance in early to midpregnancy enhances fetal growth and viability near term. We now show that this treatment promotes placental transport to the fetus, fetal substrate utilization, and nutrient partitioning near term. Pregnant guinea pigs were infused with IGF-I, IGF-II (both 1 mg.kg-1.day-1) or vehicle subcutaneously from days 20-38 of pregnancy (term=69 days). Tissue uptake and placental transfer of the nonmetabolizable radio analogs [3H]methyl-D-glucose (MG) and [14C]aminoisobutyric acid (AIB) in vivo was measured on day 62. Early pregnancy exposure to elevated maternal IGF-I increased placental MG uptake by>70% (P=0.004), whereas each IGF increased fetal plasma MG concentrations by 40-50% (P<0.012). Both IGFs increased fetal tissue MG uptake (P<0.048), whereas IGF-I also increased AIB uptake by visceral organs (P=0.046). In the mother, earlier exposure to either IGF increased AIB uptake by visceral organs (P<0.014), whereas IGF-I also enhanced uptake of AIB by muscle (P=0.044) and MG uptake by visceral organs (P=0.016) and muscle (P=0.046). In conclusion, exogenous maternal IGFs in early pregnancy sustainedly increase maternal substrate utilization, placental transport of MG to the fetus, and fetal utilization of substrates near term. This was consistent with the previously observed increase in fetal growth and survival following IGF treatment.     

Uterine blood flow, oxygen and glucose uptakes at mid-gestation in the sheep

In early ovine fetal development, the placenta grows more rapidly than the fetus so that at mid-gestation the aggregate weight of placental cotyledons exceeds fetal weight. The purpose of this study was to compare two separate methods of measuring uterine blood flow and glucose and oxygen uptakes in seven mid-gestation ewes, each carrying a single fetus. Uterine blood flow to both uterine horns was measured by microsphere and by tritiated water steady-state diffusion methodology. Calculations of tritiated water blood flows and oxygen and glucose uptakes were based on measurements of arteriovenous concentration differences across each uterine horn. The distribution of blood flow and oxygen uptake between the two uterine horns was strongly correlated with placental mass distribution. The two methods gave comparable results for uterine blood flow (457 +/- 35 vs 476 +/- 35 ml/min), oxygen uptake (457 +/- 35 vs 476 +/- 35 mumol/min), and glucose uptake (63 +/- 8 vs 64 +/- 6 mumol/min). Uterine blood flow was approximately 38% of the late gestation value and 56.1 +/- 1 times higher than umbilical blood flow. Uteroplacental oxygen consumption was about 58% of late gestation measurements and 3.9 +/- 0.5 times higher than fetal oxygen uptake. We confirm that the large placental mass of mid-gestation is associated with high levels of maternal placental blood flow and placental oxidative metabolism.     

Numerical simulation of a glucose sensitive composite membrane closed-loop insulin delivery system

Closed-loop insulin delivery system works on pH modulation by gluconic acid production from glucose, which in turn allows regulation of insulin release across membrane. Typically, the concentration variation of gluconic acid can be numerically modeled by a set of non-linear, non-steady state reaction diffusion equations. Here, we report a simpler numerical approach to time and position dependent diffusivity of species using finite difference and differential quadrature (DQ) method. The results are comparable to that obtained by analytical method. The membrane thickness directly determines the concentrations of the glucose and oxygen in the system, and inversely to the gluconic acid. The advantage with the DQ method is that its parameter values need not be altered throughout the analysis to obtain the concentration profiles of the glucose, oxygen and gluconic acid. Our work would be useful for modeling diabetes and other systems governed by such non-linear and non-steady state reaction diffusion equations.     

Regulation of Human Trophoblast GLUT1 Glucose Transporter by Insulin-Like Growth Factor I (IGF-I)

Glucose transport to the fetus across the placenta takes place via glucose transporters in the opposing faces of the barrier layer, the microvillous and basal membranes of the syncytiotrophoblast. While basal membrane content of the GLUT1 glucose transporter appears to be the rate-limiting step in transplacental transport, the factors regulating transporter expression and activity are largely unknown. In view of the many studies showing an association between IGF-I and fetal growth, we investigated the effects of IGF-I on placental glucose transport and GLUT1 transporter expression. Treatment of BeWo choriocarcinoma cells with IGF-I increased cellular GLUT1 protein. There was increased basolateral (but not microvillous) uptake of glucose and increased transepithelial transport of glucose across the BeWo monolayer. Primary syncytial cells treated with IGF-I also demonstrated an increase in GLUT1 protein. Term placental explants treated with IGF-I showed an increase in syncytial basal membrane GLUT1 but microvillous membrane GLUT1 was not affected. The placental dual perfusion model was used to assess the effects of fetally perfused IGF-I on transplacental glucose transport and syncytial GLUT1 content. In control perfusions there was a decrease in transplacental glucose transport over the course of the perfusion, whereas in tissues perfused with IGF-I through the fetal circulation there was no change. Syncytial basal membranes from IGF-I perfused tissues showed an increase in GLUT1 content. These results demonstrate that IGF-I, whether acting via microvillous or basal membrane receptors, increases the basal membrane content of GLUT1 and up-regulates basal membrane transport of glucose, leading to increased transepithelial glucose transport. These observations provide a partial explanation for the mechanism by which IGF-I controls nutrient supply in the regulation of fetal growth.     

Imprinted genes, placental development and fetal growth

In mammals, imprinted genes have an important role in feto-placental development. They affect the growth, morphology and nutrient transfer capacity of the placenta and, thereby, control the nutrient supply for fetal growth. In particular, the reciprocally imprinted Igf2-H19 gene complex has a central role in these processes and matches the placental nutrient supply to the fetal nutrient demands for growth. Comparison of Igf2P0 and complete Igf2 null mice has shown that interplay between placental and fetal Igf2 regulates both placental growth and nutrient transporter abundance. In turn, epigenetic modification of imprinted genes via changes in DNA methylation may provide a mechanism linking environmental cues to placental phenotype, with consequences for development both before and after birth. Changes in expression of imprinted genes, therefore, have major implications for developmental programming and may explain the poor prognosis of the infant born small for gestational age and the wide spectrum of adult-onset diseases that originate in utero.     

The effect of elevation of maternal plasma catecholamines on the fetus and placenta of the pregnant sheep

To study the effects of reduced uterine blood flow on fetal and placental metabolism, adrenaline has been infused at physiological doses (0.5 microgram/min per kg) into the circulation of the pregnant sheep. This gives a reduction of about one third of uterine blood flow at days 120-143 of pregnancy, but causes no significant change in umbilical blood flow. In contrast to the effects of constricting the uterine artery to reduce blood flow to a similar degree, placental oxygen consumption was reduced and that, together with a large increase in lactate production, indicated the placenta became hypoxic. The fetal blood gas status and hence oxygen consumption was not affected significantly. A consistent arterio-venous difference for glucose across the umbilical or uterine circulations was not detected unless the uterine blood flow was comparatively high. Glucose balance across the uterus showed a close linear relationship with uterine blood flow and more particularly with the supply of glucose to the uterus. There was clear evidence for glucose uptake by the placenta and fetus and also glucose output by both. The latter was more common when uterine blood flow was comparatively low or reduced by adrenaline infusion. The results are consistent with the concept that glucose supply has to be maintained to the placenta even at the expense of fetal stores, although lactate can substitute if there is enhanced output because of fetal hypoxia. They indicate that placental mobilisation of glycogen can lead to a net output of glucose to the mother. The manner of communicating to the fetus changes in placental state that occur during maternal adrenaline infusion is not clear. However towards the end of the 60 min infusion, elevation of fetal plasma adrenaline, probably resulting from a breakdown of the placental permeability barrier, may be an important signal.     

Adipokines, an adipose tissue and placental product with biological functions during pregnancy

Latter half of pregnancy is characterized by a "physiological diabetogenic state" since changes in insulin-sensitivity have been well documented. These changes ensure continuous supply of nutrients to the growing fetus. In the last years the role of adipocyte-derived signaling molecules, collectively known as adipokines has been object of different in vitro and in vivo studies. Of interest, adipokines and/or their receptors are expressed in the placental tissue which, therefore, can contribute to development of maternal insulin-resistance and, as a consequence, fetal growth. Leptin, adiponectin, and resistin represent the most well studied adipokines and, with the exception of adiponectin, their serum and placental levels increase as pregnancy progresses. High levels of adipokines have also been detected in umbilical plasma hence suggesting a possible role on fetal development and metabolism; however, it remains still unclear if such adipokines can directly stimulate fetal tissues development acting as growth factors. In addition to their well known metabolic effects, we also reported studies describing the role of adipokines in promoting proliferation and invasiveness of trophoblast cells and affecting local angiogenic processes. These observations strongly suggest that adipokines, by alternatively interfering with placental development, may affect pregnancy outcome and fetal growth. However, further studies are needed to better understand the local regulation of their expression. ? 2012 International Union of Biochemistry and Molecular Biology, Inc.     

Placental diffusing capacity and its relation to fetal growth.

The placental diffusing capacity for carbon monoxide was measured in unanaesthetized monkeys (M. Mulatta). Maternal and fetal blood was sampled from chronically placed catheters while the mother breathed 50 or 100 parts per million of CO. Diffusing was calculated from the amount of CO taken up by the fetus divided by the partial pressure difference across the placenta, it averaged 0.646 plus or minus 0.062 (SEM) ml x min(-1) x torr(-1) x kg(-1) of fetal weight. The significance of this index of respiratory gas exchange in the monkey placenta is discussed with respect to previous measurements in other species and with respect to fetal growth.     

Maternal lipid metabolism and placental lipid transfer

During early pregnancy, long-chain polyunsaturated fatty acids (LC-PUFA) may accumulate in maternal fat depots and become available for placental transfer during late pregnancy, when the fetal growth rate is maximal and fetal requirements for LC-PUFAs are greatly enhanced. During this late part of gestation, enhanced lipolytic activity in adipose tissue contributes to the development of maternal hyperlipidaemia; there is an increase in plasma triacylglycerol concentrations, with smaller rises in phospholipid and cholesterol concentrations. Besides the increase in plasma very-low-density lipoprotein, there is a proportional enrichment of triacylglycerols in both low-density lipoproteins and high-density lipoproteins. These lipoproteins transport LC-PUFA in the maternal circulation. The presence of lipoprotein receptors in the placenta allows their placental uptake, where they are hydrolysed by lipoprotein lipase, phospholipase A(2) and intracellular lipase. The fatty acids that are released can be metabolized and diffuse into the fetal plasma. Although present in smaller proportions, maternal plasma non-esterified fatty acids are also a source of LC-PUFA for the fetus, their placental transfer being facilitated by the presence of a membrane fatty acid-binding protein. There is very little placental transfer of glycerol, whereas the transfer of ketone bodies may become quantitatively important under conditions of maternal hyperketonaemia, such as during fasting, a high-fat diet or diabetes. The demands for cholesterol in the fetus are high, but whereas maternal cholesterol substantially contributes to fetal cholesterol during early pregnancy, fetal cholesterol biosynthesis rather than cholesterol transfer from maternal lipoproteins seems to be the main mechanism for satisfying fetal requirements during late pregnancy.     

Ontogeny of somatomedin during development in the mouse. Serum concentrations, molecular forms, binding proteins, and tissue receptors

The purpose of this study was to assess the ontogeny of serum concentrations and molecular forms of somatomedin during fetal and postnatal development and to define the changes in serum binding proteins for somatomedin-C during various stages of development. The finding that fetal, placental, and decidual mouse tissues possess receptors for somatomedin suggests a role for somatomedin in fetal growth and possibly in the maintenance of pregnancy. Serum somatomedin-C was measured using a highly specific, heterologous radioimmunoassay (RIA) and a less specific membrane binding assay (MBA) which is more sensitive to the influence of somatomedins other than somatomedin-C. The assays were validated for mouse serum by showing that serum concentrations were reduced in genetically growth hormone-deficient mice and in hypophysectomized mice and were increased by growth hormone therapy. As in the human, the RIA measures only a portion of the somatomedin-C present in mouse serum. This “covering up” of somatomedin is attributed to the presence of serum binding proteins and is corrected by treatment of serum samples with acid. By both RIA and MBA, serum somatomedin concentrations are low in fetal and newborn mice, begin to rise in the fourth postnatal week, and reach adult values by 7 weeks of age. The chromatographic pattern of adult mouse serum on Sephacryl 200 is similar to that observed with human sera: The immunoreactive material elutes at apparent molecular weights of 140,000 and 30,000–40,000. The elution profile of ¹²⁵I-labeled somatomedin-C bound to components of serum is nearly identical to the pattern of endogenous activity. As with human serum, somatomedin-C in acidified mouse serum elutes at a lower molecular weight, coincident with insulin and purified somatomedin-C. Maternal serum somatomedin declines in the last half of gestation at the time when placental lactogen levels rise. Along with the absolute decline in somatomedin content is the appearance of unsaturated sites on somatomedin binding proteins. These findings are unexpected and unexplained since somatomedin rises late in pregnancy in humans and several lines of evidence suggest that placental lactogen has the capacity to stimulate somatomedin production. We previously have presented evidence that explants of multiple fetal mouse tissues synthesize somatomedin-C. The present study shows that the immunoreactive somatomedin-C in fetal mouse serum shares identical characteristics with those reported previously for media obtained from mouse liver explants. It seems possible that somatomedin's actions are exerted primarily at or near its site of production and that circulatory levels do not reflect the importance of somatomedin-C on fetal growth. While elucidation of the dramatic developmental changes in serum content and molecular forms of somatomedin-C and in somatomedin binding proteins may be essential to clarifying the role of somatomedin on fetal growth, proof that somatomedin stimulates fetal growth will depend in large part on studies of its biological actions on fetal tissues.     

Growth and in-vitro metabolism of placental tissues of cows from day 100 to day 250 of gestation.

Weight of placental tissues of cows increased exponentially from Day 100 to Day 250 of gestation, but at much slower relative and absolute rates than fetal weight. In addition, growth rate of fetal placental tissues was less than that of maternal placental tissues. Concentrations of DNA, RNA and protein, however, increased in fetal placental but not in maternal placental tissues. Fetal placental tissues therefore exhibited hyperplasia, which probably contributes to increased functional capacity of the placenta during late gestation. The rate of O2 uptake in vitro was greatest for maternal placental tissues, suggesting that the maternal portion of the placenta accounts for most of the large rate of placental O2 utilization in vivo. Compared with other placental tissues, rate of secretion of macromolecules by intercaruncular endometrium was high, but decreased from Day 100 to 250, suggesting that uterine glandular secretory activity may decrease as gestation advances. Rate of secretion of macromolecules also was high for intercotyledonary tissues and increased with day of gestation, suggesting a role for secretory products of chorioallantois in gravid uterine function.     

Fetal Growth Retardation and Lack of Hypotaurine in Ezrin Knockout Mice

Ezrin is a membrane-associated cytoplasmic protein that serves to link cell-membrane proteins with the actin-based cytoskeleton, and also plays a role in regulation of the functional activities of some transmembrane proteins. It is expressed in placental trophoblasts. We hypothesized that placental ezrin is involved in the supply of nutrients from mother to fetus, thereby influencing fetal growth. The aim of this study was firstly to clarify the effect of ezrin on fetal growth and secondly to determine whether knockout of ezrin is associated with decreased concentrations of serum and placental nutrients. Ezrin knockout mice (Ez^−/−) were confirmed to exhibit fetal growth retardation. Metabolome analysis of fetal serum and placental extract of ezrin knockout mice by means of capillary electrophoresis–time-of-flight mass spectrometry revealed a markedly decreased concentration of hypotaurine, a precursor of taurine. However, placental levels of cysteine and cysteine sulfinic acid (precursors of hypotaurine) and taurine were not affected. Lack of hypotaurine in Ez^−/− mice was confirmed by liquid chromatography with tandem mass spectrometry. Administration of hypotaurine to heterogenous dams significantly decreased the placenta-to-maternal plasma ratio of hypotaurine in wild-type fetuses but only slightly decreased it in ezrin knockout fetuses, indicating that the uptake of hypotaurine from mother to placenta is saturable and that disruption of ezrin impairs the uptake of hypotaurine by placental trophoblasts. These results indicate that ezrin is required for uptake of hypotaurine from maternal serum by placental trophoblasts, and plays an important role in fetal growth.     

Improving amino acid nutrition to prevent intrauterine growth restriction in mammals

Intrauterine growth restriction (IUGR) is one of the most common concerns in human obstetrics and domestic animal production. It is usually caused by placental insufficiency, which decreases fetal uptake of nutrients (especially amino acids) from the placenta. Amino acids are not only building blocks for protein but also key regulators of metabolic pathways in fetoplacental development. The enhanced demands of amino acids by the developing conceptus must be met via active transport systems across the placenta as normal pregnancy advances. Growing evidence indicates that IUGR is associated with a reduction in placental amino acid transport capacity and metabolic pathways within the embryonic/fetal development. The positive relationships between amino acid concentrations in circulating maternal blood and placental amino acid transport into fetus encourage designing new therapies to prevent or treat IUGR by enhancing amino acid availability in maternal diets or maternal circulation. Despite the positive effects of available dietary interventions, nutritional therapy for IUGR is still in its infancy. Based on understanding of the underlying mechanisms whereby amino acids promote fetal growth and of their dietary requirements by IUGR, supplementation with functional amino acids (e.g., arginine and glutamine) hold great promise for preventing fetal growth restriction and improving health and growth of IUGR offspring.     

The placental barrier: structure, resistance, asymmetry

The concept of placental barrier has been evaluated using recent advances in ultrastructure and in transport physiology. On a structural basis, the barrier effect is grounded by the syncytiotrophoblast continuity, and by basal and plasma membrane's electrical charges and by basement membrane porosity. The aqueous phase continuity for diffusion operates through intercellular gap, fenestrations (rat, rabbit) and transcellular channels (guinea pig). However, these connections are not apparent in the human syncytiotrophobast. For the molecular size selectivity, the hemochorial placentas with a pore radius of 10 nm appear much less selective than the epitheliochorial ones. The metabolic capacity of the placental cells (trophoblast, macrophages) participates to the barrier effect by metabolizing or by converting some substrates. Similarly, trophoblast asymmetry in the location of enzymes, carriers and receptors on outer (maternal side) and on basal (fetal side) plasma membranes, and in the release of secretory products, contributes to maintain separate fetal and maternal compartments. The functional polarity of trophoblast is expressed in metabolism (corticostéroids), nutrients (amino acids) and ions (iron) transport, and most of its secretions (hPL, hCG, SP1).     

Differential expression of the vascular endothelial growth factor-receptor system in the gravid uterus of yorkshire and Meishan pigs

Litter size in the pig is limited by uterine capacity, which is dependent on uterine size, placental size, and vascularity. Placentae of U.S. pig breeds, such as the Yorkshire, exhibit marked growth from mid to late gestation, increasing their surface area of endometrial attachment. In contrast, placentae of the prolific Chinese Meishan pig exhibit little growth from mid to late gestation; instead, they exhibit a marked and progressive increase in the density of placental blood vessels. Vascular endothelial growth factor (VEGF) is a potent angiogenic and permeability-enhancing factor that is produced and secreted by placentae of several species, including the pig. The activity of VEGF is mediated through two specific receptors (VEGF-R1 and VEGF-R2), both of which are expressed by placental and endometrial tissues in pigs and are thought to play a role in mediating increased vascularization and/or permeability at the fetal-maternal interface. The objectives of the present study were to determine concentrations of VEGF in fetal blood and placental fluids as well as placental and adjacent endometrial mRNA expression of VEGF, VEGF-R1, and VEGF-R2 on Days 30, 50, 70, 90, and 110 of gestation in Yorkshire and Meishan pigs. Day 90 Meishan conceptuses exhibited marked increases (P < 0.05) in placental VEGF mRNA expression as well as fetal blood and allantoic fluid concentrations of VEGF, which remained elevated through Day 110. In contrast, Yorkshire conceptuses failed to exhibit increases in placental VEGF mRNA expression or concentrations of VEGF in fetal blood or allantoic fluid until Day 110. Receptor mRNA expression patterns differed between Meishan and Yorkshire conceptuses, but no difference was found in their expression levels. Placental efficiency (fetal weight/placental weight) was higher (P < 0.05) on Days 90 and 110 in Meishan than in Yorkshire conceptuses. The earlier increase in VEGF protein and mRNA expression in the Meishan versus the Yorkshire conceptus may explain the previously reported increased vascularity and increased placental efficiency of this breed compared the Yorkshire breed.