Effects of sow nutrition during gestation on within-litter birth weight variation: a review

The increasing demand for efficiency in pork production requires great specialization of all sectors involved in this activity. In this context, the development of strategies that could reduce undesirable traits related with negative effects on piglet survival and postnatal growth and development are essential for the pig industry. Currently, special attention is given to variation in birth weight, as some evidences suggest an increased within-litter birth weight variation in modern sows. This variation has been shown to be associated with preweaning mortality, variable weights at weaning and deteriorated growth performance, which results in economic losses and lower efficiency. Therefore, understanding the factors that can influence the events that occur during gestation and that have an impact on the fetal growth and development are important to achieve better efficiency and also to develop strategies that can be used to achieve increased within-litter uniformity of piglet birth weight. This study concludes that even at a given placental size, fetal growth may vary because of differences in placental vascularization and efficiency. Feeding extra feed or energy during late gestation only marginally improves birth weight, and positive effects are not consistent between different studies. The detrimental effects of protein restriction on fetal growth during early gestation may be due to altered placental and endometrial angiogenesis and growth, which leads to a reduction in placental-fetal blood flow, nutrient supply from mother to the fetuses and ultimately to fetal growth retardation. The number of studies that attempted to influence within-litter birth weight variation by means of sow nutrition during gestation is limited. Therefore, more research concerning sow nutrition during gestation associated with the provision of balanced diets to meet requirements of the sows and fetuses are still required. This knowledge may subsequently provide starting points for the design of nutritional strategies that can influence within-litter birth variation.     

Impacts of arginine nutrition on embryonic and fetal development in mammals

Embryonic loss and intrauterine growth restriction (IUGR) are significant problems in humans and other animals. Results from studies involving pigs and sheep have indicated that limited uterine capacity and placental insufficiency are major factors contributing to suboptimal reproduction in mammals. Our discovery of the unusual abundance of the arginine family of amino acids in porcine and ovine allantoic fluids during early gestation led to the novel hypothesis that arginine plays an important role in conceptus (embryo and extra-embryonic membranes) development. Arginine is metabolized to ornithine, proline, and nitric oxide, with each having important physiological functions. Nitric oxide is a vasodilator and angiogenic factor, whereas ornithine and proline are substrates for uterine and placental synthesis of polyamines that are key regulators of gene expression, protein synthesis, and angiogenesis. Additionally, arginine activates the mechanistic (mammalian) target of rapamycin cell signaling pathway to stimulate protein synthesis in the placenta, uterus, and fetus. Thus, dietary supplementation with 0.83 % l-arginine to gilts consuming 2 kg of a typical gestation diet between either days 14 and 28 or between days 30 and 114 of pregnancy increases the number of live-born piglets and litter birth weight. Similar results have been reported for gestating rats and ewes. In sheep, arginine also stimulates development of fetal brown adipose tissue. Furthermore, oral administration of arginine to women with IUGR has been reported to enhance fetal growth. Collectively, enhancement of uterine as well as placental growth and function through dietary arginine supplementation provides an effective solution to improving embryonic and fetal survival and growth.     

Concentrations of progesterone in arterial and venous plasma of fetal pigs and their dams in late gestation

Blood samples were drawn from uterine arteries and veins of pregnant gilts and from the umbilical artery and vein of each of their fetuses during laparotomy at Day 80. Concentrations of progesterone (P) were greater in fetal than maternal plasma. Uptake of P from the placenta by the fetal blood was evident but was not equivalent to the maternal uterine arterial-venous difference in P concentration. No correlation between plasma P and fetal weight was noted. Concentrations of P in both umbilical vessels of female fetuses were higher than in male fetuses. These data indicate that fetal sex affects the rate of transport and/or synthesis of P in the utero/placental compartment and/or the rate of metabolism of P in the fetus. The relative importance of de novo synthesis and transplacental transport of P in establishing concentrations of P in fetal blood remains to be elucidated.     

The impact of induced stress during days 13 and 14 of pregnancy on the composition of allantoic fluid and conceptus development in sows

Stress due to regrouping of breeding females is difficult to avoid completely in loose-housing systems. The effects of stress during the maternal recognition of pregnancy on fetal development and survival at Day 30 of pregnancy was, therefore, studied in 17 sows allocated into one control (C-) group, one group deprived of food during Days 13 and 14 (FD-), and one group (A-), which was treated with ACTH (0.01 mg/kg body weight of Synacthen Depot) every sixth hour during the same period. Total number of fetuses, fetal survival rate, volume of allantoic fluid, and the weight and length of total fetal unit, placentas, allantochorion and fetuses were determined. The concentrations of progesterone (P4), PGFM, PGF2, PGE, estrone-sulfate, and estradiol-17beta in the allantoic fluid were analyzed. No significant differences between groups were found for any parameter measured except for P4. Food deprivation increased P4 concentration in the allantoic fluid, and there was a positive correlation between the P4 concentration and the weight of the placenta. It is, therefore, suggested that P4 influences the placenta size among food-deprived sows.     

Stanniocalcin (STC) in the endometrial glands of the ovine uterus: regulation by progesterone and placental hormones

Stanniocalcin (STC) is a hormone in fish that regulates calcium levels. Mammals have two orthologs of STC with roles in calcium and phosphate metabolism and perhaps cell differentiation. In the kidney and gut, STC regulates calcium and phosphate homeostasis. In the mouse uterus, Stc1 increases in the mesometrial decidua during implantation. These studies determined the effects of pregnancy and related hormones on STC expression in the ovine uterus. In Days 10-16 cyclic and pregnant ewes, STC1 mRNA was not detected in the uterus. Intriguingly, STC1 mRNA appeared on Day 18 of pregnancy, specifically in the endometrial glands, increased from Day 18 to Day 80, and remained abundant to Day 120 of gestation. STC1 mRNA was not detected in the placenta, whereas STC2 mRNA was detected at low abundance in conceptus trophectoderm and endometrial glands during later pregnancy. Immunoreactive STC1 protein was detected predominantly in the endometrial glands after Day 16 of pregnancy and in areolae that transport uterine gland secretions across the placenta. In ovariectomized ewes, long-term progesterone therapy induced STC1 mRNA. Although interferon tau had no effect on endometrial STC1, intrauterine infusions of ovine placental lactogen (PL) increased endometrial gland STC1 mRNA abundance in progestinized ewes. These studies demonstrate that STC1 is induced by progesterone and increased by a placental hormone (PL) in endometrial glands of the ovine uterus during conceptus (embryo/fetus and extraembryonic membranes) implantation and placentation. Western blot analyses revealed the presence of a 25-kDa STC1 protein in the endometrium, uterine luminal fluid, and allantoic fluid. The data suggest that STC1 secreted by the endometrial glands is transported into the fetal circulation and allantoic fluid, where it is hypothesized to regulate growth and differentiation of the fetus and placenta, by placental areolae.     

Ultrasound fetal measurements and pregnancy associated glycoprotein secretion in early pregnancy in cattle recipients carrying somatic clones

Somatic cloning in the bovine species leads to high levels of fetal losses which occur throughout pregnancy. These losses are most often associated with fetal overgrowth, a syndrome known as large offspring syndrome (LOS), and excessive maternal plasma pregnancy serum protein 60 (PSP60), a protein similar to a pregnancy-associated glycoprotein of 67 kDa (PAG I67) produced by the bovine placenta. Predicting the outcome of pregnancies initiated from cloned embryos has become an important issue both to prevent potential harm to the mother because of excessive fetal size at birth and also to get a better understanding of the relationships between growth, differentiation and placental functions in developing cloned fetuses. Here, we report on a systematic analysis of fetal and placental development in the first trimester of pregnancy performed by ultrasonographic imaging and by measurement of the maternal concentrations of pregnancy associated glycoproteins (PAGS), using four different radioimmunoassays (RIA) (two homologous RIA systems with PSP60 and PAG I67; two heterologous RIA systems with PAG I67 as standard and tracer, and antisera anti-caprine PAGs). We showed that crown-rump length (CRL) in clones appeared smaller than controls at 35, 50 and 62 days (P<0.05). At 62 days of pregnancy, CRL in cloned fetuses that died before 90 days was smaller compared to the other cloned fetuses (P<0.05) whereas the width of the fetal sack and the biparietal diameter (BPD) was larger in fetuses that developed LOS in late gestation (P<0.05). Maternal PAGs concentrations were statistically different between controls and all clone recipients as early as Day 34, suggesting early abnormal placental glycoprotein synthesis for clone pregnancies regardless of pregnancy outcome. This work provides a practical, non-invasive tool to follow up clone pregnancies and suggests that primary growth retardation and abnormal placental function precedes excessive fetal and placental growth at later stages of pregnancy.     

Developmental changes in polyamine levels and synthesis in the ovine conceptus

Polyamines (putrescine, spermidine, and spermine) are essential for placental growth and angiogenesis. However, little is known about changes in polyamine synthesis associated with development of the ovine conceptus (embryo/fetus and associated placental membranes). We hypothesized that rates of placental polyamine synthesis were maximal during the rapid placental growth that occurs in the first half of pregnancy. This hypothesis was tested using ewes between Days 30 and 140 of gestation. Columbia cross-bred ewes were hysterectomized on Days 30, 40, 60, 80, 100, 120, or 140 of gestation (Day 0 = mating; n = 4 ewes/day) to obtain placentomes, intercotyledonary placenta, intercaruncular endometrium, and allantoic as well as amniotic fluids. The tissues were analyzed for ornithine decarboxylase (ODC) and arginase activities; arginine, ornithine, and polyamine concentrations; and polyamine synthesis using radiochemical and chromatographic methods. Maximal ODC and arginase activities and the highest rates of polyamine synthesis were observed in all tissues on Day 40 of gestation. Concentrations of ornithine and polyamines in placentomes and intercaruncular endometrium also peaked on Day 40 of gestation. In ovine allantoic and amniotic fluids, polyamines were most abundant during early (Days 40-60) and late (Days 100-140) gestation, respectively. Amniotic fluid spermine increased progressively with advancing gestation. Results of the present study indicate metabolic coordination among the several integrated pathways that support high rates of polyamine synthesis in the placenta and endometrium during early pregnancy. Our findings may have important implications for both intrauterine growth retardation and fetal origins of diseases in adults.     

Development of beta-cell mass in fetuses of rats deprived of protein and/or energy in last trimester of pregnancy

Fetal malnutrition is now proposed as a risk factor of later obesity and type II diabetes. We previously analyzed the long-term impact of reduced protein and/or energy intake strictly limited to the last week of pregnancy in Wistar rats. Three protocols of gestational malnutrition were used: 1) low-protein isocaloric diet (5 instead of 15%) with pair feeding to the mothers receiving the control diet, 2) restricted diet (50% of control diet), and 3) low protein-restricted diet (50% of low-protein diet). Only isolated protein restriction induced a long-term beta-cell mass decrease. In the present study, we used the same protocols of food restriction to analyze their short-term impact (on day 21.5 of pregnancy) on beta-cell mass development. A 50% beta-cell mass decrease was present in the three restricted groups, but low-protein diet, either associated or not to energy restriction, increased fetal beta-cell insulin content. Among all the parameters analyzed to further explain our results, we found that the fetal plasma level of taurine was lowered by low-protein diet and was the main predictor of the fetal plasma insulin level (r = 0.63, P < 0.01). In conclusion, rat fetuses exposed to protein and/or energy restriction during the third part of pregnancy have a similar dramatic decrease in beta-cell mass, and their ability to recover beta-cell mass development retardation depends on the type of malnutrition used. Moreover, our results support the hypothesis that taurine might play an important role in fetal beta-cell mass function.     

Fetal and placental traits at day 35 of pregnancy in relation to the estrogen receptor genotype in pigs

Fetuses from gilts with estrogen receptor (ESR) genotype AA (AA-AA and AA-AB) and BB (BB-AB and BB-BB) were compared at Day 35/36 of pregnancy, to examine whether fetal ESR genotype nested within maternal ESR genotype would affect fetal traits. Furthermore the relation of fetal body weight and fetal heart weight to various placental traits were evaluated relative to ESR genotype. Fetal and placental weight and length, and implantation surface area were not affected by fetal ESR genotype nested within maternal ESR genotype. Fetal weight was related similarly to placental length, placental weight, and implantation surface area: up to a certain threshold value (40 cm, 40 g and 250 cm2, respectively), an increase in the trait was associated with an increase of fetal weight. Thereafter, fetal weight did not change anymore. Thus, at Day 35/36 of pregnancy porcine fetuses seem to have a maximum growth potential. The percentage of AA-AA fetuses that had not reached this maximum growth potential was larger than of the other three genotype combinations studied, and therefore a higher subsequent fetal mortality may be expected in this group. Hearts of AA-AB fetuses were significantly heavier than those of BB-AB and BB-BB fetuses and tended to be heavier than those of AA-AA fetuses. The reason for this hypertrophy is unclear, but might be related to a difference in placental vascularity. Heart weight of fetuses from BB gilts increased with fetal weight, while heart weights of fetuses from AA gilts did not. Heart weight increased with an increase of placental length and implantation surface area up to 51 cm and 437 cm2, respectively, and thereafter decreased again. For BB-AB fetuses a similar relation was found between heart weight and placental weight, while heart weight of the other three genotype combinations remained unaffected as placental weight increased. The fetus and placenta are continuously changing during early pregnancy, therefore different mechanisms may change the demands for cardiac output. However, keeping in mind that placental size and blood volume are relatively large, placental vascularity and vascular development may play a major role. Therefore, further research on heart size, placental size and vascularity, relative to ESR genotype, is recommended.     

Chronic Protein Restriction in Mice Impacts Placental Function and Maternal Body Weight before Fetal Growth

Mechanisms of resource allocation are essential for maternal and fetal survival, particularly when the availability of nutrients is limited. We investigated the responses of feto-placental development to maternal chronic protein malnutrition to test the hypothesis that maternal low protein diet produces differential growth restriction of placental and fetal tissues, and adaptive changes in the placenta that may mitigate impacts on fetal growth. C57BL/6J female mice were fed either a low-protein diet (6% protein) or control isocaloric diet (20% protein). On embryonic days E10.5, 17.5 and 18.5 tissue samples were prepared for morphometric, histological and quantitative RT-PCR analyses, which included markers of trophoblast cell subtypes. Potential endocrine adaptations were assessed by the expression of Prolactin-related hormone genes. In the low protein group, placenta weight was significantly lower at E10.5, followed by reduction of maternal weight at E17.5, while the fetuses became significantly lighter no earlier than at E18.5. Fetal head at E18.5 in the low protein group, though smaller than controls, was larger than expected for body size. The relative size and shape of the cranial vault and the flexion of the cranial base was affected by E17.5 and more severely by E18.5. The junctional zone, a placenta layer rich in endocrine and energy storing glycogen cells, was smaller in low protein placentas as well as the expression of Pcdh12, a marker of glycogen trophoblast cells. Placental hormone gene Prl3a1 was altered in response to low protein diet: expression was elevated at E17.5 when fetuses were still growing normally, but dropped sharply by E18.5 in parallel with the slowing of fetal growth. This model suggests that nutrients are preferentially allocated to sustain fetal and brain growth and suggests the placenta as a nutrient sensor in early gestation with a role in mitigating impacts of poor maternal nutrition on fetal growth.     

Effect of a high maternal dietary intake during mid-gestation on components of the utero-placental insulin-like growth factor (IGF) system in adolescent sheep with retarded placental development

The aim of the present study was to investigate the effects of administering a high plane diet during early to mid-gestation on the uterine and placental insulin-like growth factor (IGF) system and on systemic IGF-I concentrations in pregnant adolescent ewes with restricted placental growth. Embryos recovered from superovulated ewes inseminated by a single sire were transferred in singleton to the uterus of adolescent recipients. After transfer ewes were offered a high (H) or moderate (M) amount of a complete diet calculated to promote rapid or normal maternal growth rates, respectively. Five ewes from each group were switched from either M to H or H to M diets at day 52 of gestation. Maternal and fetal blood samples and placental tissues were collected from all animals at day 104. Ewes on the high plane diet from mid-gestation (HH, MH groups) had restricted placental mass (P < 0.01) and tended to have smaller fetuses. This was associated with increased maternal plasma IGF-I concentrations (P < 0.001). The pattern of expression of components of the IGF system in the uterus and placenta was studied by in situ hybridization. IGF-I mRNA concentrations were below the limit of detection. IGF-II mRNA expression was high in the fetal mesoderm and present in maternal stroma, but was not influenced by nutritional treatment. In contrast, IGF binding protein 1 (IGFBP-1) mRNA expression was higher (P < 0.05) and IGFBP-3 mRNA expression was lower (P < 0.05) in the endometrial glands of ewes in HH and MH groups. In the fetal trophoblast, IGFBP-3 mRNA expression was higher in the MH group. Type 1 IGF receptor expression was increased (P < 0. 01) in the luminal epithelium of the HM group and IGFBP-2 mRNA expression was highest in the placentome capsule of ewes in the HH group. Together, these results indicate that reprogramming of the uterine and placental IGF axis by maternal nutrition could contribute to placental growth retardation in growing adolescent sheep.     

Perturbations in the biochemical composition of fetal fluids are apparent in surviving bovine somatic cell nuclear transfer pregnancies in the first half of gestation

Amniotic and allantoic fluid volumes and composition change dynamically throughout gestation. Cattle that are pregnant with somatic cell nuclear transfer (NT) fetuses show a high incidence of abnormal fluid accumulation (particularly hydrallantois) and fetal mortality from approximately midgestation. To investigate fetal fluid homeostasis in these pregnancies, Na, K, Cl, urea, creatinine, Ca, Mg, total PO(4), glucose, fructose, lactate, total protein, and osmolalities were measured in amniotic and allantoic fluids collected at Days 50, 100, and 150 of gestation from NT pregnancies and those generated by the transfer of in vitro-produced embryos or by artificial insemination. Deviations in fetal fluid composition between NT and control pregnancies were apparent after placental and fetal organ development, even when no gross morphological abnormalities were observed. Individual NT fetuses were affected to varying degrees. Elevated allantoic Na was associated with lower K and increased allantoic fluid volume or edema of the fetal membranes. Total PO(4) levels in NT allantoic and amniotic fluid were elevated at Days 100 and 150. This was not accompanied by hypophosphatemia at Day 150, suggesting that PO(4) acquisition by NT fetuses was adequate but that its readsorption by the kidneys may be impaired. Excessive NT placental weight was associated with low allantoic glucose and fructose as well as high lactate levels. However, the fructogenic ability of the NT placenta appeared to be normal. The osmolality of the fetal fluids was maintained within a narrow range, suggesting that the regulation of fluid composition, but not osmolality, was impaired in NT pregnancies.     

Diagnosis and monitoring of pregnancy in mice: correlations between maternal weight, fetal and placental mass and the maternal serum levels of progesterone, pregnancy-associated murine protein-2 and alpha-fetoprotein

Outbred Bom:NMRI mice were weighed daily for 18 days from observation of a vaginal plug. In a separate experiment, fetuses and placentae were weighed on each day of pregnancy. Pregnancy can be determined with 99% certainty on day 12 of gestation by the maternal body weight increase from day 1. The pregnancy-specific proteins alpha-fetoprotein (m-AFP) and pregnancy-associated murine protein-2 (PAMP-2), of fetal and placental origin respectively, were detectable on days 8 and 10 in the maternal circulation. Significant correlations were observed between m-AFP and fetal weight and PAMP-2 and placental weight. These markers may therefore be useful in the monitoring of fetal growth and placental growth respectively.     

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.     

Studies on experimental growth retardation in sheep. The effects of a small placenta in restricting transport to and growth of the fetus

Fetal and placental growth rate in sheep has been manipulated by removal of endometrial caruncles prior to conception. This produced two groups of fetuses, one in which prenatal growth rate was similar to normal and a second group in which the fetuses were about half of the normal size. The mortality in the latter group was high, particularly after catheterisation, and there was evidence of early intra-uterine death and fetal reabsorption. Prior to 125 days the relationship between fetal and placental size was poor, but after 126 days a close correlation between the two was apparent. The small fetuses had comparably small placentas and in all cases there was a close relationship between fetal and placental weight. The experimental growth retardation was associated with hypoglycaemia, hypoxia and hypoinsulinaemia. Plasma T3, T4 and particularly prolactin were very low in the small fetuses whilst levels of cortisol and alanine were high. In contrast to the controls these fetuses showed little evidence of net glucose, alanine or lactate consumption. Infusion of 50% glucose into the pregnant ewe, sufficient to elevate maternal plasma glucose concentrations 2 to 3 fold, caused a comparable increase in the plasma concentrations of normal fetuses but only a 50% rise in the concentration in small fetuses. In contrast administration of 50% O2 to the ewes sufficient to cause a 2 to 3-fold increase in maternal PO2 caused only a small increase of arterial PO2 of normal fetuses but doubled that to normal levels in small fetuses. The results are discussed in relation to the effect of reduced placental size causing a fall in placental and transport and transport capacity and significance of this to the associated fetal growth retardation.     

Effect of interleukin-10 null mutation on maternal immune response and reproductive outcome in mice

Interleukin-10 (IL-10) is an anti-inflammatory and immune-deviating cytokine expressed in the endometrium and placenta. IL-10 null mutant (IL-10-/-) mice have been employed to examine the role of IL-10 in regulating immune events in early pregnancy and its significance in implantation and pregnancy success. The inflammatory response elicited in endometrial tissue by insemination was amplified in IL-10-/- mice, with a 66% increase in leukocytes in the endometrial stroma on Day 3 of pregnancy. Despite this, no evidence of abnormal type 1/type 2 skewing was seen in T-lymphocytes from lymph nodes draining the uterus. On Day 18 of gestation, IL-10-/- females mated with IL-10-/- males had 15% more implantation sites and 27% more viable fetuses than pregnant wild-type (IL-10+/+) mice. Placental weight was unaffected, but fetal weight and the fetal:placental weight ratio were higher in IL-10-/- pregnancies. Similar data were obtained in allogeneic pregnancies when IL-10-/- females were mated with major-histocompatibility complex (MHC) disparate IL-10-/- males. Pups delivered by IL-10-/- mothers had increased birth weight and followed an altered growth trajectory, with growth impairment evident from early postnatal life into adulthood, which was reflected in alterations in body composition at 14 wk of age. This study shows that although IL-10 is not essential for maternal immune tolerance or successful pregnancy irrespective of MHC disparity in the fetus, maternal IL-10 is a determinant of growth trajectory in progeny in utero and after birth.     

Relationship between placental vascular endothelial growth factor expression and placental/endometrial vascularity in the pig

We investigated the temporal association between placental vascular endothelial growth factor (VEGF), a potent stimulator of angiogenesis and vascular permeability, and changes in placental/endometrial vascularity on selected days throughout gestation in the pig. Placental and endometrial tissues were collected from sows on Days 25 (n = 4), 36 (n =6), 44 (n = 6), 70 (n =5), 90 (n =5 ), and 112 (n = 7) of gestation. Cross sections of the placental/endometrial interface of each conceptus were used to estimate the number of blood vessels per unit area via image analysis and the intensity of VEGF staining via immunohistochemistry. Placental tissues were also collected on these days to evaluate VEGF mRNA expression. Placental VEGF mRNA expression and the numbers of blood vessels per unit area of placental and adjacent endometrial tissue were low and decreasing from Day 25 to Day 44, before increasing (P < 0.05) markedly and progressively through Day 112. These data are consistent with the marked increase in VEGF immunostaining in the chorionic and uterine luminal epithelium from early to late gestation. Further, these increases in placental VEGF mRNA were positively correlated with fetal weight (r = 0.73; P < 0.0001) and placental efficiency (fetal weight/placental weight ratio; r = 0.66, P < 0.0001). These data are consistent with a role for VEGF in increasing the number of blood vessels at the placental endometrial interface, resulting in an increased capacity for nutrient transfer from the maternal to the fetal compartment.     

Secreted and placental membrane forms of folate-binding protein occur sequentially during pregnancy in swine

The objective was to understand how two forms of folate-binding protein interact to accomplish folate transport during pregnancy in swine. Specific folate binding was measured in uterine flushings during the estrous cycle and early pregnancy and in allantoic fluid (secreted form) and placental membranes (membrane form) throughout later pregnancy. In addition, the localization of the secreted form of folate-binding protein (sFBP) in uterine wall sections was assessed. Uterine flushings were collected on Days 10, 13, and 15 of the estrous cycle and pregnancy. Allantoic fluid and placentas were collected on Days 20, 35, 50, 70, 90, and 105 of pregnancy. Uterine-wall sections were collected on all days of the experiment. Folate binding was measured by incubation of aliquots of uterine flushings, allantoic fluid, or placental microsomal membranes with 0.5-4 nM [(3)H]folate. Uterine-wall sections were incubated with purified anti-FBP IgG or normal rabbit serum IgG to localize sFBP. Folate binding did not differ between early pregnancy and the estrous cycle in uterine flushings, was greatest from Day 50 to 70 of pregnancy in allantoic fluid, and was greatest from Day 50 of pregnancy onward in placental microsomal membranes. Staining for sFBP was present in the endometrial glands from Day 10 to 15 in cyclic gilts and from Day 10 to 20 in pregnant gilts. The pattern of folate binding and sFBP staining supports the concept that sFBP transports folate to the developing conceptus until placentation and then the placental form takes over folate transport.     

Regional blood flow distribution in fetal sheep with intrauterine growth retardation produced by decreased umbilical placental perfusion

To determine the capacity of the fetus to adapt to chronic O2 deficiency produced by decreased placental perfusion in the early development of growth retardation, we embolized the umbilical placental vascular bed of fetal sheep for a period of 9 days. Fetal umbilical placental embolization decreased arterial O2 content by 39%, decreased total placental blood flow by 33%, and produced a 20% reduction in mean fetal body weight. Neither the combined ventricular output nor the regional blood flow distribution was significantly different between the 8 growth-retarded and 7 normally grown fetuses despite the 39% decrease in fetal arterial O2 content. Thus a 33% reduction in total placental blood flow restricts normal fetal growth, but does not exceed the placental circulatory reserve capacity necessary to maintain normal basal metabolic oxygenation. Because the proportion of combined ventricular output to the placenta at rest is decreased in late IUGR fetuses but not in early IUGR fetuses, despite chronic oxygen deficiency, we conclude that the growth retarded fetus maintains a normal regional blood flow distribution until the placental circulatory reserve capacity is depleted.