Apoptosis in rat placenta is zone-dependent and stimulated by glucocorticoids

Apoptosis, or physiological cell death, is elevated in the placenta of human pregnancies complicated by fetal growth retardation, suggesting that placental apoptosis may be a key factor in the overall control of feto-placental growth. The present study used DNA internucleosomal fragmentation analysis to characterize apoptosis in the two morphologically and functionally distinct regions of the rat placenta, the basal and labyrinth zones, during the last week of pregnancy (Days 16, 22, and 23). In addition, because glucocorticoids are potent inhibitors of feto-placental growth and can stimulate apoptosis in other tissues, we examined whether dexamethasone treatment in vivo induces placental apoptosis. DNA fragmentation was clearly evident in both placental zones at each stage of pregnancy, with higher levels evident in the basal zone compared with the labyrinth zone on Days 22 and 23. TUNEL analysis, which identifies dying cells in situ, demonstrated positive staining of cells in the basal zone, particularly giant trophoblast cells. Dexamethasone treatment increased DNA fragmentation in the basal zone but not the labyrinth zone. Similarly, maternal treatment with carbenoxolone, which can enhance local concentrations of endogenous glucocorticoid by inhibition of 11 beta-hydroxysteroid dehydrogenase, also increased DNA fragmentation in the basal zone but not in the labyrinth zone. These effects of dexamethasone and carbenoxolone on placental apoptosis were associated with reduced placental and fetal weights. In conclusion, this study shows that apoptosis occurs in both zones of the rat placenta, particularly in the basal zone near term, and is elevated after increased glucocorticoid exposure in vivo. These data support the hypothesis that placental apoptosis is an important player in the regulation of feto-placental growth, and establish the rat as a useful model to study the endocrine control of placental apoptosis.     

Epigenetics of hypoxic pulmonary arterial hypertension following intrauterine growth retardation rat: epigenetics in PAH following IUGR

Background

Accumulating evidence reveals that intrauterine growth retardation (IUGR) can cause varying degrees of pulmonary arterial hypertension (PAH) later in life. Moreover, epigenetics plays an important role in the fetal origin of adult disease. The goal of this study was to investigate the role of epigenetics in the development of PAH following IUGR.

Methods

The IUGR rats were established by maternal undernutrition during pregnancy. Pulmonary vascular endothelial cells (PVEC) were isolated from the rat lungs by magnetic-activated cell sorting (MACS). We investigated epigenetic regulation of the endothelin-1 (ET-1) gene in PVEC of 1-day and 6-week IUGR rats, and response of IUGR rats to hypoxia.

Results

The maternal nutrient restriction increased the histone acetylation and hypoxia inducible factor-1α (HIF-1α) binding levels in the ET-1 gene promoter of PVEC in IUGR newborn rats, and continued up to 6 weeks after birth. These epigenetic changes could result in an IUGR rat being highly sensitive to hypoxia later in life, causing more significant PAH or pulmonary vascular remodeling.

Conclusions

These findings suggest that epigenetics is closely associated with the development of hypoxic PAH following IUGR, further providing a new insight for improved prevention and treatment of IUGR-related PAH.     

The effect of nicotine on in vitro placental perfusion pressure

Cigarette smoking throughout pregnancy is associated with several negative outcomes, of which an increased incidence of intra-uterine growth restriction (IUGR) is most pronounced. Gestationally age-matched infants born to smoking mothers are, on average, 200 g lighter at birth, per pack smoked per day. The mechanisms and specific tobacco compounds responsible for the increased risk of IUGR among smokers have yet to be identified; however, it is widely accepted that smoking women have compromised placental perfusion throughout gestation due to the vasoconstricting effect of nicotine on uterine and placental blood vessels. Despite the universal acceptance of this theory, very little work has been completed to date examining the vasoactive properties of nicotine within the human placenta. The objective of this study was to determine the effect of nicotine on placental vascular function. Normal-term human placentae were obtained after elective cesarean sections. An in vitro placental perfusion system was used; increasing doses of nicotine (20-240 ng/mL) were added to either the maternal (n = 5) or fetal (n = 3) circulation. The basal feto-placental perfusion pressure was 39.87 +/- 4.3 mmHg and was not affected by nicotine. This finding supports the hypotheses that nicotine does not directly affect placental microvascular function and that any contribution to fetal growth restriction is likely at the level of placental function (i.e., amino acid transport) and (or) uterine vascular function.     

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.     

Bradykinin promotes migration and invasion of human immortalized trophoblasts

Maternal undernutrition (MUN) during pregnancy may lead to fetal intrauterine growth restriction (IUGR), which itself predisposes to adult risk of obesity, hypertension, and diabetes. IUGR may stem from insufficient maternal nutrient supply or reduced placental nutrient transfer. In addition, a critical role for maternal stress-induced glucocorticoids (GCs) has been suggested to contribute to both IUGR and the ensuing risk of adult metabolic syndrome. While GC-induced fetal organ defects have been examined, there have been few studies on placental responses to MUN-induced maternal stress. Therefore, we hypothesize that 50% MUN associates with increased maternal GC levels and decreased placental HSD11B. This in turn leads to decreased placental and fetal growth, hence the need to investigate nutrient transporters. We measured maternal serum levels of corticosterone, and the placental basal and labyrinth zone expression of glucocorticoid receptor (NR3C1), 11-hydroxysteroid dehydrogenase B 1 (HSD11B-1) predominantly activates cortisone to cortisol and 11-dehydrocorticosterone (11-DHC) to corticosterone, although can sometimes drive the opposing (inactivating reaction), and HSD11B-2 (only inactivates and converts corticosterone to 11-DHC in rodents) in control and MUN rats at embryonic day 20 (E20). Moreover, we evaluated the expression of nutrient transporters for glucose (SLC2A1, SLC2A3) and amino acids (SLC38A1, 2, and 4). Our results show that MUN dams displayed significantly increased plasma corticosterone levels compared to control dams. Further, a reduction in fetal and placental weights was observed in both the mid-horn and proximal-horn positions. Notably, the placental labyrinth zone, the site of feto-maternal exchange, showed decreased expression of HSD11B1-2 in both horns, and increased HSD11B-1 in proximal-horn placentas, but no change in NR3C1. The reduced placental GCs catabolic capacity was accompanied by downregulation of SLC2A3, SLC38A1, and SLC38A2 expression, and by increased SLC38A4 expression, in labyrinth zones from the mid- and proximal-horns. In marked contrast to the labyrinth zone, the basal zone, which is the site of hormone production, did not show significant changes in any of these enzymes or transporters. These results suggest that dysregulation of the labyrinth zone GC "barrier", and more importantly decreased nutrient supply resulting from downregulation of some of the amino acid system A transporters, may contribute to suboptimal fetal growth under MUN.     

miR-34a expression,epigenetic regulation,and function in human placental diseases

Preeclampsia (PE) is the major pregnancy-induced hypertensive disorder responsible for maternal and fetal morbidity and mortality that can be associated with intrauterine growth restriction (IUGR). PE and IUGR are thought to be due to a placental defect, occurring early during pregnancy. Several placental microRNAs (miRNAs) have been shown to be deregulated in the context of placental diseases and could thus play a role in the pathophysiology of PE. Here, we show that pri-miR-34a is overexpressed in preeclamptic placentas and that its placental expression is much higher during the first trimester of pregnancy than at term, suggesting a possible developmental role. We explored pri-miR-34a regulation and showed that P53, a known activator of miR-34a, is reduced in all pathological placentas and that hypoxia can induce pri-miR-34a expression in JEG-3 cells. We also studied the methylation status of the miR-34a promoter and revealed hypomethylation in all preeclamptic placentas (associated or not with IUGR), whereas hypoxia induced a hypermethylation in JEG-3 cells at 72 h. Despite the overexpression of pri-miR-34a in preeclampsia, there was a striking decrease of the mature miR-34a in this condition, suggesting preeclampsia-driven alteration of pri-miR-34a maturation. SERPINA3, a protease inhibitor involved in placental diseases, is elevated in IUGR and PE. We show here that miR-34a overexpression in JEG-3 downregulates SERPINA3. The low level of mature miR-34a could thus be an important mechanism contributing to SERPINA3 upregulation in placental diseases. Overall, our results support a role for miR-34a in the pathophysiology of preeclampsia, through deregulation of the pri-miRNA expression and its altered maturation.     

Investigation of the pathogenesis of transplacental transmission of Aleutian mink disease parvovirus in experimentally infected mink.

The transplacental transmission of Aleutian mink disease parvovirus (ADV) was studied in experimental infection of 1-year-old female non-Aleutian mink. The ADV-seronegative female mink were inoculated with ADV prior to mating or after the expected implantation of the embryos during pregnancy. A group of uninfected females served as a control group. Animals from each group were killed prior to or shortly after parturition. The in situ hybridization technique with radiolabeled strand-specific RNA probes was used to determine target cells of virus infection and virus replication. In both infected groups, ADV crossed the endotheliochorial placental barrier, although animals infected before mating already had high antibody titers against ADV at the time of implantation. The percentage of dead and resorbed fetuses was much higher in dams infected before mating. In the placentae of these mink, virus DNA and viral mRNA were detected in cells in the mesenchymal stroma of the placental labyrinth and hematoma but only occasionally in the cytotrophoblast of the placental hematoma. Placentae of animals infected during pregnancy showed in addition very high levels of virus and also viral replication in a large number of cytotrophoblast cells in the placental hematoma, which exhibited distinct inclusion bodies. In both groups, neither virus nor virus replication could be detected in maternal endothelial cells or fetal syncytiotrophoblast of the placental labyrinth. Fetuses were positive for virus and viral replication at high levels in a wide range of tissues. Possible routes of transplacental transmission of ADV and the role of trophoblast cells as targets for viral replication are discussed.     

Placental Underperfusion in a Rat Model of Intrauterine Growth Restriction Induced by a Reduced Plasma Volume Expansion

Lower maternal plasma volume expansion was found in idiopathic intrauterine growth restriction (IUGR) but the link remains to be elucidated. An animal model of IUGR was developed by giving a low-sodium diet to rats over the last week of gestation. This treatment prevents full expansion of maternal circulating volume and the increase in uterine artery diameter, leading to reduced placental weight compared to normal gestation. We aimed to verify whether this is associated with reduced remodeling of uteroplacental circulation and placental hypoxia. Dams were divided into two groups: IUGR group and normal-fed controls. Blood velocity waveforms in the main uterine artery were obtained by Doppler sonography on days 14, 18 and 21 of pregnancy. On day 22 (term = 23 days), rats were sacrificed and placentas and uterine radial arteries were collected. Diameter and myogenic response of uterine arteries supplying placentas were determined while expression of hypoxia-modulated genes (HIF-1α, VEGFA and VEGFR2), apoptotic enzyme (Caspase -3 and -9) and glycogen cells clusters were measured in control and IUGR term-placentas. In the IUGR group, impaired blood velocity in the main uterine artery along with increased resistance index was observed without alteration in umbilical artery blood velocity. Radial uterine artery diameter was reduced while myogenic response was increased. IUGR placentas displayed increased expression of hypoxia markers without change in the caspases and increased glycogen cells in the junctional zone. The present data suggest that reduced placental and fetal growth in our IUGR model may be mediated, in part, through reduced maternal uteroplacental blood flow and increased placental hypoxia.     

The effect of ventral nerve cord severance and male castration on female mating behavior, clutch size, and maternal care in the ring-legged earwig

Mating is critical for the expression of oviposition and maternal care in the earwig, Euborellia annulipes; additionally, mating diminishes receptivity to additional mating and promotes a decline in juvenile hormone synthesis at the end of the gonadotrophic cycle (in contrast to most insect species wherein mating stimulates juvenile hormone production). We report here that severance of the ventral nerve cord of virgin females similarly promoted egg deposition and maternal care of eggs, diminished mating receptivity, and elicited a timely decline in juvenile hormone biosynthesis. Mating of intact females to adult males that were castrated as larvae did not abolish oviposition; however, clutch size was reduced, and no eggs developed. Such castrated males had smaller seminal vesicles than did intact males, presumably attributable to lack of sperm in castrated males. In contrast, mating of intact females to males castrated on day 1 of adult life did not reduce clutch size compared with those of sham-operated animals and did not abolish fertilization; in fact, these castrated males produced viable offspring after six matings. These results are consistent with the notion that ventral nerve cord severance mimicked mating in intact animals. Following mating, the ventral nerve cord likely is a conduit to release the brain from inhibiting oviposition and maternal care. The presence of sperm in the spermatheca is not necessary for release of this inhibition but may modulate clutch size.     

Effects and mechanisms of action of sildenafil citrate in human chorionic arteries

Objectives  

Sildenafil citrate, a specific phosphodiesterase-5 inhibitor, is increasingly used for pulmonary hypertension in pregnancy. Sildenafil is also emerging as a potential candidate for the treatment of intra-uterine growth retardation and for premature labor. Its effects in the feto-placental circulation are not known. Our objectives were to determine whether phosphodiesterase-5 is present in the human feto-placental circulation, and to characterize the effects and mechanisms of action of sildenafil citrate in this circulation.     

Fetal growth restriction triggered by polycyclic aromatic hydrocarbons is associated with altered placental vasculature and AhR-dependent changes in cell death

Maternal cigarette smoking is considered an important risk factor associated with fetal intrauterine growth restriction (IUGR). Polycyclic aromatic hydrocarbons (PAHs) are well-known constituents of cigarette smoke, and the effects of acute exposure to these chemicals at different gestational stages have been well established in a variety of laboratory animals. In addition, many PAHs are known ligands of the aryl hydrocarbon receptor (AhR), a cellular xenobiotic sensor responsible for activating the metabolic machinery. In this study, we have applied a chronic, low-dose regimen of PAH exposure to C57Bl/6 female mice before conception. This treatment caused IUGR in day 15.5 post coitum (d15.5) fetuses and yielded abnormalities in the placental vasculature, resulting in significantly reduced arterial surface area and volume of the fetal arterial vasculature of the placenta. However, examination of the small vasculature within the placental labyrinth of PAH-exposed dams revealed extensive branching and enlargement of these vessels, indicating a possible compensatory mechanism. These alterations in vascularization were accompanied by reduced placental cell death rates, increased expression levels of antiapoptotic Xiap, and decreased expression of proapoptotic Bax, cleaved poly(ADP-ribose) polymerase-1, and active caspase-3. AhR-deficient fetuses were rescued from PAH-induced growth restriction and exhibited no changes in the labyrinthine cell death rate. The results of this investigation suggest that chronic exposure to PAHs is a contributing factor to the development of IUGR in human smokers and that the AhR pathway is involved.     

Temporal relationships among uterine pituitary adenylate cyclase-activating polypeptide, decidual prolactin-related protein and progesterone receptor mRNAs expressions during decidualization and gestation in rats

Pituitary adenylate cyclase-activating peptide (PACAP), a novel compound with vasoactive intestinal polypeptide-like activity, was recently shown to be localized in the neuronal endings of the human uterus. The purpose of the present study was to assess the functional presence of PACAP mRNA in the decidual endometrium and its relationship to the expression levels of decidual prolactin-related protein (dPRP) and the progesterone receptor mRNAs during decidualization and pregnancy in Sprague-Dawley rats. PACAP was constitutively and temporally expressed in the decidual endometrium and gravid uterus. The time-dependent correlated expression levels of PACAP, dPRP and the progesterone receptor were induced by the neurogenic reproductive signals, i.e. the vagino-cervical/deciduogenic stimuli of decidualization and by the normal equivalent stimuli of mating/blastocyst implantation of gestation. Correlation among the mRNA expression levels of PACAP, dPRP and the progesterone receptor and the coordinated inhibitory actions of the anti-progesterone (RU-486) suggest that there is also correlated time-dependent steroid regulation of the mRNA levels of PACAP, dPRP and the progesterone receptor in the decidual and pregnant uteri. One possible functional meaning for the time-related localization of endometrial/uterine PACAP could be to facilitate endometrial blood flow and increase the availability of metabolic substrates to the developing deciduoma or embryo. The study demonstrates the potential importance of PACAP expression in the regulation of the maternal feto-placental component and suggests a prominent reproductive role for the neuropeptide in mammalian pregnancy.     

In vivo treatment with interferon-gamma during early pregnancy in mice induces strong expression of major histocompatibility complex class I and II molecules in uterus and decidua but not in extra-embryonic tissues.

Allopregnant (NFR/N [Swiss-derived] H-2q females x 57/Bl H-2b males) and syngeneically pregnant (NFR/N x NFR/N) mice were subjected to daily injections (10(5) U/mouse/day, from Day 5.5 of gestation) of recombinant rat or mouse interferon-gamma (IFNg) in order to investigate its ability to induce extra-embryonic major histocompatibility complex (MHC) expression and antipaternal immune reactions if administered during the first part of the gestation period. In addition, a limited number of IFNg-treated embryo-transferred NFR/N mice carrying C57/B1 embryos (representing a complete allogenic pregnancy) were investigated. Mouse and rat IFNg caused the same type of histological and physiological changes, and most of the experiments were performed by using rat IFNg. Several IFNg-treated mice (irrespective of type of mating) showed a drop in weight and a high rate of resorptions at Day 12.5 of gestation. This interference with pregnancy appeared not to be caused by immunological reactions against the feto-placental unit (no leukocyte infiltration and no significant effect on serum levels of antipaternal antibodies in preimmunized allopregnant IFNg-treated mice). Immunohistochemical stainings of cryosectioned tissues at Day 9.5 of pregnancy revealed that IFNg treatment caused a strong induction of MHC class I and class II expression on most cells in the uterus and on several cells in the maternal decidua, while there was a complete absence of detectable MHC class I and class II expression in the extra-embryonic tissues. Characteristic for a Day 12.5 placenta of an IFNg-treated mouse (including embryo-transferred mice) was a strongly MHC class II-induced maternal decidua and a completely MHC class II-negative fetal placenta. The pattern of IFN-induced MHC class I expression was similar to that of class II, with the exception of class I expression on scattered cells within the basal zone. Thus, the present study provides immunohistological evidence that IFNg administered in vivo during the first part of gestation is not capable of inducing MHC expression on murine extra-embryonic cells despite an extremely high expression of MHC molecules on decidual cells in intimate contact with extra-embryonic tissues. It is likely that the resistance to IFNg-mediated induction of MHC expression on extra-embryonic cells is of basic importance for the protection of mammalian semi-allogeneic fetuses.     

Colocalization of leptin receptor (OB-R) mRNA and placental lactogen-II in rat trophoblast cells: gestational profile of OB-R mRNA expression in placentae

The present study was designed to clarify the cellular localization and expression of leptin receptor(s) [OB-R(s)] mRNA including its splice variants and their correlation with the cells which secrete placental hormone, placental lactogen-II (PL-II), in rat placentae. By in situ hybridization analysis, hybridization signals for OB-Rb and the common extracellular domain of OB-R were first detectable in some cells of the labyrinth zone of the placentae on day 14 of pregnancy and then a lot of cells dispersed in the entire area of the labyrinth zone expressed OB-Rb during the latter half of pregnancy. However, no expression was observed in the decidua and the junctional zone of the placentae during pregnancy. Double staining study revealed that signals for OB-R expressing trophoblast cells showed PL-II immunoreactivity in the labyrinth zone of the placentae. In Northern blot analysis, two bands (2.8 kb and 5.1 kb) of OB-R mRNA expression were observed in the placentae from day 17 to 21 of pregnancy and the expression of both increased markedly up to day 21 of pregnancy. RT-PCR analysis revealed that OB-Rb, OB-Ra, and OB-Re are expressed in the placentae on days 19 and 21 of pregnancy. These results suggest that the OB-R may have a physiological significance in the placental function during the latter half of pregnancy.     

Expression of the corticotropin-releasing hormone (CRH) gene in human placenta and amniotic membrane

Immunoreactive corticotropin-releasing hormone (IR-CRH) in maternal plasma increases progressively during pregnancy and decreases rapidly after delivery, suggesting that IR-CRH is produced in the placenta. We studied the expression of the CRH gene in developing human chorionic tissue, the amniotic membrane, the uterine myometrium and a fresh surgical specimen of hydatidiform mole by Northern blot analysis. Our results were as follows: (1) CRH mRNA was demonstrated in the placenta in the third trimester and at term, but under detectable level in the first and second trimesters. (2) CRH mRNA expression was observed in the amniotic membrane, but its expression in the myometrium in normal pregnancy was under detectable level at term. (3) CRH mRNA was also under detectable level in trophoblasts of a hydatidiform mole. These results suggest that the sources of the increased level of IR-CRH in human plasma and amniotic fluid during pregnancy are the placenta and amniotic membrane, and that gene expression of placental CRH increases during pregnancy.     

Induction of abortion in mice with a monoclonal antibody specific for suppressor T-lymphocyte molecules

A monoclonal antibody, mAb 14-30, which binds T-cell produced suppressor factors (TsF) was used to study the possibility that molecules produced by suppressor T-cells play a role in maintaining pregnancy, presumably by protecting the fetus from the maternal immune system. Female mice were injected with mAb 14-30 at various times after mating. Overall, only 14% of the expected 68% of the mated and treated females were pregnant at term. In addition, Western blots were used to demonstrate the presence of TsF in fetuses, placentae, uteri and spleen of pregnant animals and its presence only in the spleen of non-pregnant animals. These experiments help to confirm results that indicate the importance of immune suppressor factors in maintaining pregnancy and extend these previous observations to include suppressor T-cell molecules.     

Severe feto-placental abnormalities precede the onset of hypertension and proteinuria in a mouse model of preeclampsia

Preeclampsia is a prevalent and potentially devastating disorder of pregnancy. Characterized by a sudden spike in blood pressure and urinary protein levels, it is associated with significant obstetric complications. BPH/5 is an inbred mouse model of preeclampsia with borderline hypertension before pregnancy. BPH/5 mice develop hypertension, proteinuria, and endothelial dysfunction during late gestation (after E14.5). We hypothesized that BPH/5 mice might exhibit early feto-placental abnormalities before the onset of maternal disease. All placental cell lineages were present in BPH/5 mice. However, the fetal and placental weights were reduced, with abnormalities in all the placental zones observed starting early in gestation (E9.5-E12.5). The fractional area occupied by the junctional zone was significantly reduced at all gestational timepoints. Markedly fewer CDKN1C-stained trophoblasts were seen invading the proximal decidual zone, and this was accompanied by reductions in Cdkn1c gene expression. Trophoblast giant cell morphology and cytokeratin staining were not altered, although the mRNA levels of several giant cell-specific markers were significantly downregulated. The labyrinth layer displayed decreased branching morphogenesis of endothelial cells, with electron microscopy evidence of attenuated trophoblast layers. The maternal decidual arteries showed increased wall-to-lumen ratios with persistence of actin-positive smooth muscle cells. These changes translated into dramatically increased vascular resistance in the uterine arteries, as measured by pulse-wave Doppler. Collectively, these results support the hypothesis that defects at the maternal-fetal interface are primary causal events in preeclampsia, and further suggest the BPH/5 model is important for investigations of the underlying pathogenic mechanisms in preeclampsia.     

Changes in the temporal and spatial expression of H beta 58 during formation and maturation of the chorioallantoic placenta in the Rat

Cloning and sequencing of a cDNA amplified by RNA fingerprinting at the implantation site of pregnant rats revealed 80% similarity with H beta 58, previously shown to be essential for formation of the chorioallantoic placenta in the mouse. H beta 58 mRNA was detected in the endometrium of hormonally sensitized rats stimulated to undergo decidualization and in the contralateral uterine horns lacking a decidual stimulus, indicating that uterine expression of H beta 58 mRNA did not require decidualization or the presence of a blastocyst. Immunodetection in the early postimplantation uterus (Days 6-8 of pregnancy) showed H beta 58 localized in the luminal and glandular epithelia and some stromal cells. Decidual cells at Day 6 of pregnancy expressed H beta 58, and by Day 9 of pregnancy, the protein localized throughout the maternal decidua. The temporal and spatial distribution of H beta 58 in the developing chorioallantoic placenta was assessed at Days 10, 12, and 14 of pregnancy. Immunoreactive H beta 58 localized to erythroid cells within the developing fetal vasculature of the chorioallantoic primordia at Day 10 of pregnancy. By Day 12, the fetal vasculature extended into the placental labyrinth, and the erythroid stem cells continued to strongly express H beta 58. At Day 14 of pregnancy, immunoreactivity became evident in the trophoblast giant cells and syncytiotrophoblast of the fetal placenta. As the chorioallantoic placenta matured (Day 18), H beta 58 mRNA was 3.6-fold higher in the labyrinth compared with the junctional region. Stable cell lines (HRP/LRP) isolated from the rat labyrinthine placenta expressed H beta 58 mRNA and protein. The expression pattern of H beta maternal and fetal placental tissues and its early expression in fetal erythroid stem cells during formation and maturation of the chorioallantoic placenta suggest that H beta 58 plays key roles in the regulatory networks that control hematopoietic development and placentation.