Cellular dynamics of growth in sheep and goat synepitheliochorial placentomes: an autoradiographic study

The Ruminant Placental Trophoblast Binucleate cell - an evolutionary breakthrough

Viviparity and the development of a placenta are two of the major reasons for the success of the mammals in colonizing all habitats, both terrestrial and aquatic. The placenta is an apposition of fetal to maternal tissue which serves two main, but competing functions: to maximize oxygen transfer and the acquisition of nutrients from the mother, but to minimize immunological rejection by the maternal immune system. This has resulted in the evolution of four main types differing in the degree of loss of the maternal uterine epithelial (UE) barrier: epitheliochorial, synepitheliochorial, endotheliochorial, and hemochorial, all providing a successful safe balance between the needs of mother and fetus. Epitheliochorial is the least invasive, a simple apposition and microvillar interdigitation of the apices of uterine epithelium and trophoblast. It is suggested to have evolved as a response to the increase in the size of the animal to provide a sufficiently long gestation to produce a single altricial (run/swim-soon-as-born) neonate as in the Cetartiodactyla. The mother needs to have good control of the fetal demands so the UE barrier is maintained. However, in the synepitheliochorial placenta, characteristic of all ruminants, the fetus has evolved a means of increasing, or at least maintaining, demand without the need for invasion. This has been achieved by the development of the trophoblast binucleate cell which, uniquely, can fuse with a UE cell to form fetomaternal hybrid tissue. This can maintain some maternal barrier function but also deliver fetally synthesized immunomodulatory and metabolic messages to the maternal circulation. This review provides the evidence for this remarkable evolutionary step and also considers an alternative explanation for the formation of the structure of the ruminant placenta.

Morphology of placentome in Korean water deer Hydropotes inermis argropus

The placenta of the Korean water deer was anatomically examined to accumulate basic information regarding its reproductive system. The convex placentomes with five to nine well-developed pedicles were observed in the whole uterine horns, and therefore, the placenta was classified as oligocotyledonary. The evidence indicating the migration of binucleate cells (BNCs) from trophectoderm to the uterine epithelium led to the histological classification of the placenta as synepitheliochorial. The number of fetuses was markedly higher than that in other ruminant species. However, the number of placentomes was found to be similar to the other Cervidae species. Therefore, these results suggest that the Korean water deer may possess special mechanisms or structures at the fetus attachment site to maintain this unusally high number of fetuses.

Trinucleate uterine epithelial cells as evidence for White-tail Deer trophoblast binucleate cell migration and as markers of placental binucleate cell dynamics in a variety of wild ruminants

INTRODUCTION

The unicellular trophoblast epithelium of all ruminants so far investigated contains 15-20% binucleate cells with numerous secretory granules. Electron microscope (EM) studies of the domesticated cow, ewe, goat and deer species have established that these BNC migrate out of the trophoblast epithelium to fuse with the apposed maternal uterine epithelial cells or derivative to form fetomaternal tissue throughout pregnancy. However there is one careful EM study of the trophoblast of a wild ruminant, the White-tail deer, which found the usual number of BNC but no evidence of any migration or fusion. Since there are up to 200 species of wild ruminants, it was important to establish whether there really are two possible scenarios for BNC function.

MATERIALS AND METHODS

This paper reports a light microscope (LM) immunocytochemical study of cell dynamics in ruminant placentas using 1-2 mμ deresinated sections.

RESULTS

The results clearly demonstrate that the White-tail deer and all of the other 15 (see Table 1) randomly selected wild ruminants show the same BNC migration and fusion pattern.

DISCUSSION

These results suggest that this remarkable cellular behaviour is fundamental to the ruminant evolutionary success.

The Roles of Syncytin-Like Proteins in Ruminant Placentation

Recent developments in genome sequencing techniques have led to the identification of huge numbers of endogenous retroviruses (ERV) in various mammals. ERVs, which occupy 8%–13% of mammalian genomes, are believed to affect mammalian evolution and biological diversity. Although the functional significance of most ERVs remains to be elucidated, several ERVs are thought to have pivotal roles in host physiology. We and other groups recently identified ERV envelope proteins (e.g., Fematrin-1, Syncytin-Rum1, endogenous Jaagsiekte sheep retrovirus Env) that may determine the morphogenesis of the unique fused trophoblast cells, termed trinucleate cells and syncytial plaques, found in ruminant placentas; however, there are still a number of outstanding issues with regard to the role of ERVs that remain to be resolved. Here, we review what is known about how these ERVs have contributed to the development of ruminant-specific trophoblast cells.

Early dexamethasone treatment induces placental apoptosis in sheep

Glucocorticoid treatment given in late pregnancy in sheep resulted in altered placental development and function. An imbalance of placental survival and apoptotic factors resulting in an increased rate of apoptosis may be involved. We have now investigated the effects of dexamethasone (DEX) in early pregnancy on binucleate cells (BNCs), placental apoptosis, and fetal sex as a determinant of these responses. Pregnant ewes carrying singleton fetuses (n = 105) were randomized to control (n = 56, 2 mL saline/ewe) or DEX treatment (n = 49, intramuscular injections of 0.14 mg/kg ewe weight per 12 hours over 48 hours) at 40 to 41 days of gestation (dG). Placentomes were collected at 50, 100, 125, and 140 dG. At 100 dG, DEX in females reduced BNC numbers, placental antiapoptotic (proliferating cell nuclear antigen), and increased proapoptotic factors (Bax, p53), associated with a temporarily decrease in fetal growth. At 125 dG, BNC numbers and apoptotic markers were restored to normal. In males, ovine placental lactogen-protein levels after DEX were increased at 50 dG, but at 100 and 140 dG significantly decreased compared to controls. In contrast to females, these changes were independent of altered BNC numbers or apoptotic markers. Early DEX was associated with sex-specific, transient alterations in BNC numbers, which may contribute to changes in placental and fetal development. Furthermore, in females, altered placental apoptosis markers may be involved.

Cytokeratin antibodies as differential markers of trophoblast and fetomaternal syncytial plaques in the sheep placentome

The sheep placenta is an often used model in placental research. Uterine epithelium and trophoblast of this synepitheliochorial placenta form a complex, intensely interdigitating epithelial barrier separating maternal and fetal organisms. The close topographical relation and additionally the presence of hybrid syncytia formed by focal fusion of both epithelia hamper identification of the various cellular constituents. Therefore we aimed to find a specific immunohistochemical marker differentiating between the fetomaternal syncytial plaques and trophoblast. A monoclonal antibody directed against type II cytokeratins strongly stained unicellular trophoblast. The syncytial plaques were only weakly stained while binucleate trophoblast remained unstained. This antibody proved to be a useful tool for easy histological orientation in the sheep placenta. In combination with other antibodies in double immunohistochemistry it facilitates exact localization of antigens.

A light and electron microscopical study of the Tragulid (mouse deer) placenta

The Tragulidae are the living relics of the basal ruminant stock. They have a diffuse placenta, with no aggregations of the placental villi into localised placentomes characteristic of all other ruminants. Despite this difference, this ultrastructural and immunocytochemical investigation demonstrates that in Tragulus the trophoblast binucleate cell (BNC) plays the same central role in development and structure as in all other ruminants. It shows an identical development and ultrastructure, produces granules reactive with bovine placental lactogen and pregnancy associated glycoprotein antibodies, and migrates when mature through the trophoblast tight junction to fuse into a mosaic of syncytial plaques from which the granules are released to the mother and which have replaced the uterine epithelium. Unlike the persistent plaques in the sheep and goat placenta, in Tragulus they are transient, dying by apoptosis with the fragments phagocytosed by the trophoblast. This brings the trophoblast into direct endotheliochorial apposition to maternal tissue until BNC migration and fusion replace the dead plaque. This intimate fetomaternal confrontation has not been shown in any other ruminant, and could be a relic of the evolutionary development of the synepitheliochorial from the original basic eutherian endo- or hemo-chorial placenta.

Cellular localization and function of the antiviral protein, ovine Mx1 (oMx1): I. Ovine Mx1 is secreted by endometrial epithelial cells via an 'unconventional' secretory pathway

PROBLEM

Embryonic loss is a major contributor to infertility. Understanding factors contributing to embryonic loss will aid in development of technologies to improve/regulate fertility in animals and humans.

METHOD OF STUDY

We tested the hypothesis that the antiviral protein, ovine Mx1 (oMx1), is secreted by uterine epithelial cells. Uterine flushes were obtained from cyclic and early pregnant ewes and examined for levels of oMx1 protein. The pathway for ovine Mx1 secretion in ovine glandular epithelial (oGE) cells was determined using brefeldin A (BFA), an inhibitor of the conventional secretory pathway. Effects of BFA were determined using beta2-microglobulin (beta2MG) as a marker for the conventional secretory pathway, and interferon stimulated gene 15 (ISG15) and Galectin-1 (Gal-1) as markers for the unconventional secretory pathways.

RESULTS

Ovine Mx1 protein levels were low in uterine flushes from cyclic ewes and levels increased in pregnant ewes after D 15. Ovine GE cells secreted oMx1 in response to interferon and secretion was not reduced by BFA, suggesting oMx1 was secreted via an unconventional secretory pathway. beta2MG secretion was reduced by BFA, whereas ISG15 and Gal-1 were not.

CONCLUSION

This is the first report that the antiviral protein, oMx1, is secreted and provides evidence that secretion occurs via unconventional secretory pathway(s).

PrP(Sc) accumulation in placentas of ewes exposed to natural scrapie: influence of foetal PrP genotype and effect on ewe-to-lamb transmission

Placentas from scrapie-affected ewes are known to be infectious. Nevertheless, placenta infectivity in such ewes is not systematic. Maternal transmission to lambs is highly suspected but contamination of the foetus in utero has not been demonstrated. Using ewes from a naturally scrapie-infected flock, it was demonstrated that abnormal prion protein (PrP(Sc)) accumulation in the placenta (i) is controlled by polymorphisms at codons 136, 154 and 171 of the foetal PrP gene and (ii) is restricted mainly to placentome foetal trophoblastic cells. In order to go deeper into the role of the placenta in scrapie transmission, the pattern of PrP(Sc) dissemination was established in susceptible lambs (genotype VRQ/VRQ) sampled from 140 days post-insemination to the age of 4 months from either VRQ/VRQ ewes with PrP(Sc)-positive placentas or ARR/VRQ ewes with PrP(Sc)-negative placentas. In both VRQ/VRQ lamb groups, PrP(Sc) spatial and temporal accumulation patterns were similar, suggesting post-natal rather than in utero contamination.

Establishment of feeder-independent cloned caprine trophoblast cell line which expresses placental lactogen and interferon tau

A feeder-independent cloned trophoblast cell line, HTS-1, was established from a mature placenta of Shiba goat (Capra hircus). During the growth phase, single HTS-1 cells exhibited ruffled membranes or lamellipodia often accompanied by elongated cell shape, indicating highly motile nature of the cells. At or near confluence, HTS-1 cells formed monolayers with few sign of cellular overlapping. Binucleate cells were found at a high frequency especially in the peripheral regions of monolayers. In small colonies and the monolayers, majority of HTS-1 cells assumed polygonally shaped cobble-stone like morphology characteristic to epithelial cells, although considerable variations in cellular morphology were observed despite of repeated cloning. Time-lapse video recordings of HTS-1 cells during culture revealed that not only the small colonies but also the monolayers near or at confluence were remarkably motile, often causing extreme elongation of the cells within them. The extremely plastic nature of HTS-1 cells in vitro is likely to be the reflection of the extraordinary capacity of caprine trophoblast cells to be stretched to extreme thinness in vivo as shown by electron microscopy. HTS-1 cells cultured on matrigel are highly invasive, and express MT1-MMP which, in the mouse, has been known to be expressed at the invasive edge of trophoblast both in vitro and in vivo. HTS-1 cells express placental lactogen (PL) and interferon-tau (IFNtau), as confirmed by immunocytochemistry, Western blotting and RT-PCR analysis. Both PL and IFNtau expression in the cells appeared to be down-regulated by cell-cell contact. In the medium conditioned by HTS-1 cells, the presence of secretory form of PL and IFNtau was confirmed by Western blotting. The HTS-1 cell line will serve as a useful in vitro model for the analysis of the molecular and/or cellular mechanisms underlying synepitheliochorial placentation in bovidae animals.

The effects of cortisol on the binucleate cell population in the ovine placenta during late gestation

Large granulated binucleate cells (BNCs) producing placental lactogen (PL) and pregnancy-associated glycoproteins (PAG) are present in all ruminant placentae throughout pregnancy. These BNC account for 15-20% of the cells in ovine trophectoderm for most of gestation but decrease in number close to term at the same time that fetal cortisol levels rise. The present study investigated the effects of cortisol on the BNC population using immunohistochemistry to count BNCs in ovine placentomes during late gestation and after experimental manipulation of the fetal cortisol level by fetal adrenalectomy and exogenous cortisol infusion. Abolition of the prepartum rise in fetal cortisol prevented the normal decline in BNC numbers towards term. Conversely, raising cortisol levels in immature fetuses to prepartum values prematurely reduced placental BNC numbers. However, a small population of BNC remained, even at the highest cortisol concentrations. When all the data were combined irrespective of treatment or gestational age, there was a significant inverse correlation between fetal plasma cortisol and the number of BNCs in the ovine placenta. These findings show that cortisol regulates the BNC population in ovine placenta during late gestation. They also have important implications for the production of PL and PAG during ovine pregnancy.

Evolution of the placenta and fetal membranes seen in the light of molecular phylogenetics

Recent analyses of nucleotide sequence data suggest that living placental mammals belong to one of four superorders. The early divergence of these groups was followed by long periods of geographical isolation, due to the break up of continental land masses, allowing for convergent evolution of similar traits in different superorders. As an example, the transition from epitheliochorial to haemochorial placentation occurred independently in bats, rodents, anthropoid primates, armadillos and others. A group of ancient African mammals is suggested by the molecular data, but is not fully supported by morphological evidence. The hypothesis is, however, consistent with some of the data on fetal membranes, suggesting that it would be worthwhile to study the early development of tenrecs, golden moles and elephant shrews. Analyses of fetal membrane traits that group the tarsiers with anthropoid primates, and separate them from the lemurs, are challenged by the molecular data. Other relatives of the primates seem to include tree shrews and flying lemurs, and little is known about the fetal membranes of the latter group. Comparative studies of placental function normally are confined to primates, rodents, lagomorphs and domestic animals: the biological diversity represented by mammals that evolved in ancient Africa and South America is not represented. Therefore, future comparative studies should strive to include species such as the rock hyrax and the armadillo.

Relaxin-like factor (RLF) mRNA expression in the fallow deer

Employing RT- and RACE-PCR on RNA isolated from testicular tissue, we have cloned the coding cDNA sequence for the RLF, also known as Insl3, of the fallow deer. The RLF coding sequence consisted of 396 bp encoding a peptide of 131 amino acids and shared highest homology with bovine, sheep and goat RLF. Northern analysis revealed a single 0.9 kb transcript in the deer testis. There is only one RLF gene in the deer genome. Nonradioactive in situ hybridization revealed the Leydig cells to be the sole source for RLF mRNA in the deer testis. In the non-pregnant uterus, RLF transcripts were located in the luminal and glandular epithelium of the endometrium. Within the ovary of the pregnant doe, follicular theca interna cells and the corpus luteum expressed RLF transcripts. In uteroplacental tissues, luminal and glandular epithelium, fetal uninucleate and binucleate trophoblast cells (BNC) of the basic villous trophoblast layer expressed RLF mRNA. BNC located at the apical trophoblast layer or the tip of the fetal villus were devoid of RLF transcripts. Pseudostratified trophoblast cells at the base of fetal villi coexpressed RLF mRNA and immunoreactive MHC class Ib molecules.

Quantitative analysis throughout pregnancy of intraepithelial large granular and non-granular lymphocyte distributions in the synepitheliochorial placenta of the cow

Intraepithelial lymphocytes are a constant feature of ruminant uterine epithelium. Light microscope quantitation on semithin sections of resin embedded perfused material from pregnant and non-pregnant cows shows that although the proportion of large granular lymphocytes to non-granular lymphocytes in interplacentomal areas increases during pregnancy, the total number of lymphocytes in these areas remains at a similar level. However all types of lymphocytes are eliminated from the caruncular uterine epithelium of the cow by 28 days of pregnancy at the initiation of placentomal development. Despite the subsequent enormous growth in area of this region during pregnancy, no lymphocytes are found in the bovine placentomal cellular uterine epithelium. This pattern is similar to that in the ewe and goat although in these species the placentomal uterine epithelium is modified to maternofetal hybrid syncytial plaques. However, in the deer, a similar cellular placentomal epithelium has numerous intraepithelial lymphocytes and large granular lymphocytes are closely associated with fetomaternal hybrid trinucleate cells formed by binucleate cell migration throughout pregnancy. Cow interplacentomal large granular lymphocytes and trinucleate cells show similar apposition but the formation and fate of cow placentomal trinucleate cells does not involve lymphocytes. Since the caruncular (placentomal) area is 10-20 times that of the intercaruncular this suggests a very different function for intraepithelial lymphocytes in the deer compared with the other ruminant species.

Structure and function in the ruminant synepitheliochorial placenta: central role of the trophoblast binucleate cell in deer

The ruminant placenta has a very uniform gross structure based on localised areas of fetomaternal membrane apposition and proliferation to form placentomes. There is no consistency, however, in the number or size of these placentomes (6-150), nor in the villus architecture of the individual placentome. The one consistent feature is the binucleate cell (BNC) population in the trophoblast. These BNC form 15-20% of the epithelium in all ruminants examined so far. They synthesise the placental lactogen hormone and other glycoproteins and migrate through tight junctions to fuse with a uterine epithelial cell to form initially fetomaternal hybrid trinucleate cells (TNC) and subsequently syncytial plaques (SP). Such SP may be transient or persist throughout pregnancy depending on the species. The wide range of deer species examined confirms the uniformity of the BNC hormone production, migration, and fusion pattern described for other ruminants. BNC migration produces predominantly transient TNC, but there are areas of SP largely restricted to the apex of the maternal crypts. Maternal large granule lymphocytes (LGL) are uniquely found in deer placentomal uterine epithelium; they are usually closely associated with TNC and SP sites, but the significance of the interactions remains to be established.

Membrane dynamics during migration of placental cells through trophectodermal tight junctions in sheep and goats

Binucleate cells in ruminant trophectodermal epithelium are unique in that they form part of the tight junction as they migrate across it, maintaining the ionic barrier seal to the internal milieu of the fetus. Such participation imposes considerable constraints on the cell migration because membrane cannot flow through a tight junction. We report quantitative ultrastructural immunocytochemical evidence for vesicle membrane insertion into the binucleate cell plasmalemma which allows the cells to form a pseudopodium past the tight junction. This pseudopodium increases continuously in area by vesicle insertion and develops a close apposition to the plasmalemma of the fetomaternal syncytium which constitutes the fetomaternal boundary in the placenta of the sheep and goat. Eventually the apposed membranes of the binucleate cell pseudopodium and the syncytium fuse by vesiculation and the cytoplasm and nuclei of the binucleate cell merge into the fetomaternal syncytium. The binucleate cell plasmalemma remaining on the trophectodermal side of the tight junction is blebbed off into, and phagocytosed by, the uninucleate trophectodermal cells between which the binucleate cell passed. This process permits the delivery of the binucleate cell granules to the maternal side of the placenta but none of the fetal molecules expressed on the plasma membrane of the binucleate cells are exposed to potential maternal immunological rejection.