Autoradiographic evidence for migration and fusion of cells in the sheep placenta: resolution of a problem in placental classification

Trophoblast cells of ruminant placentas—A minireview

The ruminant placenta is classified as cotyledonary and synepitheliochorial on the basis of its gross anatomical features and histological characteristics, respectively. The richly vascularized embryonic chorioallantois is lined on its external surface by cells of the trophectodermal epithelium. These cells which assume specialized functions are referred to as trophoblast cells. Two morphologically and functionally distinct cell types have been recognized in the trophectoderm of the placenta of ruminant animals. These are the mononucleate trophoblast cells and the binucleate trophoblast cells. The occurrence, morphological characteristics, and specialized functions of these trophoblast cells, in relation to conceptus nutrition and survival in utero are discussed in this review.

Immunophenotypical Characterization of Lymphocyte Subpopulations of the Uterus of Non-pregnant and Pregnant Goats

The increased susceptibility during pregnancy to certain pathogens that cause abortions may be related to changes in the distribution and phenotype of lymphocyte subpopulations in the uterus. Histological, electron microscopic and immunocytochemical techniques were used in this study to examine whether such variations occur in different stages of the reproductive cycle of goats. The study of non-pregnant goats showed that most uterine lymphocytes were T cells and displayed both an intraepithelial and stromal distribution. CD8+ T lymphocytes were more numerous than CD4+ T lymphocytes. In the endometrial epithelium two lymphocyte subpopulations were observed: non-granulated CD2+ CD8+ T lymphocytes and granulated CD2+ CD8- T lymphocytes. During gestation, no lymphocytes were observed in the placentomal area, while a decreased number of T lymphocyte subpopulations were found in the inter-placentomal area. In the inter-caruncular epithelium, non-granulated CD2+ CD8+ T lymphocytes disappeared, whereas the granulated CD2+ CD8- T lymphocyte subpopulations increased their number and changed their morphology.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

The role of trophoblast binucleate cells in implantation in the goat: a quantitative study

The number of goat trophoblastic binucleate cells, the incidence of their migration and the formation of trinucleate and syncytial cells in the maternal uterine epithelial layer was estimated quantitatively using transmission electron microscopy between 14 and 23 days postcoitum (dpc). Binucleate cells were first observed at 18 dpc and their proportions increased rapidly from less than 1 per cent to 16 per cent by 19 dpc and 22 per cent by 23 dpc. The appearance of trinucleate cells within the maternal uterine epithelial layer coincided with evidence of migration and fusion of binucleate cells with individual uterine epithelial cells, and an increased death rate among the other uninucleate uterine epithelial cells. There was also a slight increase in the incidence of intraepithelial lymphocytes close to the trinucleate cells. The quantitative studies uphold the hypothesis that at implantation in the goat, placental trinucleate cells and their subsequent multinucleate syncytial plaque derivatives are fetomaternal hybrid tissue formed by fusion of a binucleate cell(s) with a single uterine epithelial cell.

Characterization of the synthetic capacities of isolated placental binucleate cells from sheep and goats

Sheep and goat binucleate cells (BNC) play a central role in placental growth and development. This study reports a simple method for isolating 60-70% pure populations of BNC of high viability. After incubation of the isolated BNC with a brief pulse of 14C-leucine or 3H-fucose or 3H-galactose, electron microscope autoradiography showed that label was eventually incorporated into the characteristic BNC granules via the Golgi body. Fucose and galactose initially showed a much higher Golgi body label than leucine, which was at first predominantly localised in the endoplasmic reticulum. 35S-methionine incorporation by BNC suspensions was extensive enough to allow an immunoprecipitation investigation which demonstrated that the protein hormone ovine placental lactogen and the SBU-3 antigen were synthesised de novo. Previous studies with isolated BNC have shown a remarkable range of substances to be released into the incubation medium but not necessarily synthesised during the incubation. The results demonstrate unequivocally that isolated BNC's are capable of total synthesis in vitro of two of the proteins that these same cells are known to secrete in vivo.

Role of binucleate cells in fetomaternal cell fusion at implantation in the sheep

In the sheep conceptus, individual intraepithelial binucleate cells first appear at 14 days post-coitum (d.p.c.); and by 16 d.p.c. they form 15-20% of the trophectoderm where it is apposed to the caruncular sites of initial attachment to the uterine epithelium. These binucleate cells and their derivatives can be recognized in the placenta by electron microscopy after selective staining of their Golgi body and its products, the numerous characteristic granules. Between 16 and 24 d.p.c. the uterine epithelium transforms to syncytial plaques. Evidence from ultrastructure and serial semithin sections indicates that this transformation is initiated by migration of fetal binucleate cells up to the microvillar junction and their fusion with individual uterine epithelial cells, producing trinucleate fetomaternal hybrid cells. These cells then appear to release their granules to the endometrium. It is suggested that continued binucleate-cell migration and fusion with the trinucleate cells, together with displacement and/or death of the remaining uterine epithelial cells, produces the plaques of syncytium that cover the entire caruncle by 20-24 d.p.c. This caruncular syncytium, in which no nuclear division has been found, expands enormously in area during the formation and maintenance of the cotyledons, deriving its nuclei from binucleate migration.

Placentation

Placental development differs greatly among members of different taxa. Not only does blastocyst attachment take place at different times, the penetration of trophoblast varies considerably. From an epitheliochorial relationship between fetus and mother to the hemochorial placentation of taxa, such as the higher primates, the trophoblast becomes increasingly exposed to maternal immune recognition which may be one cause of rejection in interspecific embryo transfers or hybridization. Shape of uterus and endometrium and genetic factors govern the morphologic form of the placenta. The paper reviews ungulate placentation, successes and failures of interspecific embryo transfers, and the scant knowledge of genetic determinants in primate placentas.

Cell migration in the ruminant placenta: a freeze-fracture study

In the ruminant placenta, binucleate cells normally migrate out of the trophectodermal epithelium throughout pregnancy to fuse with the syncytium bounding the maternal connective tissue. This study shows that they form part of the trophectodermal tight junction as they migrate, thus maintaining the barrier function while penetrating the junction. This method of migration seems to be unique and the means by which it is achieved is briefly discussed.

Volumes of distribution of sodium and albumin in the sheep placenta

The volumes of distribution of 22Na and 24Na, 51Cr-EDTA and albumin labelled with 131I and 125I were measured in cotyledons and skeletal muscle taken from pregnant sheep and the fetuses near term. Radiolabelled substances were injected through chronically implanted catheters 0.5-3.2 hr before removal of tissues in 'short-term' experiments and 24-26 hr before in 'long-term' experiments. Volumes of distribution were expressed as percentages of tissue weights. Cotyledonary water content was 81.5 +/- 0.6% (S.E. of mean) of wet weight (n = 10 sheep). Sodium content was 76.0 +/- 4.0 m-mole kg-1 wet weight (n = 4) and potassium content was 58.8 +/- 2.4 m-mole kg-1 wet weight (n = 4). In short-term studies cotyledonary spaces following injection into the fetal circulation were: Na, 20.5 +/- 0.9% (n = 10 sheep); Cr-EDTA, 19.5 +/- 1.2% (n = 3); and albumin, 2.8 +/- 0.3% (n = 11). The equivalent maternal cotyledonary spaces were: Na, 23.5 +/- 1.6%, or 22.1 +/- 1.6% when 'corrected' for fetal radioactivity (n = 8); and albumin, 4.7 +/- 0.8% (n = 8). Maternal and fetal cotyledonary sodium spaces were additive. Larger cotyledons had larger fetal sodium spaces. In long-term studies, the value for maternal albumin spaces was considerably higher 12.3 +/- 3.2% (n = 3), whereas that for fetal albumin spaces was only 3.9 +/- 0.4% (n = 4). The higher fetal sodium space of 24.9 +/- 1.0% (n = 4) in the long-term experiments was attributable to rising sodium counts in the maternal circulation. In the short-term experiments maternal skeletal muscle spaces were Na, 13.5 +/- 1.3% (n = 7) and albumin, 1.7 +/- 0.3% (n = 7), with higher values of 22.0% (n = 2) and 3.8 +/- 0.2% (n = 3) respectively in long-term experiments. In the short-term experiments fetal skeletal muscle spaces were Na, 32.8 +/- 3.2% (n = 7) and albumin, 2.5 +/- 0.3% (n = 10), with higher values of 39.7 +/- 7.4% (n = 4) and 16.2 +/- 0.6% (n = 4) respectively in long-term experiments.