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.