Binucleate cell migration in the bovine placentome

The ultrastructure of cow placentomes, collected between 37 and 260 days of gestation, was examined. The microvillar junction and binucleate cell granules were selectively stained by phosphotungstic acid. Fetal binucleate cells interrupted the microvillar junction and penetrated as far as the basement membrane of the uterine epithelium. The uterine epithelium included not only binucleate cells which contained the distinctive granules but also non-granulated binucleate cells with pyknotic nuclei at the microvillar junction. Binucleate cells with pyknotic nuclei were also seen within chorion cells. It is suggested that the normal function of a mature chorionic binucleate cell at all the stages of bovine pregnancy is migration into the uterine epithelium to release its granules and subsequent condensaton to a cell remnant which is phagocytosed by the chorionic epithelium.