The ultrastructure of the sheep placenta around the time of parturition.

Placental tissue, taken from seven sheep before, during and after delivery, was fixed and sectioned for study by transmission electron microscopy. Daily blood samples from the same sheep were taken from about 135 days p.c. and were analysed by radioimmunoassay for total unconjugated oestrogens. Sections of placentomal and interplacentomal areas were examined for ultrastructural variations that might be correlated with the endocrine changes observed over this period, and to determine whether placental separation preceded delivery of the fetus and whether separation occurred at the fetal-maternal junction or elsewhere. Ultrastructural correlates of the endocrine pattern were not detected. Placental separation occurred after delivery and involved disruption of the fetal tissue (trophoblast), while its microvillous attachment to the maternal symplasma remained intact.