Inter- and intraspecific placentae in sheep, goats and sheep-goat chimaeras.

These studies compared inter- and intraspecific placentae during implantation and at full-term in sheep, goats and interspecific sheep-goat chimaeras. Histological sections prepared from intra- and interspecific day-26, 32 and 36 placentae in ewes and does indicated an altered ability of the trophoblast to invade the maternal caruncle in interspecific pregnancies. Two sheep-in-goat pregnancies were less, and two goat-in-sheep pregnancies were more, invasive than homologous pregnancies. Caprine pregnancies in chimaeras generally terminated before timed samples could be obtained, but biopsy samples collected at laparotomy between days 42 and 48 demonstrated both normal and abnormal placentation in chimaeras after breedings to rams. In six of 11 full-term fetal placentae from ovine pregnancies in chimaeras, there was abnormal retention of maternal caruncular tissue to the extent that macroscopic lesions were visible on the surface of the fetal cotyledons. Histological observations indicated that proliferation of maternal septa and hyalinization of maternal vessels had occurred at the expense of the fetal villi. Overall, the results suggested that the physiological events that regulate implantation are different in the two species, despite anatomical similarities between the ovine and caprine placenta. The caprine conceptus is likely to be rejected in the ovine or chimaeric uterus because of its over-invasiveness in the early stages of implantation, whereas the ovine conceptus can survive in the potentially chimaeric uterus.