Separation of the Placenta in the Ewe: An Ultrastructural Study

Abstract

Placental separation in the ewe appears to involve an explosive degeneration of the chorionic epithelium which occurs in the short space of time between parturition and the delivery of the fetal membranes. In the goat there is a similar mechanism of separation. By contrast separation of the placenta in the cow appears to depend upon a detachment at the junctional region between fetal and maternal epithelia. There is no obvious explanation for these findings. The results are discussed in the light of the placental hormonal environment in the period which immediately precedes parturition.The placenta of the ewe, like that of other ruminants, is divided into cotyledonary and intercotyledonary areas. The cotyledons consist of large numbers of highly branched villi, which interdigitate with corresponding crypts in the uterine caruncles to form an epitheliochorial placenta [Lawn, Chiquoine and Amoroso, 1969). In the ewe the cotyledons are concave and sessile: the fetal vessels enter the substance of the cotyledon from the concavity of the central depression [Steven, 1968]. From the 8th week of gestation this central region of the cotyledon is characterized by an extravasation of maternal blood, which separates the tips of the maternal septa from the arcades of chorionic epithelium at the bases of the chorionic villi [Wimsatt, 1950]. In the zona intima of the deeper regions of the cotyledon, fetal and maternal epithelia are held together by an interdigitating microvillous junction. There are no marked differences in placental morphology between the ewe and the goat.The general arrangements of the bovine placenta are similar to those of the other two species. In the cow, however, the cotyledons are convex and pedunculated, the fetal vessels are distributed over the convex surface of the cotyledons and there is no obvious extravasation of maternal blood at the tips of the maternal septa. In cattle retention of the placenta following parturition is a common clinical condition [Bj6rkman and Sollen, 1961] which disrupts breeding programmes and leads to considerable economic loss. In sheep and goats retention of the placenta is very rare indeed.It has been shown by Bj6rkman and Soll6n [1960] that under normal circumstances separation of the bovine placenta occurs within the cotyledons at the line of microvillous attachment between fetal and maternal epithelia, and that the process of separation involves a loosening of the attachment between fetal and maternal microvilli with little damage either to fetal or maternal tissues. In this paper evidence is presented to show that placental separation in the sheep and goat is achieved by a very different mechanism. A preliminary report has already been published [Steven, 1973].

Ultrastructural Studies of the Placenta of the Ewe: Phagocytosis of Erythrocytes by the Chorionic Epithelium at the Central Depression of the Cotyledon

“…7, 9) most probably represent the PAS positive perivascular cuffs described by Wimsatt, and the granules, vesicles and other accumulations of cell debris may be the final remains of stromal cells which have disintegrated during the first 100 days of pregnancy. Such changes, however, are not entirely confined to the haemophagous region of the cotyledon since they also occur to a lesser extent in the connective tissues of the more peripheral regions of the maternal septa, where they were noted by Wimsatt [1950], Lawn, Chiquoine and Amoroso [1969] and Steven [1975]. The electron micrographs of Lawn and his colleagues appear to show that by the 42nd day ofpregnancy the stromal cells have assumed a rounder and less angular form, that they have lost their desmosomal interconnexions and that in some cases they are in process of breakdown.…”

“…obliterated by the formation of a microvillous junction between foetal and maternal tissues (see Figs. 1, 4;Steven, 1975). As in the haemophagous region of the cotyledon the chorionic epithelium of the intermediate zone rests upon a basement membrane of considerable thickness.…”

Bovine retained placenta: aetiology, pathogenesis and economic loss

“…However, as the process occurs over a period of one to three hours, separation cannot be a purely vascular phenomenon (Roberts 1986). In sheep, the process of detachment is completed by the explosive degeneration of the fetal uninucleate cells, but in the cow the majority of the chorionic villus layer is expelled intact (Steven 1975). This lack of damage to the fetal membranes during their expulsion rules out the possibility of detachment being a purely physical process; placental separation therefore requires a reduction in feto-maternal adherence.…”

Evolutionary implications of fetal and maternal microvillous surfaces in epitheliochorial placentae

“…In this type of placenta there is one surface area of the fetal trophoblast cells and one of the maternal endometrial cells and some intercellular space in between (Hasselager, ). Typically, both epithelia are covered with apical microvilli, which interdigitate with each other, but also regions of flat apposition of fetal and maternal membranes exist (sheep: Steven, ; camel: Abd‐Elnaeim et al, ).…”