Synepitheliochorial Placentation : Ruminants (Ewe and Cow)

In Grosser’s syndesmochorial category (exclusively ruminants) the uterine epithelium was said to be lost so the trophectoderm was apposed to the maternal connective tissue directly. It has now been shown by a variety of techniques that throughout pregnancy characteristic ruminant fetal trophectodermal binucleate cells (BNCs) modify the uterine epithelium by apical fusion to form fetomaternal hybrid syncytial plaques at the junction of the fetal and maternal tissue (Wooding 1982a,b, 1984) (Figs.6.1 and 6.2). This fusion delivers fetal hormones and effectors to the maternal circulation.

The synepitheliochorial placenta has three identifying characteristics: the BNC in the fetal trophectoderm (Chap. 1.6.5) which form a structurally separate population among the uninucleate trophectodermal cells from which they are derived; the fetomaternal syncytium formed by BNC fetal migration and fusion with some or all of the maternal uterine epithelial cells; and the placentomal [or cotyledonary] chorioallantoic placental organization, with the restriction of fetal and maternal villus elaboration to discrete regions (placentomes) based on pre-existing non-glandular caruncular areas on the uterine epithelium (Amoroso 1952) which are well developed in the fetal uterus by mid-pregnancy (lamb, Wiley et al. 1987). A ‘placentome ’ is formed by enmeshed fetal villi and maternal crypts; the tuft of fetal villi is correctly referred to as a ‘cotyledon ’ and the corresponding maternal structure is a ‘caruncle’ (Fig. 6.3).