Structure and function in the ruminant synepitheliochorial placenta: central role of the trophoblast binucleate cell in deer.

The ruminant placenta has a very uniform gross structure based on localised areas of fetomaternal membrane apposition and proliferation to form placentomes. There is no consistency, however, in the number or size of these placentomes (6-150), nor in the villus architecture of the individual placentome. The one consistent feature is the binucleate cell (BNC) population in the trophoblast. These BNC form 15-20% of the epithelium in all ruminants examined so far. They synthesise the placental lactogen hormone and other glycoproteins and migrate through tight junctions to fuse with a uterine epithelial cell to form initially fetomaternal hybrid trinucleate cells (TNC) and subsequently syncytial plaques (SP). Such SP may be transient or persist throughout pregnancy depending on the species. The wide range of deer species examined confirms the uniformity of the BNC hormone production, migration, and fusion pattern described for other ruminants. BNC migration produces predominantly transient TNC, but there are areas of SP largely restricted to the apex of the maternal crypts. Maternal large granule lymphocytes (LGL) are uniquely found in deer placentomal uterine epithelium; they are usually closely associated with TNC and SP sites, but the significance of the interactions remains to be established.