An electron microscopic study of the chorioallantoic placenta and the subplacenta of the cane rat (Thryonomys swinderianus Temminck).

The ultrastructure of the cane rate chorio-allantoic placenta has been studied. In its thinnest regions the interhaemal membrane is composed of a single layer of syncytiotrophoblast, a common basal lamina, and the fetal capillary endothelium. Hence this is a haemomonochorial placenta. The cytotrophoblastic cells show many mitotic figures but they are never seen directly bounding the maternal blood spaces. The syncytiotrophoblast of the spongy zone is rich in dilated cisterns of granular endoplasmic reticulum and is also characterized by many 'intrasyncytial canaliculi'. The subplacenta consists of both cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast has numerous short microvilli projecting into the intercellular spaces, granular endoplasmic reticulum, a well-developed Golgi complex, and numerous electron-dense granules of uniform size. The intercellular spaces or lacunae contain a precipitate of moderate electron density which is similar in texture to the intracellular precipitate in the 'brush border' region of the syncytiotrophoblast. The similarities between the ultrastructure of the cane rat placenta and those of the guinea pig and chinchilla are discussed.