Fine structural changes in the placental membrane of the marmoset with increasing gestational age

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Abstract
Marmoset placentas were obtained surgically from early, mid and late pregnancy and examined with the electron microscope. At each stage both inner cytotrophoblastic and outer syncytiotrophoblastic layers were identified at the surfaces of the labyrinthine trabeculae. The trabecular cores contain very little connective tissue and are made up primarily of allantoic blood vessels of the peripheral umbilical circulation. Although a subepithelial basal lamina consistently is seen underlying the trophoblast, no subendothelial basal lamina has been identified at any stage. In early stages, the cytotrophoblast cells form a contiguous layer; consequently, the syncytium does not reach the basal lamina; and the placental membrane is hemodichorial. In the latest stage, since cytotrophoblast, though present, forms an interrupted layer and syncytiotrophoblast reaches the lamina, the membrane properly can be considered hemomonochorial. Not only in the gradual reduction in numbers of cytotrophoblast cells, but also in the fine structure of each constituent cellular layer, the membrane of this primitive anthropoid is strikingly like that of higher primates.