Electron microscopy of the placenta and related structures of the marmoset

The marmoset has recently become an important animal in oncologic research, especially with respect to herpes deoxyribonucleic acid viral tumors. As a prototype of the primitive New World monkey, it is also valuable for the study of primate evolution. For these reasons this ultrastructural description of its placenta is presented, including the first electron micrographs of the basal plate and amnion. The main placental mass is pseudo-labyrinthine, or trabecular. The ultrastructural features of the cellular and syncytial trophoblast and endothelium closely resemble those of higher primates. The junctional region shows numerous clumps of moderately well-differentiated trophoblast, an acellular junctional zone, and a poorly developed decidual reaction. Trabecular connective tissue is sparser and transitional trophoblastic elements are less prominent than those in the human placenta. The amnion is ultrastructurally indistinguishable from that of the higher primates. Although the marmoset's placenta retains certain primitive gross and histologic features, such as the large persistent yolk sac and the trabecular condition, it is ultrastructurally quite similar to that of man.