AbstractPlacental villi were obtained from monkeys at the Wisconsin Primate Research Center during 50–150 days gestation. As in the human, the villi consist of a vascular mesenchymal core surrounded by inner cytotrophoblastic and outer syncytiotrophoblastic layers.The syncytiotrophoblast contains abundant rough endoplasmic reticulum and a well developed Golgi complex consisting of stacked lamellae and diffuse vesicles. An asymmetrical distribution of ribosomes occurs on RER cisternae associated with mitochondria. Branched microvilli frequently bear dilated tips and resemble stereocilia. Cytoplasmic tufts and microvilli increase the surface area, and the base of the tuft usually contains a large membrane‐bound vacuole with a flocculent inclusion of low electron density. These vacuoles are lacking after 100 days. In later stages the syncytiotrophoblast bears irregular microvillous processes and specialized attachment sites at its contact with the basal lamina.Hofbauer cells in the villus core contain numerous large vacuoles, coated pinocytotic vesicles and smooth vesicles apparently derived from ER; the latter are sometimes fused to the large vacuoles. Hofbauer cells appear to be derived from undifferentiated mesenchymal ce
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