Ultrastructure of the chorionic villus of the human placenta of the second and third trimester pregnancy.

38 human placentae of the 14th week of normal gestation were studied ultrastructurally. The thickness of the syncytial layer of villi varied markedly, and the syncytial cytoplasm showed marked vacuolization, which increased in the course of pregnancy in association with intensification of processes of resorption and absorption and hormonal function of the syncytium. Toward the end of the 2nd and at the beginning of the 3rd trimester of pregnancy, hypertrophic cytotrophoblastic cells appeared, whose ultrastructural character indicated functional stimulation. This observation contradicts the widely held view that cytotrophoblastic cells undergo gradual reduction in the course of pregnancy.