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Anatomical Relationship between Fetus and Mother

This paper will briefly examine the relationship between trophoblast and maternal tissues. It will primarily refer to the overall topic of this volume, the transport of fetal cells to the maternal blood. The human placenta is a hemochorial organ. This statement implies that the maternal blood comes into direct contact with the fetal trophoblastic cells that cover the placental villi. Although the majority of primates have this type of placenta, it is not true for the Lemuridae, for instance. They have an epitheliochorial placentation. The different types of approximation between trophoblast and mother, that is, the number of layers separating fetal from maternal blood, were first enumerated by Grosser.’ He differentiated among four major types of placentas. But it was not only the thickness of the “placental membrane” that he examined, but also the great variability in the shape of the placenta among mammals. Many different shapes of placenta have been described, and they have been used to divide species. There are membranous, cotyledonary, discoid, ring-shaped placentas, and others. The human placenta is a discoid organ, and it does not differ much in structure from that of apes. A review of all of this knowledge is contained in the comprehensive book of Mossman.* An extensive overview of the human placenta, including the electron microscopy and physiology, is to be found in Boyd and Hami1ton.j The fate of trophoblast cells will first be described briefly so as to arrive at an understanding of the type of placenta that we possess. The still-primitive blastocystic shell, containing the inner cell mass (the future embryo), attaches to the endometrium approximately six to seven days after ovulation and fertilization. During this stage of implantation, the trophoblast dissociates the endometrium with specific proteases, and the blastocyst wall thus initiates its future “interstitial” implantation: During the subsequent growth of the placenta and the embryonic cavities, the membranes develop, primarily through the enlargement of the embryonic cavity. In a way, the membranes herniate out into the endometrial cavity as the cavity enlarges. After the setting-aside of the free membranes, the chorion laeve, the superficial trophoblast, and the primitive villous structures that cover this portion of the membranes atrophy. They will not be discussed further here because no cell traffic occurs between fetus and mother at this point. A continuing point of disagreement here relates to whether