As the embryo develops, the invasive activity of the syncytiotrophoblast erodes the maternal capillaries and anastomoses them with the tro-phoblast lacunae, forming the maternal blood sinusoids. These communicate with each other and form a single blood compartment, lined by syn-cytiotrophoblasts, called the intervillous space. At the end of the second week of development, cytotrophoblast cells form primary chorionic villi. They project into the maternal blood space. In the third week of development, invasion of the extra embryonic mesenchyme into the primary chori-onic villi creates secondary chorionic villi. At the end of the third week, core mesenchyme differentiates into connective tissue and blood vessels that connect with the embryonic circulation. These tertiary chorionic villi constitute functional units for exchange of gases, nutrients, and waste products between maternal and fetal circulation without direct contact with each other. This separation of fetal and maternal blood is referred to as the placental barrier. Each tertiary villus consists of a connective tissue core surrounded by two distinct layers of trophoblast-derived cells. The outermost layer consists of the syncytiotrophoblast; immediately under it is a layer of cytotrophoblast cells. Starting at the fourth month, these lay-ers become very thin to facilitate the exchange of products across the placental barrier. The thinning of the wall of the villus is due to the loss of the inner, cytotrophoblastic layer. At this stage, the syncytiotrophoblast forms numerous trophoblastic buds that resemble the primary chorionic villi; however, the cytotrophoblast and the connective tissue grow very rapidly into these structures, transforming them into tertiary villi. At term, the placental barrier consists of the syncytiotrophoblasts; a spare, discontinuous, inner cytotrophoblast layer; the basal lamina of the trophoblast; the connective tissue of the villus; the basal lamina of the endothelium; and the endothelium of the fetal placental capillary in the tertiary villus.
Tertiary chorionic villi, placenta, full-term, human, H&E ×280.
This photomicrograph shows a section through the intervillous space of the placenta at term. It includes chorionic villi (CV) of different sizes and the surrounding intervillous space (IS). The connective tissue of the villi contains branches and tributaries of the umbilical vein (UV) and arteries. The intervillous space usually contains maternal blood (only a few maternal blood cells are seen here). The outermost layer of each chorionic villus derives from the fusion of cytotrophoblast cells. This layer, known as the syncytiotrophoblast (S), has no intercellular boundaries, and its nuclei are rather evenly distributed, giving this layer an appearance similar to that of cuboidal epithelium. In some areas, nuclei are gathered in clusters (arrowheads); in other regions the
Tertiary chorionic villi, placenta, mid-term, human, H&E ×320.