This higher-magnification photomicrograph shows a cross section through immature chorionic tertiary villi surrounded by the intervillous space (IS). At this stage, chorionic villi are growing by proliferation of their core mesenchyme, syncy tiotrophoblast (S), and fetal endothelial cells. The syncytiotrophoblast syncytiotrophoblast layer appears relatively free of nuclei  (arrows). These stretches of the syncytiotrophoblast may be so attenuated in places that the villous surface appears devoid of a covering. The syncytiotrophoblast contains microvilli that project into the intervillous space. In well-preserved specimens they may appear as a striated border (see inset below). The cytotrophoblast consists of an irregular layer of mono-nucleated cells that lies beneath the syncytiotrophoblast. In immature placentas, the cytotrophoblasts form an almost complete layer of cells. In this full-term placenta, only occasional cytotrophoblast cells (C) can be discerned. Most of the cells within the core of the villus are typical connective tissue fibroblasts and endothelial cells. Other cells have a visible amount of cytoplasm that surrounds the nucleus. These are considered to be fetal placental antigen–presenting cells or placental macrophages (PM) historically known as the Hofbauer cells.
Secondary chorionic villi, placenta, human, H&E x320; inset ×640.
This micrographs shows the secondary chorionic villi in the third week of embryonic development. These villi are composed of a mesenchymal core (MC) surrounded by two distinct layers of the trophoblast. Secondary villi have a much larger number of cytotrophoblast cells (C) than the mature tertiary villi and form an almost complete layer of cells immediately deep to the syncytiotrophoblast (S) (see inset). The syncytiotrophoblast covers not only the surface of the chorionic villi, but also extends into chorionic plate. Maternal red blood cells are present in the intervillous space.
surrounding the chorionic villus (center of the image) forms trophoblastic bud (TB), which will be invaded subsequently by cells of the cytotrophoblast (C), connective tissue, and rapidly developing new blood vessels. In addition to fibroblasts, several fetal placental antigen–presenting cells (placental macrophages) (PM) can be identified by the amount of cytoplasm surrounding their nuclei.
C, cytotrophoblast cells CV, chorionic villiI S, intervillous space MC, mesenchymal core PM, placental macrophages S, syncytiotrophoblast TB, trophoblastic bud UV, umbilical vein KEY