FIGURE 23.20 • Stratified squamous epithelium of the ectocervix. The stratified squamous epithelium and underlying fibrous connective tissue within the lower rectangle in Figure 23.22 is shown here at higher magnification. The more mature epithelial cells have a clear cytoplasm (arrowheads), a reflection of their high glycogen content. Also, note the connective tissue papillae protruding into the epithelium (arrows). The bulk of the cervix is made up of dense, fibrous connective tissue with relatively little smooth muscle. 120.
FIGURE 23.21 • Transformation zone of the cervix. The site of the squamocolumnar junction from the upper rectangle in Figure 23.19 is shown here at higher magnification. Note the abrupt change from stratified squamous epithelium to simple columnar epithelium (arrow). Neoplastic changes leading to development of cervical cancer most frequently begin in this transformation zone. Within the connective tissue are the branched, mucus-secreting cervical glands (CG) composed of a simple columnar epithelium that is continuous with the lining epithelium of the cervical canal. 120.
FIGURE 23.22 • Metaplastic stratified squamous epithelium of the cervical canal. This photomicrograph shows an island of the fully mature stratified squamous epithelium surrounded by the simple columnar epithelium normally found in the cervical canal. 450. (Courtesy of Dr. Fabiola Medeiros, Mayo Clinic.)
Several types of cells are recognized in the connective tissue stroma of the villi: mesenchymal cells, reticular cells, fibroblasts, myofibroblasts, smooth muscle cells, macrophages, and fetal placental antigen–presenting cells, historically also known as Hofbauer cells (Fig. 23.24 and Plate 100, page 888). Fetal placental antigen–presenting cells are the specific villous macrophages of fetal origin that participate in the placental innate immune reactions. In response to antigen, they proliferate and upregulate specific surface receptors that recognize and bind to a variety of pathogens. Like other antigen-presenting cells, if stimulated, they increase the number of MHC II (major histocompatibility complex) molecules on their surface.
They are more common in the early placenta. The vacuoles in these cells contain lipids, glycosaminoglycans, and glycoproteins. Recent studies of HIV-infected placentas indicate that HIV is primarily localized within the fetal placental antigen–presenting cells and in the syncytiotrophoblast.
Early in development, the blood vessels of the villi become connected with vessels from the embryo.
Blood begins to circulate through the embryonic cardiovascular system and the villi at about 21 days. The intervillous spaces provide the site of exchange of nutrients, metabolic products and intermediates, and wastes between the maternal and fetal circulatory systems.
During the first 8 weeks, villi cover the entire chorionic surface, but as growth continues, villi on the decidua capsularis begin to degenerate, producing a smooth, relatively avascular surface called the chorion laeve. The villi adjacent to the decidua basalis rapidly increase in size and number and become highly branched. This region of the chorion, which is the fetal component of the placenta, is called the chorion frondosum or villous chorion. The layer of the placenta from which the villi project is called the chorionic plate (Plate 99, page 886).