FIGURE 3.

Diagrammatic representation of the gross morphology of the placenta and the histology of the interhemal membrane in the human, mouse, and sheep. In each case, the bottom panel represents detail of the area outlined by the square in the top panel. A: in the human, the fetal villi arise as a series of lobules (L) from the chorionic plate (CP). The basal plate abutting the maternal decidua (D) is thrown into a series of folds forming septae (S) that partially compartmentalize the placenta into lobes. Each lobe may contain one or more lobules. Maternal blood enters the intervillous space (IVS) from the spiral arteries (SA), passes between the villi, and drains into the openings of the uterine veins on the septae. B: a single layer of syncytiotrophoblast (Stb) covers each villus and is generated from underlying cytotrophoblast (Ctb) cells. It is bathed by maternal blood in the IVS from the start of the 2nd trimester onwards. Fetal capillaries (FC) within the stromal core (Str) invaginate to reduce the length of the diffusion pathway (arrowed). C: the mouse placenta is divided into an exchange labyrinth zone (Lz) and an endocrine junctional zone (Jz). The visceral endoderm layer of the inverted yolk sac (YS) is exposed to the decidua (D) after the outer parietal layer breaks down (dotted line). This represents an important route of nutrient exchange during early pregnancy and may continue until term. D: in the labyrinth, the syncytiotrophoblast (Stb) is two-layered, and an additional layer of sinusoidal giant cells (SGC) lines the maternal blood spaces (MBS). Little stromal tissue (Str) is interposed between the fetal capillaries (FC) and the trophoblast. E: in sheep, fetal villi (FV) interdigitate with maternal crypts within specialized areas of the endometrium (E), the caruncles, to form placentomes. In between placentomes, the trophoblast forms areolae (Ar) opposite the openings of the endometrial glands (EG). Histotroph from the glands is taken up by the trophoblast, representing another route for maternal-fetal transfer. F: within a placentome, there are six tissue layers interposed between the maternal (MC) and fetal (FC) capillaries: the maternal endothelium, maternal stromal tissue (MStr), the uterine epithelium which is converted into a synepithelium by the migration and fusion of fetal binucleate cells, the trophoblast (Tr), the fetal stroma (FStr), and the fetal endothelial cells. Differences in the nature of the interhemal interface mean that extrapolation of transport data from one species to another may not always be justified.