Developmental Changes in the Mouse Amnion: An SEM Study

In the mouse, the amnion is a barrier between fluid contained in the amnionic and exocoelomic cavities. Although there is considerable interest in the role of fetal membranes in non-placental transfer, most studies with rodents have focused on the yolk sac. Since there is a paucity of morphologic studies on the mouse amnion, we have examined the amnionic and exocoelomic surfaces at different stages of gestation. Animals were sacrificed at 13, 16 and 18 days of gestation and at term. Uteri were opened and placentas with attached fetal membranes were removed and processed for scanning electron microscopy. At 13 days the exocoelomic surface is covered by flattened fibroblasts which have an epithelial appearance or fine, coiled fibers. There appears to be an accumulation of fibers throughout gestation so that by 21 days the cells are obscured by thick fibers which are arranged in a woven pattern. At 13 days the surface lining the amnionic cavity is characterized by squamous epithelial cells with sparse irregular patches of short microvilli. There is an apparent increase in microvilli so that by 16 days the cells are covered by these surface specializations. At 18 days and at term, some cells resemble those seen at 13 days; however, many cells are covered by microfolds. These observations suggest the amnion functions primarily to protect the developing fetus by forming a sac reinforced by a fibrous coat. The inner layer may function in absorption during gestation.