Scanning electron microscopy of the decidual stalk and decidua basalis in the mouse.

Placentas either in situ or mechanically separated from their uterine beds were surveyed by scanning electron microscopy to determine the changing relationship of the placenta to its uterine bed with special reference to alteration in the decidual basalis accompanying parturition. At 13 days gestation the placenta is connected to the uterus by a short, broad decidual stalk which becomes longer and more constricted by term. The stalk is covered by a layer of squamous epithelium. Mechanical separation of the placenta and uterus in early gestation reveals that the entire decidual basalis of the decidual stalk is composed of large coarse fibers. As gestation progresses, a relatively smooth acellular capsule forms around the base of the placenta. However, the center of the decidua basalis, the core of the stalk, continues to be composed of large coarse fibers throughout gestation and appears to be the only region penetrated by maternal vessels.