The muscle coat morphology of the mouse oviduct during the estrous cycle.

Light and electron microscopic examination of the muscle coat of the mouse oviduct reveals differences in various regions concerning both the architectural pattern and cytological features of the smooth muscle cells. A network with large meshes made up of thin smooth muscle cells with unusual features, characterizes the infundibulum and upper ampulla. In the lower ampulla and isthmus these cells gradually stratify into two distinct muscle layers and acquire the typical characteristics of smooth muscle cells. In the uterotubal junction, the smooth muscle cells show a structure quite similar to that of myometrial cells. Exclusively at this level do cells of the circular layer form a ring at the opening of the oviduct into the uterine cavity. Independent of their location in one or the other region, the oviductal smooth muscle cells undergo striking changes during the estrous cycle. A substantial increase in organelle content, an enlargement in the extent of the contact area and the occurrence of myoblasts characterize proestrous and estrous. During metestrous all these features regress, until a picture typical of diestrous is reestablished. The peculiar structure of smooth muscle cells and their different arrangement in the upper oviductal region with respect to that in the lower one are in keeping with zonal differences in contractility of the oviductal wall and together with the striking cytological changes occurring during the estrous cycle, indicate their active involvement in reproductive processes.