The fate of thecal smooth muscle cells in postovulatory hamster follicles.

Smooth muscles cells (SMC) have been identified in the theca externa of preovulatory follicles in several mammals. The function of these cells is not clear. It has been suggested that they are involved in ovulation and may play a role in the collapse of ruptured follicles and their transformation into corpora lutea (CL). In this study we used transmission electron microscopy (TEM) to examine the ultrastructure of SMC at the base of hamster follicles during the formation and regression of CL. For the first three days following ovulation, the CL are surrounded by a distinct theca externa composed of 1-3 layers of spindle-shaped cells. The cells were identified as SMC by the masses of filaments in the cytoplasm, dense attachment plaques and caveolae along the plasma membrane and an external lamina surrounding each cell. On the fourth day after ovulation, the theca externa is fragmented and TEM observations suggest the following three fates for theca externa cells: 1) many cells degenerate, 2) some cells persist as SMC, 3) some cells possess few or no filaments and appear similar to fibroblasts. The significance of these results is discussed in regard to ovarian contractions, the fate of CL, and the recycling of ovarian tissues.