An ultrastructural study of smooth muscle‐like cells in the theca externa of ovarian follicles in the rat

The cells of the theca externa of large, antral ovarian follicles in the rat were studied by electron microscopy. Many resembled fibroblasts, while others possessed cytoplasmic filaments and dense bodies characteristic of smooth muscle cells. Filament‐containing cells of intermediate structure appeared to represent transitional forms between the two previous types. Small numbers of smooth muscle‐like cells were found also around small antral follicles, and many corpora lutea possessed locally well‐defined coats of smooth muscle. These observations indicate that cells with a probable contractile function are present in the theca externa of ovarian follicles in the rat. These cells may play a role in the process of ovulation.