Occurrence of a contractile tissue in the theca externa of atretic follicles in the mouse ovary.
In mature mouse ovaries, smooth muscle cells (SMC) are as a rule concentrated in the perifollicular stroma of developing follicles but are also present in different ovarian regions. In growing antral follicles SMC form a conspicuous investment ('capsule') of concentric layers of stretched and elongated elements located in the zones of the theca externa. On the contrary, in antral follicles reaching approximately the same growth phase but with clear signs of atresia, the contractile capsule of the follicles has an irregular outline due to an apparent dislocation and shortening of SMC. The ultrastructural findings show that in atretic follicles the majority of the cells forming this capsule possess the cytological characteristics of SMC fixed in contraction. The occurrence of compressed SMC in the wall of atretic follicles suggests that some relationship exists between follicle contraction and the atretic process. In the case of the follicle at ovulation, it may be suggested that the activation of such perifollicular contractile tissue, even if incapable of producing alone the dehiscence of the follicle, may nevertheless be capable of squeezing the follicular wall. Then, if other factors do not simultaneously occur to cause a normal ovulatory process, the contraction of this tissue, instead of assisting in the rupture of the follicle, may favor its collapse and subsequent involution during atresia.
D000818:Animals; D005260:Female; D005614:Freeze Fracturing; D051379:Mice; D009130:Muscle, Smooth / Q000648:ultrastructure; D006080:Ovarian Follicle / Q000648:ultrastructure*; D010053:Ovary / Q000033:anatomy & histology / Q000502:physiology; D010060:Ovulation; D013799:Theca Cells / Q000648:ultrastructure*