Oocyte Maturation in the Brachiopod Terebratalia transversa: Role of Follicle Cell-Oocyte Attachments During Ovulation and Germinal Vesicle Breakdown

In the brachiopod Terebratalia transversa, each ovarian follicle consists of (i) a prophase-arrested oocyte with an enlarged nucleus (= the germinal vesicle, or GV), and (ii) a surrounding sheath of follicle cells that attach to the oocyte by means of junctional complexes. Within about 80 min after removing a follicle from the ovary, the follicular sheath undergoes a microfilament-mediated retraction, and the ovulated oocyte that emerges from the retracted sheath subsequently completes germinal vesicle breakdown (GVBD). Based on the experimental manipulations reported here, it appears that the follicle must be detached from the ovary for such ovulation and GVBD to occur. Moreover, GVBD can be prevented if the oocytes are mechanically stripped of their follicle cells up to 30 min after being isolated from the ovary. GVBD proceeds normally, however, if follicle cells are removed more than 40 min after the follicle is obtained from the ovary. The percentage of oocytes that undergo GVBD is also diminished following treatment with drugs that uncouple gap junctions. Collectively, these data suggest that removing a follicle from the ovary stimulates follicle cells to produce a maturation-inducing factor that uses the follicle cell-oocyte junctional complexes to reach the oocyte within about 30-40 min after follicular removal. The significance of these findings is discussed relative to previous reports on oocyte maturation in brachiopods and other animals.