Relative roles of the pituitary, follicle cells, and progesterone in the induction of oocyte maturation in <i>Rana pipiens</i>

Abstract The responses of ovarian oocytes obtained from mature frogs ( Rana pipiens ) to pituitary hormone were examined after removal of the follicular envelope. When the follicular envelope was removed manually or after treatment with collagenase, the apparently naked oocyte can still respond to pituitary hormone by germinal vesicle breakdown and by polar body formation. However, when the follicular envelope was removed after treatment with EDTA or Ca‐Mg‐free solution, the responsiveness of the oocyte was completely lost. Microscopic examination revealed that removal of the follicular cells was incomplete unless Ca‐Mg‐free solutions or EDTA were used. Complete removal of follicle cells prevents the oocyte from responding to pituitary hormone. Moreover, the naked oocytes packed together with previously separated follicular envelopes showed maturation responses when treated with pituitary hormone solution in the presence of large amounts of ovarian tissue. Naked oocytes which are unable to respond to pituitary hormone can nevertheless full respond to progesterone to undergo maturation reactions. In fact, germinal vesicle berakdown and polar body formation were brought about within intact ovarian tissue by treatment with progesterone alone. Oocytes responding to progesterone alone always remained within the follicular envelope. The degree of maturation of the oocytes after progesterone treatment was tested by means of nuclear transplantation. When nuclei obtained from blastula or early gastrula cells were implanted into treated oocytes, still within the follicular envelope, normal cleavage occurred. These results indicate that pituitary gonadotropin acts on the follicle cells to stimulate them to release a hormone that directly acts on the oocyte to induce maturation.