Promotion of follicular antrum formation by pig oocytes in vitro

Pig oocyte-cumulus-granulosa cell complexes (OCG complexes) from pig early antral follicles reorganise an antrum under the stimulation of FSH. The purpose of this study was to examine the role of the oocytes in antrum formation. In the first experiment, oocyte-cumulus complexes were removed from pig OCG complexes, and the antrum formation of parietal granulosa cells themselves (PGs) was examined. Antrum formation by sham-operated OCG complexes (OC/G complexes), in which the connections between the oocyte-cumulus complexes and the parietal granulosa cells had been disrupted, was also examined. The complexes were cultured for 8 days in collagen gels in the presence of 10 ng/ml FSH. Antra were formed in about 60% of the intact OCG complexes and the sham-operated OCG complexes, while only 20% of the PGs formed antra. In the second experiment, oocyte-cumulus complexes in the OCG complexes were replaced by denuded oocytes (O/G complexes) or Sephadex G-25 beads (B/G complexes) similar in diameter to the oocytes, and the two types of complexes were cultured under the same conditions. The O/G complexes formed antra to a similar extent as the OC/G complexes, whereas the B/G complexes scarcely formed any antra. The histological sections showed that the granulosa cells in the OC/G and O/G complexes were in intimate contact with each other and retained a shape similar to those in the ovarian follicles, while the granulosa cells in the PGs and B/G complexes became quite irregular in shape. These results suggest that pig oocytes promote contact between the granulosa cells to induce antrum formation in a physiological manner.