Follicle somatic cells influence pig oocyte penetrability and cortical granule distribution

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Abstract
The influence of somatic cells on oocyte penetrability was studied during in vitro maturation. Four experiments were carried out. In the first, pig oocytes were fertilized in vitro immediately after collection (immature oocytes) or after being cultured for 44 hr with cumulus cells connected to the whole wall of the extroverted follicle (follicle oocytes) or without cumulus cells (denuded oocytes) (Mattioli et al.: Gamete Res 20:177–183, 1988a). In follicle and immature oocytes, 12 hr after insemination, the sperm were equally distributed between zona and ooplasm; in denuded oocytes, the majority (90.5%) of the sperm were located in the zona. In the second experiment, zonae pellucidae of follicle and denuded oocytes, obtained after 44 hr of culture, were incubated with a sperm suspension for 2 hr at 39°C to evaluate the zona binding. The different number of sperm found on the zona pellucida of follicle or denuded oocytes (49.3 ± 2.97 vs. 37.8 ± 1.77) did not achieve statistical significance (P > 0.05). In the third experiment, follicle and denuded oocytes were denuded of their zonae after maturation and then fertilized in vitro. The number of sperm recorded in the ooplasm of zona‐free follicle oocytes was significantly higher than that recorded in zona‐free denuded oocytes (9.2 ± 0.93 vs 1.19 ± 0.28) (P<0.01). In the last experiment the influence of somatic cells on the distribution of cortical granules was evaluated. Pig oocytes were denuded at different stages of culture and, after completion of maturation, processed for electron microscopy. The removal of somatic cells at 5–20 hr of culture resulted in a premature migration of cortical granules underneath the oolemma. An increase of exocytotic events was also recorded. These data demonstrate that the presence of cumulus cells during maturation is fundamental for the pig oocyte to maintain its penetrability. The persistence of the interaction between cumulus cells and oocyte stabilizes the distribution of cortical granules, and the interruption of this coupling induces cortical granule exocytosis and may be responsible for the reduced penetrability of oocytes matured without cumulus cells.