Cell coupling and maturation-promoting factor activity in in vitro-matured prepubertal and adult sheep oocytes

We examined some differences between prepubertal and adult ovine oocytes; in particular we analyzed the functional status of the cumulus-oocyte complex, protein synthesis during in vitro maturation, and because no information is available on prepubertal and adult sheep, maturation-promoting factor (MPF) fluctuations throughout meiotic progression both in prepubertal and adult sheep oocytes. After 24 h of maturation, percentages of MII oocytes were similar between prepubertal and adult animals. Electron microscopy examinations showed that prepubertal oocytes had fewer transzonal projections than adult oocytes. Methionine uptake was significantly lower in prepubertal cumulus-enclosed oocytes examined through meiotic progression. On the contrary, denuded prepubertal oocytes showed a higher methionine incorporation in the first 4 h of incubation compared with adult oocytes. We also found some differences in MPF activity between prepubertal and adult oocytes at MII stage. In fact, prepubertal MII oocytes had a significantly lower level of MPF activity than adult oocytes did and, after fusion with germinal vesicle oocytes, they were unable to induce nuclear breakdown and chromosome condensation 1-2 h post-fusion, whereas adult MII oocytes could induce these processes. Our findings show that the lesser competence of prepubertal oocytes could be due to morphological anomalies and alterations in physiological activity and that oocytes do not reach full developmental competence until puberty.