The complete nuclear maturation and fertilization in vitro of oocytes from 30–40-day-old juvenile lambs, and their ability to develop up to the blastocyst stage when transferred into recipient ewes after fertilization in vivo and culture was studied. Cumulus–oocyte complexes were recovered from juvenile ovaries and only those with several cumulus cell layers were selected and compared with oocytes from adult sheep. The rate of meiotic progression was significantly lower (P < 0.001) for juvenile oocytes (8%) than for adult oocytes (58%) without gonadotrophins in the culture medium. However, a similar maturation rate was observed in oocytes of both juvenile (76%) and adult sheep (84%) in the presence of gonadotrophins. Eighteen hours after in vitro insemination, the fertilization rates for juvenile oocytes were not significantly different from those of adult oocytes (64% and 72%, respectively). Parthenogenetic activation and polyspermy were higher in juvenile than in adult oocytes (P < 0.001). The proportion of blastocysts produced was lower for juvenile (20% than for in vitro matured adult oocytes (49%) after their transfer into transitory recipients for 5.5 days (P < 0.01). However, the viability of blastocysts (67%) derived from in vitro matured juvenile oocytes showed a rate of hatching similar to that obtained from adult oocytes (74%). Pregnancy rates for recipient ewes at 90 days were similar for both juvenile (57%) and adult (61%) oocytes. The results indicate that it is possible to mature and fertilize in vitro matured juvenile oocytes to produce viable embryos.

We examined some differences between prepubertal and adult ovine oocytes; in particular we analyzed the functional status of the cumulus-oocyte comple