New insights into the causes of low developmental ability of oocytes from prepubertal lambs.

Various aspects of cytoplasmatic deficiency of oocytes derived fiom prepubertal animals werepreviously reported. The novelty of our contribution is the demonstration of nuclear immaturity oflamb oocytes. Our developmental studies consist of nuclear exchange between lamb oocytesdivided in two groups: first, with diameter equal to adult oocytes, and second, smaller than adultones. Al1 reconstructed MI1 oocytes were fertilized by ICSI and cultured in vitro ti11 the blastocyststage. Using this approach we revealed that when lamb oocytes smaller fiom adult ones were usedfor nuclear transfer, the development of reconstructed embryos in both groups was blocked beforethe blastocyst stage while surprisingly the embryo progression of embryos generated fiom nuclearexchange between equal in diameter oocytes recovered from adult and lamb donor was significantlydifferent for adult- and lamb-nucleus derived embryos (18% vs. O%, respectively). The ability toprogress to the blastocyst stage was maintained only for embryos reconstructed with adult nucleus.These results demonstrated that smaller oocytes lack both, cytoplasmic and nuclear competentewhile those reaching the diameter equal to adults are still incompetent in term of nuclearpreparation. Further observation of nucleolar ultrastructure revealed fibrillogranular materia1indicative for transcriptional activity in smaller oocytes. Altogether, our developmental andmorphological studies support evidence for uncompleted nuclear maturation in prepubertal oocytes.[...]