Advances in production of embryos in vitro from juvenile and prepubertal oocytes from the calf and lamb.

The use of juvenile donors in embryo-transfer (ET) programmes offers considerable potential for accelerated genetic gain in domestic livestock through reduced generation interval. The present paper reviews recent research aimed at optimizing embryo production from oocytes collected from young calves and lambs using in vitro methods of embryo production. Emphasis is placed on criteria for donor selection, oocyte-collection methods, and hormone-stimulation methods designed to produce maximum yields of viable oocytes. In vitro fertilization (IVF) rates of calf and lamb oocytes did not differ significantly whether matured in vivo or in vitro, and rates of development of blastocyst stages in culture were similar to those observed for embryos derived from adult donors. Blastocysts produced by IVF of lamb and calf oocytes established ET pregnancies at rates of 30-45%. Pregnant recipients have reached full term and delivered normal offspring at rates similar to those expected following ET of embryos produced in vivo from superovulated donors. On the basis of current follicle-stimulation protocols, on rates of blastocyst production in vitro under optimal conditions, and on observed pregnancy rates from fresh transfer of IVF embryos, 8-10 pregnancies may be expected per oocyte collection from 10-12-week-old calves and from 6-8-week-old lambs.