Bovine follicular development and its effect on the in vitro competence ofoocytes

The current knowledge is reviewed concerning correlations between follicula r development in the cow and the competence of matured oocytes to develop i nto an embryo following IVF and IVC. At the follicular size of 3 mm, some o ocytes become competent and the proportion of competent oocytes does not in crease during development up to 7 mm. The proportion of competent oocytes i ncreases greatly in follicles >8 mm in both untreated and gonadotropin-stim ulated cows. The competence of in vitro-matured oocytes hem these large fol licles is lower than the competence of in vivo-matured oocytes. These obser vations lead to the following concept. Oocytes have acquired an intrinsic c apacity to develop into an embryo after IVM-IVF-IVC at the follicular stage of 3 mm, but require an additional "prematuration" to express this compete nce. In vivo, this prematuration occurs during preovulatory development bef ore the occurrence of the LH surge. In follicles of 3-7 mm, a low level of atresia appears to improve the in vitro competence of oocytes which may act via a prematuration-like effect. A thorough understanding, however, of the effect of atresia and other factors on the competence of this highly heter ogeneous oocyte population is still missing. Two routes to improve the embr yo yield in ovum pick-up (OPU) practice are discussed. (C) 1999 by Elsevier Science Inc.