Developmental capacity acquisition requires exposure of uncondensed chromosomes before maturation of bovine oocytes

Immature bovine oocytes are forming an heterogeneous population since only one third of all morphologically normal cumulus-oocyte-complexes will result in a developing embryo after in vitro culture. To increase developmental competence, it is essential to improve the quality of the 2/3 not responding to the in vitro maturation context. To achieve this goal we can address the cytoplasmic domain or the nucleus capacity to control future events leading to normal fertilization and development. To influence for example the quantity and quality of RNA present at meiotic resumption (MR), we need access to uncondensed chromatin before that period. Maintenance of the germinal vesicle for a minimum of 24 h can be achieved with protein synthesis or phosphodiesterase (PI) inhibitors or with cAMP accumulation at a level higher that the one required in the cumulus-granulosa portion to stimulate MR. It is suspected that in vivo, the granulosa layer is responsible for the arresting signal since follicular fluid (BFF) alone is not sufficient for a complete effect in vitro. Low concentrations of granulosa cells in vitro (10(6)/ml) stimulate MR but monolayers with their conditioned medium provide inhibitory conditions (33% GV) to oocytes adhering to the monolayers. To achieve a complete meiotic arrest for 24 h (greater than 75% GV), a high concentration of granulosa cells must be used (10(7)/ml). The embedding of cumulus-enclosed oocytes in agar prior to culture with the cells prevents the significant effect of the cells on MR indicating either a need for contact stimulation or a very labile product.(ABSTRACT TRUNCATED AT 250 WORDS)