Chromosome configurations and time sequence of the first meiotic division in bovine oocytes matured in vitro.

Bovine oocytes aspirated from small follicles were cultured for various time periods. Subsequently, the oocytes were fixed and stained with Giemsa and analyzed for their chromosome configuration. The appearance of the various chromosome configurations and their time sequence were documented. The influence of the presence or absence of follicle-stimulating hormone (FSH) during culture was studied. No difference was found in the proportion of oocytes completing meiotic maturation in the presence or absence of FSH. On average, 75% of the oocytes reached metaphase II after 20 h. FSH showed two main effects: 1) all the cumulus oocyte complexes incubated longer than 10 h in the presence of FSH showed cumulus expansion, and 2) the time period required for chromosome condensation was prolonged for 3 h in the presence of FSH. However, the time sequence in vitro in the presence as well as in the absence of FSH paralleled the time sequence found in vivo, where variations of several hours have been reported. The delay in chromosome condensation in the presence of FSH was assumed to be due to a transient increase in cyclic adenosine 3',5'-monophosphate in the cumulus oocyte complexes. As demonstrated for FSH, the described culture system allowed the study of individual factors for their influence on meiotic maturation of bovine oocytes.