Research articleEffect of follicular aging on ATP content and mitochondria distribution in bovine oocytes

Hereford heifers were assigned randomly to three superstimulation groups and given FSH for 4 days in the short FSH group (n = 5) and FSH starvation group, (n = 5) or for 7 days in the long FSH group (n = 4). In vivo oocyte maturation was induced with LH given 12 hours after the last FSH treatment in short and long FSH groups and 84 hours after the last FSH treatment in the FSH starvation group. The ovaries were removed by colpotomy 18 to 20 hours after LH treatment to aspirate cumulus-oocyte complexes (COCs) from follicles 8 mm or greater. Cumulus-oocyte complexes were graded morphologically, and oocytes were processed for either mitochondrial staining or for ATP assay. Collection efficiency was similar among treatment groups, but a greater proportion of COCs were expanded (P < 0.01) and oocyte ATP content of the expanded COC tended to be greater (P < 0.09) in the long FSH group than other two groups. Oocytes in the FSH starvation group had a greater proportion (P = 0.01) of mitochondrial clusters (i.e., fewer scattered individual mitochondria). Individual mitochondria and mitochondrial clusters in oocytes from the long FSH and FSH starvation groups had twice the relative staining intensity (P < 0.01) compared to oocytes from the short FSH group. In summary, the long FSH superstimulation protocol yielded a greater proportion of expanded COCs that had oocytes with a scattered mitochondrial population and a greater ATP content than other two protocols. FSH starvation of 84 hours yielded a high proportion of grade 4 COCs characterized by a greater proportion of mitochondrial clusters within the oocyte.