Differences in follicular function of 3-month-old calves and mature cows

After in vitro maturation, fertilization and development, the percentage of fertilized eggs developing to the blastocyst stage is usually lower in cal ves compared with cows. It is unknown whether this low ability to develop i n vitro is inherent to calf oocytes or is caused by altered follicular matu ration. The latter possibility was explored in the present study using two markers of follicle function: in vitro steroidogenesis by intact follicles and aromatase activity of follicular walls. Calf follicles > 9 mm in diamet er had a low ability to produce oestradiol (ten times reduction compared wi th cows) despite a testosterone output by theca cells which was similar to that observed in cows. This finding is in agreement with the low aromatase activity of granulosa cells of calf follicles measured by tritiated water r elease assay. Qualitative and quantitative differences between calf and cow follicular fluids were assessed using western blotting (inhibin and activi n, heat shock protein 90, Mullerian inhibiting substance) and assays (inhib in and activin) to determine whether this defective aromatase could be prod uced by alterations in the amounts of follicular proteins modulating aromat ase (inhibin and activin, heat shock protein 90, Mullerian inhibiting subst ance). Western blotting of Collicular fluid proteins demonstrated three mai n bands (59, 57 and < 30 kDa) and one minor band (34 kDa) with the anti-alp ha inhibin antibody, whereas a single 18 kDa band was detected when an anti -beta inhibin antibody was used. Calf follicular fluid contained similar am ounts of all main inhibin forms (alpha and beta) but a 34 kDa alpha inhibin form was missing. The amounts of dimeric inhibin were similar between cows and calves but small follicles from calves contained more activin. Single bands at 70 kDa (Mullerian inhibiting substance) and 90 kDa (heat shock pro tein 90) were detected by western blotting. Mullerian inhibiting substance was missing from calf follicular fluid and heat shock protein 90 was presen t in smaller amounts in calf versus cow follicular fluid. None of the above differences could explain the defective aromatase of calf follicles. Two-d imensional separation of the [S-35]-labelled proteins secreted by follicula r walls originating from calf or cow follicles matched for size and follicl e health was performed and 151 spots were observed on the master gel, which summarized all the spots present at least once. Fifteen spots were present in calves and not in cows. Quantitative differences were also detected wit h three spots containing more proteins in cows than in calves. Whether some of these proteins can alter maturation of follicles or oocytes requires fu rther investigation.