Introduction The ovaries of calves contain large numbers of growing and large follicles (Erickson, 1966), the largest of which have a wave-like pattern of growth (Adams et al., 1994; Evans et al., 1994). The use of oocytes contained in some of these follicles could increase the rate of genetic gain in livestock breeding programmes through a reduction of the generation interval (Betteridge et al., 1989; Georges and Massey, 1991). Indeed, these oocytes have the potential to resume meiosis, reach metaphase II and be fertilized (Armstrong et al., 1992, 1994; Duby et al., 1995; Revel et al., 1995). However, after development in vitro, the percentage of fertilized eggs developing to the blastocyst stage is usually lower in calves compared with cows (Kajihara et al., 1991; Levesque and Sirard, 1994; Duby et al., 1995, 1996; Revel et al., 1995), although a few studies did not confirm this finding (Armstrong et al., 1992; Irvin et al., 1993). Treat- ment with exogenous gonadotrophins does not improve the ability of fertilized eggs of calves to develop to the blastocyst stage (Revel et al., 1995). In contrast, ageing of calves was associated with improved rates of development (Pressice et al., 1997). The low ability of calf oocytes to develop may be a consequence of inherent defects in oocyte development or function (Khatir et al., 1997). Alternatively, an altered follicular maturation, which would decrease the ability of the oocyte to develop, is suggested by the reduced cytoplasmic maturation of oocytes matured in the presence of calf follicular fluid (Khatir et al., 1997). The aim of the Differences in follicular function of 3-month-old calves and mature cows M. A. Driancourt 1 *, K. Reynaud 1 and J. Smitz 2 1 INRA-URA CNRS 1291, PRMD, 37380 Monnaie, France; and 2 AZ-VUB, Centre for Reproductive Medicine, Vrije Universiteit Brussel, Laarbeeklaan 101, B-1090 Brussels, Belgium Reproduction (2001) 121, 463–474 Research After in vitro maturation, fertilization and development, the percentage of fertilized eggs developing to the blasto- cyst stage is usually lower in calves compared with cows. It is unknown whether this low ability to develop in vitro is inherent to calf oocytes or is caused by altered follicular maturation. 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 diameter had a low ability to produce oestradiol (ten times reduction compared with 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 release assay. Qualitative and quantitative differences between calf and cow follicular fluids were assessed using western blotting (inhibin and activin, heat shock protein 90, Müllerian inhibiting substance) and assays (inhibin and activin) to determine whether this defective aromatase could be produced by alterations in the amounts of follicular proteins modulating aromatase (inhibin and activin, heat shock protein 90, Müllerian inhibiting substance). Western blotting of follicular fluid proteins demonstrated three main bands (59, 57 and < 30 kDa) and one minor band (34 kDa) with the anti-α inhibin antibody, whereas a single 18 kDa band was detected when an anti-β inhibin antibody was used. Calf follicular fluid contained similar amounts of all main inhibin forms (α and β) but a 34 kDa α 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 (Müllerian inhibiting substance) and 90 kDa (heat shock protein 90) were detected by western blotting. Müllerian inhibiting substance was missing from calf follicular fluid and heat shock protein 90 was present in smaller amounts in calf versus cow follicular fluid. None of the above differences could explain the defective aromatase of calf follicles. Two-dimensional separation of the [ 35 S]-labelled proteins secreted by follicular walls originating from calf or cow follicles matched for size and follicle 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 with three spots containing more proteins in cows than in calves. Whether some of these proteins can alter maturation of follicles or oocytes requires further investigation. © 2001 Journals of Reproduction and Fertility 1470-1626/2001 *Present address: Intervet Pharma R&D, BP 67131, 49071 Beaucouzé, France Email: marc-antoine.driancourt@intervet.com Downloaded from Bioscientifica.com at 02/18/2023 03:47:25AM via free access