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Effect of Ageing on Hormone Secretion and Follicular Dynamics in Sheep with and without the Booroola Gene

Departamento de Reproduccion Animal, Instituto Nacional de Investigaciones Agrarias (INIA) (A.G.-B.), 28040 Madrid, Spain; Department of Obstetrics and Gynaecology, Centre for Reproductive Biology, University of Edinburgh Chancellor’s Building (A.G.-B., C.J.H.S., D.T.B.), Edinburgh, United Kingdom EH16 4SB; and the School of Human Development (B.K.C.), University of Nottingham, Queen Medical Centre, Nottingham NG7 2UH, United Kingdom

Address all correspondence and requests for reprints to: Antonio Gonzalez-Bulnes, Departamento de Reproduccion Animal, Instituto Nacional de Investigaciones Agrarias (INIA), Avenida Puerta de Hierro s/n, 28040 Madrid, Spain. E-mail: bulnes{at}inia.es.

It has been suggested that ewes carrying the Booroola gene (Fec^B) consistently ovulate more follicles because they recruit more primordial follicles and/or have a lower rate of atresia. If the former is correct, the pool of follicles would be depleted sooner in Fec^B animals. We have studied follicular dynamics and endocrine function during follicular and early luteal phases of the estrous cycle of older ewes with or without the fecundity gene and compared this data with data obtained 6 yr previously in the same animals. Older sheep carrying the Booroola gene maintained a significantly higher ovulation rate than noncarrier ewes [4.2 ± 0.8 vs. 2.2 ± 0.6 corpora lutea (CL), respectively; P < 0.05], and in keeping with data from young animals, both ovulatory follicles and CL (4.7 ± 0.3 vs. 6.9 ± 0.7 mm and 12.8 ± 0.5 vs. 16.7 ± 0.8 mm, respectively) were smaller than those of noncarrier ewes (P < 0.05). The interval from luteolysis to the onset of the LH surge increased with age in all the animals (from 52.0 ± 8.0 to 67.0 ± 7.5 h in gene carrier sheep and from 56.0 ± 2.0 to 79.5 ± 9.6 h in noncarrier sheep, P < 0.05). The concentration of estradiol and inhibin A in the early luteal phase was lower in older noncarrier ewes (P = 0.08 and P < 0.05, respectively), and the level of inhibin A was inversely related to the level of FSH in aged sheep of both genotypes (P < 0.0001). In contrast, the number of developing follicles in older ewes of both genotypes was similar to the number found in younger ewes, suggesting that increased ovulation rate in sheep carrying the Fec^B mutation is related to a reduced rate of atresia.