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Age-related changes in peripheral hormone concentrations and their relationships with testis size and number of Sertoli and germ cells in yearling beef bulls

A study was conducted to determine whether basal or GnRH-stimulated concentrations of FSH, LH, testosterone, androstenedione and oestradiol measured from 2 months to 12 months of age were correlated with size and histology of the testes of Angus bulls at 12 months of age. Three blood samples per month were taken from 24 calves at 1.5 h intervals to establish basal hormone concentrations. On the following day, calves received 0.05 µg of GnRH kg^–1 body mass and were bled 1.5 h and 3.0 h later. At 12 months of age, bulls were castrated and testes were measured and prepared for histological analysis. Yearling testis diameter was related to GnRH-stimulated testosterone at 3 months (r = 0.48, P < 0.05). Basal FSH at all ages between 2 months and 12 months was correlated with yearling testis diameter (r = –0.53, P < 0.05, to r= –0.74, P < 0.01). In addition, basal FSH, measured at 2 and 3 months and between 8 and 12 months, was related to number of Sertoli cells per testis (r= –0.47, P < 0.05, to r= –0.53, P < 0.05), number of round spermatids per A1 spermatogonium (r = –0.48, P < 0.05, to r = –0.60, P < 0.01) and number of round spermatids per Sertoli cell (r=–0.50, P < 0.05, to r=–0.64, P < 0.01). Similarly, prepubertal concentrations of GnRH-stimulated FSH between 2 months and 12 months of age were correlated with testis diameter (r = non-significant, to r= –0.71, P < 0.01) and cell ratios in the seminiferous tubules (r = non-significant, to r= –0.61, P < 0.01). Thus, prepubertal hormone secretion could serve as a meaningful index of the potential sperm-producing capacity of the yearling bull.