Reproduction in Domestic Animals

Puberty in the Bull 297increase in LH secretion. The later, rapid increase inserum concentrations of testosterone occurs after theearly rise in LH secretion and once the production ofadult Leydig cells is virtually complete, but importantly,over a period of time when gonadotropin secretion islow. Rapid growth of the testes and the androgendependentsexual accessory structures occurs after20 weeks of age during the rapid increase in testosteronesecretion. When LH and FSH receptor concentrationsper milligram of protein, in the testes, were studied every4 weeks from 5 to 33 weeks of age and again at56 weeks of age, the major finding was a transientdecrease in concentrations from 13 to 25 weeks of age(Bagu et al. 2006a). For these bulls (Hereford · Charolais)this transient decrease in gonadotropin receptorconcentrations occurred as serum gonadotropin concentrationsdeclined from their high postnatal levels andas numbers of adult Leydig and Sertoli cells increasedquite rapidly at the onset of rapid testicular growth.Proliferation of immature Sertoli cells occurs duringthe early postnatal period in the bull calf while serumFSH concentrations are high. Maturation of immatureto mature Sertoli cells starts around the time of the earlypostnatal rise in LH secretion and continues as gonadotropinsecretion declines. It is interesting that thephase of most active production of spermatogonia andforward progression of spermatogenesis, in the developingbull, occurs at the end of the early postnatalincrease in LH secretion and as serum concentrations ofFSH decline from their postnatal zenith. Again, theperiod of rapid testicular growth and development ofspermatogenesis, occur after 20–25 weeks of age whenserum concentrations of LH and FSH are lower than inthe early postnatal period. However, pulsatile secretionof LH is still important for testis growth in this period ofrapid testicular growth, as suppression of LH secretionwith oestradiol slows testicular growth, but treatmentwith LHRH restores it (Schanbacher 1981; Schanbacheret al. 1982).The Critical Role of the Early PostnatalIncrease in Gonadotropin SecretionWhen 48 Hereford · Charolais bull calves were splitinto two groups, based on age at puberty as defined byWolf et al. (1965), serum LH concentrations were higherduring the postnatal increase in LH secretion, in earlymaturing (42 weeks age) compared to late maturingbulls (48 weeks of age; Evans et al. 1995; Aravindakshanet al. 2000b). Serum FSH concentrations did notdiffer between groups. We were surprised to see subsequently,that late maturing bull calves had higher serumconcentrations of LH in response to LHRH at 4 and20 months of age (at the onset and end of the earlypostnatal increase in LH secretion) than early maturingbull calves (Bagu et al. 2006b). This was perhapsbecause early maturing bull calves experienced an earlierand greater increase in GnRH secretion postnatally, thatcaused depletion of LH stores in the early developingpituitary, giving a lower response to exogenous LHRH,compared to late maturing bull calves. Treatment ofHereford · Charolais bull calves with a GnRH agonistdesigned to lower LH secretion, from 6 to 18 weeks ofage (Chandolia et al. 1997a), delayed the peak in theearly postnatal rise in LH secretion from 20 (controlcalves) to 25 weeks of age. Serum concentrations ofFSH were lower in treated calves at 14, 16, 18 and26 weeks of age compared to control calves. Serumtestosterone concentrations were significantly suppressedat 14, 16 and 18 weeks of age. Scrotal circumferencewas smaller in treated calves from birth to50 weeks of age. At 50 weeks of age spermatid andspermatocyte numbers, but not Sertoli cell numbers,were lower in tubule cross sections for tubules in stageVI of spermatogenesis in treated compared to controlbulls. In another study, similar bull calves were givenLHRH intravenously, every 2 h for 14 days, between 4and 6 weeks of age (Chandolia et al. 1997b). Thistreatment increased the frequency of secretory pulsesof LH during the period of treatment and resulted in agreater scrotal circumference. Spermatogenesis wasenhanced and numbers of Sertoli cells were increasedbased on histology of testicular tubular cross sections instage VI of spermatogenesis, from testes collected at54 weeks of age. Serum concentrations of FSH were notaffected by treatment. In subsequent studies, in Hereford· Charolais bull calves, treatments were given lessfrequently; GnRH was given (i.m.) twice every day from4 to 8 weeks of age (Madgwick et al. 2007) and inseparate sets of bull calves, either bovine LH or bovineFSH were given every 2 days from 4 to 8 weeks of age(Bagu et al. 2004). Based on achievement of a scrotalcircumference of 28 cm (Wolf et al. 1965; Lunstra et al.1978) GnRH treatment advanced age at puberty(42 weeks of age) compared to control bulls (47 weeksof age). The FSH treatment also advanced puberty(39 weeks of age) compared to control bulls (45 weeksof age). Schuenemann et al. (2007) gave FSH at 27 daysof age, when bull calves were immunized against inhibinand saw increased germ cell numbers at 17 weeks of age.It appears that early postnatal secretion of FSH andparticularly LH, prior to 25 weeks of age, are critical toinitiate testicular differentiation and development in thebull calf, with the process continuing after this timeagainst a background of lower serum gonadotropinconcentrations, but increasing serum concentrations oftestosterone.Regulation of Gonadotropin Secretion DuringDevelopmentThe regulation of the relatively high levels of FSHsecretions seen postnatally in the bull calf and the early,transient, postnatal rise in LH secretion are critical asthey appear to be pivotal signals for the onset of sexualmaturation (Fig. 3). When bull calves were castrated atdifferent ages there were only limited increases in FSHsecretion but LH secretion rose markedly in calves24 weeks old and older (Amann and Walker 1983;Deaver and Peters 1988). In another study, castrationdid not result in an increase in LH secretion in bullcalves at 7, 8 or 9 weeks of age, but a response was seenat 10, 11, and 13 weeks of age (Wise et al. 1987).However, Bass et al. (1978) concluded that castrationresulted in increased LH secretion at 1, 7 and 17 weeksof age. Testosterone and dihydrotestosterone suppressÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag