Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 295–301 (2008); doi: 10.1111/j.1439-0531.2008.01177.xISSN 0936-6768Sexual Maturation in the BullN Rawlings 1 , ACO Evans 2 , RK Chandolia 3 and ET Bagu 41 Department of Veterinary Biomedical Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; 2 School of Agriculture FoodSciences and Veterinary Medicine, University College Dublin, Dublin, Ireland; 3 Institute fu¨r Reproducktions Medizin, Universitat Mu¨nster, Mu¨nster,Germany; 4 Centre de recherche´ en biologie de la Reproduction, Laurier, Quebec, CanadaContentsIn this review, we describe the process of sexual maturationin the bull calf. The testes of the bull grow relatively slowlyuntil approximately 25 weeks of age and then a rapid phaseof growth occurs until puberty, at 37–50 weeks of age.During the early postnatal phase of slower growth of thetestis pre-spermatogonia and some spermatogonia are established,adult Leydig cells appear and undifferentiated Sertolicells are produced. The rapid testicular growth, after25 weeks of age, consists of marked increases in thediameter and length of the seminiferous tubules, dramaticproliferation and differentiation of germ cells, with maturespermatozoa occurring between 32 and 40 weeks of age.The adult Leydig cell population is largely in place by30 weeks of age and that of Sertoli cells by 30–40 weeksof age. Serum concentrations of LH increase from 4 to5 weeks of age, to an early postnatal peak at 12–16 weeksof age, followed by a decline to 25 weeks of age.Serum FSH concentrations are high postnatally, decliningto approximately 25 weeks of age. Serum testosteroneconcentrations increase during the phase of rapid testiculargrowth. Hypothalamic opioidergic inhibition may abatetransiently to allow the early postnatal increase in LHsecretion, while testicular androgenic negative feedbackprobably contributes to the decline in gonadotropinsecretion to 25 weeks of age. Several lines of study haveled us to suggest that early postnatal gonadotropin secretionis pivotal in initiating the process of sexual maturation inthe bull calf.IntroductionA fairly widely accepted end point for pubertal developmentin the bull was defined by Wolf et al. (1965) asan ejaculate that contained at least 50 million spermwith no less than 10% progressive motility. In bullsfrom European breeds puberty is reached over a rangeof approximately 37–50 weeks of age, with dairy breedsmaturing somewhat earlier than beef breeds (Lunstraet al. 1978; Amann 1983; Evans et al. 1995). Althoughscrotal circumference at puberty, as defined by Wolfet al. (1965), varies somewhat between breeds, a scrotalcircumference of 28 cm is often used as the point of, orage at puberty (Wolf et al. 1965; Lunstra et al. 1978).However, sperm output and quality increase for sometime after the age of puberty (Almquist and Cunningham1967; Lunstra and Echternkamp 1982). Thispresent review will focus on development of the reproductivesystem in the bull, with particular emphasis onregulatory mechanisms. As much as possible we havetried to describe trends and mechanisms for bulls ingeneral, regardless of breed, season of birth andnutrition.Testicular Growth and Development of theReproductive TractTesticular growth follows a sigmoid pattern in the bullcalf, with more rapid growth after approximately25 weeks of age through puberty, slowing as the bullachieves adult sperm output (Fig. 1; Abdel-Raouf 1960;Macmillan and Hafs 1968; Amann 1983). Seminiferoustubule diameter increases gradually until 20–25 weeks ofage and then more rapidly (Abdel-Raouf 1960; Macmillanand Hafs 1969; Amann 1983; Evans et al. 1996).Increased diameter and length of the tubules accountsfor most of the increase in testis size up to 32 weeks ofage, but tubule length predominates during later stagesof development (Curtis and Amann 1981). Luminationof the tubules occurs at approximately 25 weeks of age(Evans et al. 1996). Fetal Leydig cells degenerate byapproximately 8 weeks after birth and adult Leydig cellnumbers increase rapidly from 4 to 30 weeks of age(Wrobel 1990). Undifferentiated Sertoli cells proliferatefrom 4 weeks of age up until 13–20 weeks of age andthen the number decreases as these cells differentiateinto mature, adult Sertoli cells; differentiation of matureSertoli cells appears to be complete somewhere between30 and 40 weeks of age (Abdel-Raouf 1960; Curtis andAmann 1981; Sinowatz and Amselgruber 1986; Wrobel2000; Bagu et al. 2006a). The number of Sertoli cells is acritical determinant of daily sperm production in thebull (Berndtson et al. 1987). At birth the solid seminiferoustubules or chords contain the primordial germcells or gonocytes; these largely disappear by 30 weeksof age (Curtis and Amann 1981; Wrobel 2000; Baguet al. 2006a). Proliferation of pre-spermatogonia andsome spermatogonia occurs from 4 to 5 weeks of ageonwards (Abdel-Raouf 1960; Curtis and Amann 1981;Wrobel 1990; Evans et al. 1996; Bagu et al. 2006a). Prespermatogonialcell numbers decrease after 24 weeksof age (Curtis and Amann 1981), but numbers ofspermatogonia increase rapidly until 44 weeks of age(Abdel-Raouf 1960). Primary spermatocytes appear atapproximately 20 weeks of age, secondary spermatocytesat 20–30 weeks of age, round spermatids between25 and 30 weeks of age, long spermatids at 25–35 weeksof age and finally mature spermatozoa are clearly seenbetween 32 and 40 weeks of age (Abdel-Raouf 1960;Macmillan and Hafs 1968; Curtis and Amann 1981;Evans et al. 1996; Bagu et al. 2006a). It is interestingthat rapid testicular growth, differentiation and development,occur after 20–25 weeks of age.As with testicular development, epididymal growth isslow initially, but increases rapidly after 28 weeks of ageÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag