Reproduction in Domestic Animals

296 N Rawlings, ACO Evans, RK Chandolia and ET BaguFig. 1. Growth and developmentof some segments of the reproductivesystem of the bull calf frombirth to puberty based on Abdel-Raouf (1960), Macmillan and Hafs(1969), Curtis and Amann (1981),Sinowatz and Amselgruber (1986),Evans et al. (1996), Wrobel (2000)and Bagu et al. (2006a). Horizontallines indicate approximate periodsfor the activities shown; lines witharrow heads indicate start pointsfor continued activities(Fig. 1; Abdel-Raouf 1960; Macmillan and Hafs 1969).Rapid development of the vesicular glands occurs afterapproximately 2 months of age (Lindner and Mann1960; Macmillan and Hafs 1969; Chandolia et al. 1997c)in the bull. Content of the two principle secretoryproducts of the vesicular glands, fructose and citric acid,increase markedly in bulls after 20–24 weeks of age(Abdel-Raouf 1960; Macmillan and Hafs 1969) but inone report (Lindner and Mann 1960) this increase wasseen as early as 8 weeks of age.Endocrine Changes During Sexual Maturation;Alignment with Testicular DevelopmentSerum concentrations of LH are low in the first 4–5 weeks after birth but increase thereafter to a peak atapproximately 12–16 weeks of age and then concentrationsdecrease to approximately 25 weeks of age,remaining low but variable through the time of puberty(Fig. 2; Rawlings et al. 1978; Lacroix and Pelletier1979; McCarthy et al. 1979; Amann and Walker 1983;Rodriguez and Wise 1989; Rawlings and Evans 1995;Fig. 2. The temporal patterns of serum concentrations of reproductivehormones from birth to puberty in the bull calf based on Evans et al.(1996), Chandolia et al. (1997c), Aravindakshan et al. (2000a) andWrobel (2000). Horizontal lines indicate approximate periods for theactivities shownAravindakshan et al. 2000a; Bagu et al. 2006a). Thisearly, postnatal increase in LH secretion is caused by atransient increase in the frequency of peaks or episodesin LH secretion (Rawlings and Evans 1995). The earlypostnatal increase in LH pulse frequency is clearlydriven by an increase in the frequency of pulses ofGnRH secretion (Rodriguez and Wise 1989) and isaccompanied by an increase in pituitary LH stores(Rodriguez and Wise 1991), pituitary GnRH receptors(Amann et al. 1986; Rodriguez and Wise 1991) and LHrelease in vitro (McAndrews et al. 1994). The pattern ofserum concentrations of FSH during development in thebull is less clear ranging from no real trend (McCarthyet al. 1979; Amann and Walker 1983) to elevatedconcentrations postnatally, decreasing to 25 weeks ofage; FSH secretion in the bull does not appear to bepulsatile (Miyamoto et al. 1989; Evans et al. 1996; Baguet al. 2006a).Interestingly, serum concentrations of inhibin arehigh postnatally declining to 7 or 8 months of age(Miyamoto et al. 1989; Matsuzaki et al. 2001). Serumconcentrations of testosterone increase slowly after birthand up to approximately 20 weeks of age; subsequently,concentrations increase rapidly, starting anywhere fromapproximately 20–35 weeks of age (Secchiara et al.1976; Lacroix et al. 1977; Sundby and Velle 1980;Miyamoto et al. 1989; Rawlings and Evans 1995; Evanset al. 1996). During the early postnatal period, up to20 weeks of age, the testis also secretes significantamounts of androstenedione and dihydrotestosterone,albeit at lower levels than testosterone (Rawlings et al.1972; McCarthy et al. 1979; Sundby et al. 1984; Rawlingsand Cook 1986).The slow increase in testosterone secretion andproduction of androstenedione and dihydrotestosterone,seen prior to 20 weeks of age, occur during thephase of the rapid increase in number of adult Leydigcells in the testes and the early postnatal transientÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag