In this review, we describe the process of sexual maturation in the bull calf. The testes of the bull grow relatively slowly until approximately 25 weeks of age and then a rapid phase of growth occurs until puberty, at 37-50 weeks of age. During the early postnatal phase of slower growth of the testis pre-spermatogonia and some spermatogonia are established, adult Leydig cells appear and undifferentiated Sertoli cells are produced. The rapid testicular growth, after 25 weeks of age, consists of marked increases in the diameter and length of the seminiferous tubules, dramatic proliferation and differentiation of germ cells, with mature spermatozoa occurring between 32 and 40 weeks of age. The adult Leydig cell population is largely in place by 30 weeks of age and that of Sertoli cells by 30-40 weeks of age. Serum concentrations of LH increase from 4 to 5 weeks of age, to an early postnatal peak at 12-16 weeks of age, followed by a decline to 25 weeks of age. Serum FSH concentrations are high postnatally, declining to approximately 25 weeks of age. Serum testosterone concentrations increase during the phase of rapid testicular growth. Hypothalamic opioidergic inhibition may abate transiently to allow the early postnatal increase in LH secretion, while testicular androgenic negative feedback probably contributes to the decline in gonadotropin secretion to 25 weeks of age. Several lines of study have led us to suggest that early postnatal gonadotropin secretion is pivotal in initiating the process of sexual maturation in the bull calf.

In the ewe, ovarian follicular waves emerge every 4 to 5 days and are preceded by a peak in FSH secretion. It is unclear whether large antral follicle(s) in a wave suppress the growth of other smaller follicles during the inter-wave interval, as is seen in cattle. In this study, anestrous (n = 6; experiment 1) and cyclic (n = 5; experiment 2) Western white face ewes were given ovine FSH (oFSH) (0.5 microg/kg; two s.c. injections, 8 h apart) during the growth phase (based on ultrasonography) of a follicular wave (wave 1). Control ewes (n = 5 and 6, respectively) received vehicle. In oFSH-treated ewes, serum FSH concentrations reached a peak (P < 0.05) by 12 h after oFSH treatment, and this induced FSH peak did not differ (P > 0.05) from the endogenous FSH peaks. In all ewes, emergence of follicular waves 1 and 2 was seen (P > 0.05). However, in oFSH-treated ewes, an additional follicular wave emerged approximately 0.5 days after treatment: during the interwave interval of waves 1 and 2 without delaying the emergence of wave 2. The growth characteristics and serum estradiol concentrations did not differ (P > 0.05) between oFSH-induced waves and waves induced by endogenous FSH peaks. We concluded that, unlike in cattle, the largest follicle of a wave in sheep has limited direct effect on the growth of other follicles induced by exogenous oFSH. In addition, the largest follicle of a wave may possibly not influence the rhythmicity of follicular wave emergence, as it does in cattle.

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