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5 Endocrine Control of Testicular Development and Initiation of Spermatogenesis in Bulls
Leonardo F.C. Brito
STgenetics, Middleton, WI, USA
Introduction
The process of testicular development that leads to initiation of spermatogenesis in bulls involves complex maturation mechanisms of the hypothalamus–pituitary–testes axis. Sexual development can be divided into three periods according to changes in gonadotropin and testosterone concentrations, namely the infantile, prepubertal, and pubertal periods. These changes are accompanied by changes in testicular cell proliferation and differentiation (Figure 5.1).
Figure 5.1 Mean serum luteinizing hormone (LH), follicle‐stimulating hormone (FSH), and testosterone concentrations in bulls during the infantile (a), prepubertal (b), and pubertal/postpubertal (c) periods. Data from 2 to 6 weeks are adapted from Hereford × Charolais bulls [1]. Data from 10 to 70 weeks are from Angus and Angus × Charolais bulls receiving adequate nutrition [2–5]. Infancy is the period that extends from birth until approximately eight weeks of age. During this period gonadotropin and testosterone concentrations are low, the testicular parenchyma is occupied mostly by interstitial tissue, seminiferous cords are lined by undifferentiated Sertoli cells, and centrally located gonocytes with large nuclei can be observed. The prepubertal period is characterized by a dramatic increase in gonadotropin concentrations (the early gonadotropin rise) and by slowly increasing testosterone secretion. The prepubertal period extends from approximately 10 to 26 weeks of age and during this period a cord lumen begins to develop and gonocytes migrate toward the basement membrane, differentiating into spermatogonia. Gonadotropin concentrations decrease concomitantly with a rapid increase in testosterone concentration during the pubertal period, which also coincides with the start of a phase of rapid testicular growth. Formation of the tubular lumen is evidence of Sertoli cell differentiation and development of a functional blood–testis barrier that precedes the appearance of primary spermatocytes and spermatids around 32 weeks of age. With continuous increase in diameter, seminiferous tubules occupy most of the testicular parenchyma and start to produce mature sperm at approximately 40 weeks of age.
Sources: Data from [2,4–6].
Infantile Period
The infantile period is characterized by low gonadotropin and testosterone secretion and relatively few changes in testicular cellular composition. This period extends from birth until approximately two months of age in Bos taurus bulls.
Gonadotropin secretion during the infantile period is low