Endocrine Changes Associated with Onset of Spermatogenesis in Holstein Bulls

Abstract

After birth, a bull enters a period of infancy during which the reproductive organs are relatively quiescent. This is followed by the prepubertal period, which starts at 10 to 12 wk in well-fed Holstein bulls, characterized by profound changes of hypothalamic, pituitary, and gonadal function that culminate in puberty. The prepubertal sequence of events probably is: a) initiation of spon- taneous discharge of luteinizing hormone; b) hormone induced differentiation of Leydig cells with increased secretion of androstenedione in response to luteini- zing hormone stimulation; c) further differentiation of Leydig ceils resulting in luteinizing hormone-stimulated secretion of testosterone; d) testosterone-induced differentiation of indifferent supporting cells to Sertoli cells concomitant with testosterone-induced differentiation of gonocytes to prespermatogonia and A- spermatogonia; e) increased sensitivity of the hypothalamus-anterior pituitary to negative feedback of gonadal steroids; f) diminished frequency and amplitude of luteinizing hormone discharge; g) forma- tion of junctional complexes between Sertoli cells and establishment of the bood-testis barrier; h) formation of pri- mary spermatocytes and ultimately spermatids and spermatozoa; and i) con- tinued increase of efficiency of sperma- togenesis until sufficient sperm are pro- duced to provide the first ejaculum around 37 to 38 wk. Following puberty,