The relationship between prepuberal hormone levels and sertoli cell number of spermatid production in the beef bull

Producers of beef cattle must become more efficient and the bull is often the component in beef cattle production that either limits or enhances efficiency. Testicular size is positively correlated with sperm producing ability in the bull. Factors affecting testicular size are, consequently, potential indicators of sperm producing capacity. To determine whether prepuberal hormonal levels and Sertoli cell numbers were related to postpuberal testicular size, groups of crossbred bulls were injected with 10 ug of GnRH ethyamide when either 60 (n=8), 90 (n=14), 120 (n=18), or 180 (n=14) days of age. Jugular blood was collected at 0, 1, 2, and 4 hours after GnRH injection and plasma was assayed for gonadotrophins, testosterone and estradiol. Animals were then unilaterally castrated. The testis was weighed and measured. Sections (5 urn) were evaluated for the number of Sertoli cells per testis. A second challenge was administered when the bulls were approximately 14 months of age; blood was collected and assayed as before. The second testis was removed, weighed and measured. Sections (5 um) were evaluated for percentage of tubular areas occupied by spermatids. Prepuberal FSH, testosterone and estradiol did not vary significantly with age (p >0.1) nor were they correlated with Sertoli cell number (p >0.1). Basal LH levels differed significantly with age (p >0.01). Basal LH levels were correlated (r = -.57) with Sertoli cell number. Basal prepuberal levels of LH were negatively correlated (r = -.31) and basal testosterone levels were positively correlated (r = .51) with the number of cross sections containing spermatids. Basal prepuberal LH levels were not significantly different between high and low testicular weight (p >0.1). Sertoli cell number was higher in animals with greater testicular weight (p <0.1). Sertoli cell number was not correlated with spermatid production. These results were probably due to the fact that approximately one-half of the animals in the study were still in the prepuberal state at 14 months, as determined by histological examination. The response of hormonal levels to GnRH stimulation was not a useful predictor of spermatid production of testicular weight, as stimulated levels were correlated with neither. It is concluded that prepuberal LH and testosterone Were correlated with degree of testicular development but not with testicular weight, another criterion of testicular development. Studies of bulls of more advanced age are required to determine if prepuberal LH and testosterone levels are potentially useful as indicators of full testicular function.