Testicular vascular cone development and its association with scrotal temperature, semen quality, and sperm production in beef bulls.

The objectives of this study were to characterize development of the testicular vascular cone using ultrasonography and to determine associations of vascular cone morphology with scrotal temperature, semen quality, and sperm production. Beef bulls (n=70) were examined from 10 to 70 wk of age in two years, and a third group of bulls (n=44) was examined only at 74 wk of age. Testicular vascular cone diameter increased until approximately 13.5 mo of age, or until 1 to 8 wk before maximum scrotal circumference was observed. Vascular cone fat thickness also increased with age and followed a pattern similar to that observed for backfat. Testicular artery wall thickness and the distance from the arterial to the venous blood in the vascular cone decreased with proximity to the testis. Vascular cone diameter was negatively correlated with scrotal surface temperatures and with the percentage of sperm head defects and detached sperm heads, but positively correlated with the percentage of normal sperm. The arterial-venous blood distance was negatively correlated with the percentage of normal sperm and positively correlated with percentage of sperm head defects and proximal droplets. In conclusion, testicular vascular cone diameter increased with age following testicular development, whereas vascular cone fat thickness increased similar to a pattern observed for backfat. Increased testicular vascular cone diameter and decreased distance between arterial and venous blood were associated with increased percentage of normal sperm and decreased percentages of sperm defects.

Effect of nutrition during calfhood and peripubertal period on serum metabolic hormones, gonadotropins and testosterone concentrations, and on sexual development in bulls.

The objective of the present study was to characterize the effects of nutrition on circulating concentrations of metabolic hormones, gonadotropins, and testosterone during sexual development in bulls. Nutrition regulated the hypothalamus-pituitary-testes axis through effects on the GnRH pulse generator in the hypothalamus and through direct effects on the testes. Pituitary function (gonadotropin secretion after GnRH challenge) was not affected by nutrition. However, nutrition affected LH pulse frequency and basal LH concentration during the early gonadotropin rise (10-26 weeks of age). There were close temporal associations between changes in insulin-like growth factor-I (IGF-I) concentrations and changes in LH pulse frequency, suggesting a role for IGF-I in regulating the early gonadotropin rise in bulls. The peripubertal increase in testosterone concentration was delayed in bulls with lesser serum IGF-I concentrations (low nutrition), suggesting a role for IGF-I in regulating Leydig cell function. Serum IGF-I concentrations accounted for 72 and 67% of the variation in scrotal circumference and paired-testes volume, respectively (at any given age), indicating that IGF-I may regulate testicular growth. Bulls with a more sustained elevated LH pulse frequency during the early gonadotropin rise (high nutrition) had greater testicular mass at 70 weeks of age relative to the control group (medium nutrition), despite no differences in metabolic hormone concentrations after 26 weeks of age. Therefore, gonadotropin-independent mechanism regulating testicular growth might be dependent on previous gonadotropin milieu.

Effect of improved nutrition during calfhood on serum metabolic hormones, gonadotropins, and testosterone concentrations, and on testicular development in bulls.

The objective of the present study was to evaluate the effects of improved nutrition during calfhood on serum metabolic hormones, gonadotropins and testosterone concentrations, and on sexual development in bulls. Bulls received high (n=17) or control nutrition (n=16) diets from 10 to 30 week of age and the same control nutrition diet from 31 to 74 week of age. Improved nutrition during calfhood resulted in a more sustained period of elevated LH secretion (pulse frequency and total secretion in 10h) during the early gonadotropin rise. GnRH-stimulated LH secretion was not affected by diet, indicating that pituitary responsiveness was not altered; therefore, improved nutrition had direct effects on GnRH secretion by the hypothalamus. Insulin and insulin-like growth factor-I (IGF-I) concentrations were greater during calfhood in bulls receiving high nutrition, indicating that these metabolic hormones might be involved in regulating GnRH and LH secretion. Improved nutrition also resulted in increased testosterone secretion that was associated with greater circulating IGF-I concentrations, suggesting a role for this metabolic hormone in regulating Leydig cell number and function. Furthermore, improved nutrition during calfhood resulted in greater testicular weight and sperm production in mature bulls, indicating that increased LH secretion during calfhood, and increased IGF-I and testosterone concentrations during calfhood and peripubertal period were associated with greater testicular cell proliferation and enhanced function.