Thymic weight changes and endocrine relationships during maturation in cattle: effects of age, sex, and castration

Elucidation of cause and effect relationships between gonadal and thymic function can be particularly challenging due to the complexity of interactions among various hormones on thymic function, many of which are largely unknown. To monitor gonadal-thymic interaction with the endocrine changes associated with maturity, bulls, steers, heifers, and ovariectomized heifers were slaughtered at 4, 6, 10, 12, or 14 months of age (n = 5-7/sex/age). Steers and ovariectomized heifers were castrated before 1 week of age. Thymic weights were increased in castrated animals as early as 4 months of age (p < 0.01), and differences were more pronounced in males. Thymic secretory peptide, thymosin beta4, increased up to six months of age in all groups (p < 0.05) and then decreased with age, similar to the pattern of growth hormone. Prepuberal testosterone concentrations increased to 10-12 months of age and then decreased in both bulls and heifers (p < 0.05), coordinate with decreases of thymosin beta4 at 10 months of age. Growth hormone concentrations increased up to 6 months of age in all groups (p < 0.01) and then declined. Concentrations of prolactin peaked at 10 months of age (p < 0.01) but were not different between groups. Relationships of luteinizing hormone (LH) were highly related to thymosin beta4 changes in heifers during maturation. Changes of thymosin beta4 and LH were coordinate in ovary-intact heifers and inversely related in ovariectomized heifers. Gonadal effects on thymic weight were evident as early as 4 months of age, and no abrupt declines in thymic weight were detected at puberty.