Factors controlling growth and age involution of the rat thymus

Thymic development and somatic growth were investigated in three strains of rat. The well established influence of sex and age on somatic growth was confirmed. It was also found that the rate of thymic growth was not influenced by the sex of the animal until four weeks of age, but thereafter, the thymus grew faster in males compared with females of the same strain. Gonadectomy had a marked effect on the thymus, resulting in a higher rate of growth than in normal animals; but a slower initial rate of involution. The maximum thymus weights attained in gonadectomized animals were found to be independent of the age at which the animals were gonadectomized or of the sex of the animal. Gonadectomy did not influence the point at which involution began. Thymic mitotic indices of gonadectomized animals were found to be higher than those of normal animals. The influence of sex hormones on somatic growth and the development of the thymus is discussed, and it is concluded that cell loss is the regulator of thumus growth and involution.