Summary of Thymus Gland Biochemistry and Physiology

A summary of the major characteristics of the well-defined thymic factors is shown in Table 2. It should be appreciated from the wealth of data presented in this chapter that all of the traditional criteria required for categorizing the thymus as a true endocrine organ have now been satisfied. Nevertheless, the complex mechanisms by which the thymus controls the development and expression of immunity are still not well defined, nor is our full understanding of T-cell development, function, and regulation. 

Probably the two most perplexing and controversial issues in thymus endocrinology are the multiplicity of apparently unique thymic factors that have been described and the pleiotropic effects on precursor T cells, as well as on mature T cells, that each of the putative thymic hormones possess (cf. Stutman, 1983). Many of the biologic activities of thymic hormones can be mimicked by nonspecific agents such as endotoxin, prostaglandins, and cyclic AMP. In addition, it is not clear why multiple different thymic factors should exhibit identical effects in some of the biological assays. 

Agent Usual source Chemistry Molecular weight(s) Chemically synthesized Present in thymic epithelial cells Adapted to measure serum levels Decreases in serum 

Thymic humoral Calf polypeptide, 31-amino acid 3200 No — -  

Decrease demonstrated over the first 10 years of life, but not with further aging. primarily found in Hassall s corpuscles. 

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