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Mammalian thymic histogenesis can be morphologically divided into three consecutive stages: a) epithelial, b) lymphopoietic or lympho-epithelial, and 3) differentiated cellular microenvironmental, with formation of Hassall's bodies (HBs). Immunomorphological changes characteristic of human thymic involution begin during or soon after the first year after birth, and continue progressively throughout the entire life span. The 3% to 5% annual reduction in the number of cells of the human thymic microenvironment continues until middle age, when it slows down to less than 1% per year. According to the extrapolation of these results, total loss of thymic reticulo-epithelial (RE) tissue and the associated thymocytes should occur at the age of 120 years in humans. The marked reduction of the thymic cellular microenvironment is a well- controlled physiological process and is presumably under both local and global regulation by the cells of the RE meshwork and by the neuroendocrine axis, respectively. In humans, the age related decline of facteur thymique serique (FTS) levels in blood begins after 20 years of age and FTS completely disappears between the 5th and 6th decade of life. In contrast, serum levels of thymosin-alpha 1 and thymopoietin seem to decline earlier, starting as early as 10 years of age. The influences of a variety of other hormones on the involution of the thymus have also been characterized: testosterone, estrogen, and hydrocortisone treatment results in marked involution, cortisone and progesterone administration have a slight to moderate effect while use of desoxycorticosterone has no effect. The experimental administration of thyroxin yielded dose dependent results: low doses resulted in thymic hypertrophy, higher doses produced a slight hypertrophy, while the highest employed doses caused thymic atrophy. The atrophy was of apicnotic type, very different from that detected after treatment with corticoid hormones. Thymus transplantation experiments indicate that age-related, physiological thymic involution has been genetically preprogrammed. Grafting of the thymus from one week old C3H leukemic strain mice into 6 month old hosts resulted in changes in thymic weight and involution patterns that were synchronous in all recipients, in direct correlation with the glands in the donor, but not in the host. These data strongly suggest that the stimulus for thymus cell proliferation and differentiation is genetically determined within the organ implant. Since the thymus is the primary T-lymphopoietic organ during mammalian ontogenesis, its age-related involution with typical immunomorphological alterations can be held responsible only for the decline in antigen-specific T lymphocyte immune functions. Thymic involution and diminished T lymphocyte proliferation can be partially restored by thymic tissue transplantation or use of thymic hormones. The only partial reconstitution of CD4+ T helper lymphocyte subset after antineoplastic chemotherapy and bone marrow transplantation represents a significant, therapy complicating, clinical problem. After high-dose chemotherapy, restoration of thymus dependent CD4+ T lymphocyte genesis was reported only in children. Our radiation, stem cell transplantation, and hormone treatment experiments in animals strongly suggest age and time dependent regeneration of the cytoarchitecture of the thymic cellular microenvironment, as well as intrathymic lymphopoiesis. The human body's zinc pool undergoes progressive reduction, resulting in low zinc plasma levels and a negative crude zinc balance in older rodents, as well as humans. Previous research suggests that the diminished bioavailability of zinc in older mammals may represent one of the major factors for the involution of the thymus and consequent cellular immunological dysfunction. In PBMCs, zinc induces several cytokines, predominantly IL-1, IL-6 and TNF-alpha, and therefore, has an immense immunoregulative capacity. (ABSTRACT TRUNCATED)