AGE‐RELATED CHANGES OF THYMUS – MORPHOLOGICAL and FUNCTIONAL ASPECTS –

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Abstract
The thymus involutes progressively throughout life, beginning around sexual maturation. In long-lived BC3F1 hybrid mice, the thymic capacity to induce T[thymus-derived] cell differentiation began to decline earlier than the onset of thymic involution, although the magnitude of the decline was different by the subpopulation of T cells. Morphologically, the most active secretory structure seems to be limited exclusively to the neonatal thymus and certain structural changes, reflective of a decline in secretory function, can be detected early in life and become more pronounced with age. Heterogeneity of thymic epithelial cells was suggested by the facts that age-related and radiation induced decline of immune activities were different in degree by subpopulation of T cells, and the concept was also supported from a morphological viewpoint. Age-related thymic involution results in decrease of recruitment of fresh capable T cells and increase of old exhausted T cells, eventually bringing about T cell insufficiency in the aged individuals. Such an impaired immune function in the aged mice can be effectively restored by the combined grafting of young bone marrow and newborn thymus, and the thymus is apparently the most limiting factor in the aged. The biological significance of age-related thymic involution was also discussed.