Involvement of growth factors in thymic involution.

The thymus undergoes an age-dependent degenerative process which is mainly characterized by a progressive loss of lymphoid tissue. Thymic involution is particularly important in relation to immunosenescence and its various associated diseases; this fact has prompted many studies aimed at understanding the causes and mechanisms of thymic degeneration which may, ultimately, lead to the possibility of manipulating it. In this sense, one of the aspects which has deserved most attention is the thymic microenvironment, and more precisely, the many growth factors to which the cells present in the organ are exposed. Thus, the levels of several of such factors have been reported to undergo age-dependent changes in the thymus, which may point at an influence on the regression of the organ. In this article we consider which growth factors and growth factor receptors occur in the vertebrate thymus. Then, focusing on those whose influences are better documented, i.e., neurotrophins, cytokines and IGFs, we discuss their potential role in the organ and the possibility of their being involved in thymic involution.