Ultrastructural study of thymic microenvironment involution in aging mice.

Aging involves morphological alterations of the thymus and deregulation of various immune response parameters. Altogether, these phenomena have been termed thymic involution. Using electron microscopy, we studied the morphological ultrastructure of the thymic microenvironment in aged mice. We observed cellular damages which progressively affected all the thymic stroma. At later stages (i.e., about 18-20 months old), a disappearance of the organ architecture with a drastic decrease in lymphocyte number was observed. The loss of cellular integrity of the microenvironment with lysis of cellular membranes and formation of a large and clear cytoplasmic layer engulfing a few remaining lymphocytes was noted. Extensive lipidic invasion surrounding the remaining epithelial cells grouped in nest formations and/or bordering cytics cavities was also present in these thymus from aged mice. Because the thymic microenvironment plays an important role in the "education" and functional maintenance of T cells and because the alteration of this cellular entity precedes a decline in certain immune functions, it can be suggested that membrane alterations, lack of cellular microenvironment integrity, and T cell dysfunction are correlated.