Acid phosphatase activity and cell death in mouse thymus

The distribution of acid phosphatase activity in the thymus of young (8 week) and old (42 week) mice is presented. In 8 week old mice acid phosphatase positive cells represent 1.27 +/- 0.13% of the total population whereas in 42 week old mice, showing involution of the thymus, acid phosphatase positive cells represent 2.40 +/- 0.17% of the total population. Loci of free acid phosphatase activity have been interpreted as sites of cell lysis and death. This has been confirmed at electron microscope level where free acid phosphatase has been demonstrated in the cytoplasm of lysing thymic lymphocytes. Vacuolar sites of acid phosphatase activity have been demonstrated in macrophages which appear to dispose of the lymphocytes. Extensive autophagic activity occurs in the epithelial reticular cells. The role of acid phosphatase in thymic lymphocyte deletion and in the tissue dynamics of the thymus is discussed.