Cell cycle in the aging thymus

The effect of aging on cell replication in the course of thymocyte development was analyzed by cell labeling with 5-bromo-2-deoxyuridine (BrdU) and subsequently staining with anti-BrdU antibodies and propidium iodide (PI). Cell cycle was measured on peanut agglutinin (PNA) separated, as well as unseparated thymocytes, from young (3 months) and old (24 months) C57BL/6J mice, following stimulation by concanavalin A (ConA). Splenocytes were examined in parallel, as a reference for the age-related decline in ConA-induced proliferation. Splenocytes of both age groups showed an increase in BrdU incorporation and the percent of cells entering S/G2/M, yet, the values recorded in the old were significantly lower than in the young, as expected. Whereas the PNA fractionated, and the unseparated thymocytes of the young incorporated BrdU within the first 24 hr of incubation, in the old these values were limited. Incorporation of BrdU increased during the subsequent 24 hr in the PNA^- cells of both age groups. Our study indicates that whereas the proportion of cycling thymocytes of the old is a priori lower than in the young, the mature (PNA^-) cells of the old are capable of entering cycle in response to activation, similarly to those of the young.