Differential effect of cortisol on thymocytes from mice of different ages

The influence of cortisol in vivo and in vitro was studied in thymus sections, suspensions, and smears obtained from new-born, 6 days old, and 30 days old mice. Histology as well as differential counts revealed that new-born thymocytes are far more sensitive to the cytotoxic effect of cortisol than thymocytes from older animals, and thymocytes from 30 days old mice are more sensitive than cells from 6 days old animals. Thus, the number of pyknotic cells in the thymus of new-born and 30 days old mice increased up to 20 per cent 6 h after injection of cortisol compared to 10 per cent in 6 days old animals. At 24 h after injection, the per cent of pyknotic cells in the thymus of new-born mice was 50 compared with about 1 in the two older age-groups. Supravital dye exclusion showed that thymocytes in 6 days old mice are less sensitive to the cytolytic action of cortisol in vivo than thymocytes from new-born and 30 days old mice. Incorporation of 3H-TdR and 3H-UdR into thymocyte DNA and RNA, respectively, was inhibited in all groups after treatment in vivo. DNA-synthesis of thymocytes in new-born animals was inhibited in all groups after treatment in vivo. DNA-synthesis of thymocytes in new born animals was inhibited more than DNA-synthesis in thymocytes of older animals, whereas RNA-synthesis was inhibited to nearly the same extend in the three groups studied. Incubation in vitro with 10(-6)M cortisol inhibited the DNA-synthesis to the same extent in thymocytes from the three age groups studied whereas the inhibiting effect of 10(-5)M cortisol on thymocytes from 6 and 30 days old animals was more intense than that on cells in new-born mice. The RNA-synthesis of thymocytes from 6 days old mice incubated in 10(-5)M cortisol was inhibited far less than RNA-synthesis in cells from new-born and 30 days old mice. The experiments reported, showing differential effects of cortisol on thymocytes obtained from mice of different ages, may reflect different stages of thymocytes maturation during the process of ontogenesis.