V 2–7D1 D2 J2 C V˜4J˜1 C˜1 Stem cells in adult Starting during late fetal development ° ° ° ° ° V 2–7D1 D2 J2 C° ° ° ° ° V˜7J˜ C˜ ˜:° cells home to lymph nodes and spleen and are programmed to secrete IFN-˜ ˜:° cells home to intestinal epithelium and are programmed to secrete IFN-˜ 107 106 105 104 103 15 16 17 18 19 1 2 V1 D2 J2 C V6 J1 C1 C1 Numbers of thymocytes skin epidermis uterus skin, dermis lung spleen, lymph nodes liver intestinal epithelium Days of gestation Birth Weeks of age V1 D2 J2 C V 5 J1 V 2–7D1 D2 J2 C V˜4J˜1 C˜1 Stem cells predominant in fetus and newborn, rare in adult Starting day 17 of development Stem cells in fetus Days 14–18 of development Stem cells in fetus Days 16–19 of development V˜5 V˜6 V˜4 and V˜1 V˜1,2,4 and V˜7 ° ° ° ° ˜ ˜ ˜ ˜ ˜:° thymocytes ° ° ° °˜ ˜ ° ° ° ° ° V°6D 1D 2J 2 C V˜1J˜ C˜ ° ° ° ° ˜:° T cells become established in skin epidermis and are programmed to secrete keratinocyte growth factor, and infammatory cytokines and chemokines ˜:° T cells become established in mucosal epithelia, including reproductive tract, lung, and skin dermis, and are programmed to secrete IL-17 ˜:° cells home to lymph nodes, spleen, lung, and skin dermis and are programmed to secrete IL-17 ˜:° cells home to lymph nodes, spleen, lung, and liver and are programmed to secrete IL-4 and IFN-˜