histogenesis of the lymphoid organs in the carp Cyprinus carpio L. and the ontogenetic development of allograft reactivity

A functional T cell population matures rapidly in the carp, Cyprinus carpio L. At 22 ± 1° C, the thymus first appears at two days post‐hatch and becomes actively lymphopoietic by day 5. The kidney, which contains haemopoietic tissue at day 2, shows differentiating lymphoid cells by day 6–8. The spleen first appears at day 5 but develops more slowly than the kidney. Lymphocytes are capable of effecting allograft rejection as early as day 16 post‐hatch but the number of cells invading a skin allograft is low if the graft is applied in the first month of life. Thereafter the numbers increase rapidly, possibly correlated with the large increase in lymphocytes in the pronephric and mesonephric kidneys at this time.