Ontogeny of the immune system inSebastiscus marmoratus: Histogenesis of the lymphoid organs and effects of thymectomy

Synopsis The ontogenetic development of the immune system in a marine teleostSebastiscus marmoratus was studied by histological examination and removal of the thymus. The pronephros and the spleen had been differentiated at the time of birth and contained small numbers of haemopoietic cells. In contrast to most vertebrates, the rudiments of the thymus were first visible 1 week post-birth in the dorsoposterior part of the pharynx, the same location as in the adults. However, small lymphocytes first appeared in the thymus of fish at 3 weeks of age, followed by the pronephros at 4 weeks and the spleen at 6 weeks. Complete or partial suppression of the antibody response to sheep red blood cells (SRBC) occurred in fish that were thymectomized at 1.5 months of age and immunized 2 weeks later, and a marked decrease in lymphocytes was observed in the pronephros and spleen. The thymectomy of adult fish also caused reduced serum antibody titres in fish immunized 1 month after the operation. These results suggest that the thymus plays an essential role in the development of the immune system and its functions continue into adult life.