Histogenesis of the lymphoid organs in sea bream (Sparus aurata L.)

Abstract A study of the histogenesis of the lymphoid organs in the sea bream ( Sparus aurata ) showed that the species already possessed kidney tubules at hatching though the haemopoietic tissue was limited to only a few cells until the fish were more than 54 days old. The spleen was first seen when the fish were between 5 and 12 days old and the thymus was first observed at day 29. The thymus was a paired organ in the dorso-posterior part of the branchial cavity, on each side of the fish. During the course of development the thymus and the kidney grew closer together, and an apparent migration of cells was seen between them. The variety of cells in the thymus increased as the fish aged and the thymus became vascularised by day 70. The thymus acquired some morphologically mature lymphocytes by day 47; the kidney had similar cells at day 54, but the spleen was not lymphoid by the end of the study (77 days post-hatch). By the 77th day, the thymus had distinct cortex and medullary zones. Melanomacrophage centres were not apparent by day 77 and the spleen was not divided into red and white pulp. The results are discussed in relation to the onset of immunocompetence in this species and the possibility of early vaccination.