Ontogeny of lymphoid organs and immunoglobulin producing cells in Atlantic cod (Gadus morhua L.).

The ontogeny of lymphoid organs and the development of cells expressing immunoglobulin heavy chain (IgH) mRNA as well as cells containing immunoglobulin (IgM) were studied in Atlantic cod (Gadus morhua L.), a marine teleost. Head kidney and spleen appeared as the first lymphoid organs, present at the time of hatching, whereas thymus was observed in 9 mm larvae. Fully developed lymphoid organs were not achieved until after metamorphosis. Cells expressing IgH mRNA were detected in paraffin sections of larvae and juveniles by in situ hybridization. Positive cells were not detected in fish smaller than 33 mm (58 days after hatching). IgH mRNA expression coincided with the first appearance of immunoglobulin-positive cells as revealed by immunohistochemistry in the same animals.