Leucocyte subpopulations in developing carp (Cyprinus carpioL.): immunocytochemical studies

The distribution of leucocyte subpopulations was studied in thymus, head kidney, spleen, gut and blood of developing carp by using monoclonal antibodies against early T cells, B cells, thrombocytes and macrophage-like cells. In the first week post-fertilisation (p.f.), early T cells are very numerous (approximately 77%) in thymus, but are also present in other organs, especially the head kidney; subsequently these cells gradually dis-appear in all organs, except the thymus (approximately 40%). B cells appear in head kidney from the second week p.f., and later in spleen and blood, but their number remains low in thymus and gut. Thrombocytes first appear in spleen during the first week p.f. and their percentage increases until the fourth week (approximately 30%). Monocyte/macrophage-like cells are present in all organs from the first week p.f., and their percentage gradually increases until the eighth week p.f. This study indicates that: (1) thymus may be the primary lymphoid organ for T lymphocytes; (2) head kidney could be the primary lymphoid organ for B lymphocytes; (3) spleen is a primary site for thrombo-poiesis; and (4) monocyte/macrophage-like cells are already present in early age. These findings provide new information on the development of the fish immune system.