Ontogeny of IgM‐producing cells in the lymphoid organs of rainbow trout, Salmo gairdneri Richardson: an immuno‐ and enzyme‐histochemical study

The ontogenetic development of IgM‐containing cells is described as demonstrated by immunoperoxidase staining with a mouse anti‐trout IgM monoclonal antibody and the differentiation of enzyme‐histochemical markers in the non‐lymphoid cells forming the stroma of the thymus, spleen and kidney of the rainbow trout. The first lymphoid cells staining with the monoclonal antibody occurred at day 4‐5 after hatching in the renal lympho‐haemopoietic tissue. By 1 month after hatching IgM‐positive cells also appeared in the spleen and thymus. Enzyme‐histochemical demonstration of the alkaline and acid phosphatase and non‐specific σ‐naphthyl acetate esterase enzymatic activities in the non‐lymphoid cells indicated that a certain degree of maturation of the cellular stroma of the developing lymphoid organs of trout was reached before or at the time when IgM‐expressing cells could be observed. The relationships of the stromal components of the various lymphoid organs to the development of IgM‐positive cells, and the possible role of the renal lympho‐haemopoietic tissue as a primary lymphoid organ for B‐cell differentiation in the trout are discussed.