The ontogeny of gut-associated lymphoid tissue in Oreochromis-mossambicus

The ontogenesis of gut-associated lymphoid tissue (GALT) was investigated using histological procedures. Samples were taken at various time intervals to include larval, first-feeding and juvenile stages. Whole fish and tissue samples were embedded in either paraffin wax or metha-crylate resin and sections stained with Giemsa, periodic acid-Schiff (PAS), methyl green pyronin (MGP) and methylene blue. GALT was first observed as individual lymphocyte-like and macrophage-like cells in the lamina propria, and in later stages plasma cells and granulocytes were identified. Leucocytes began to infiltrate the epithelium just prior to first feeding and then rapidly once feeding had began. By day 50, GALT was equal to that observed in the adult gut.