Post‐hatching development of the thymic epithelial cells in the rainbow trout Salmo gairdneri: An ultrastructural study

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Abstract
This study reports the ultrastructure of subpopulations of epithelial cells of the thymic parenchyma during the post‐hatching development of the rainbow trout, Salmo gairdneri, kept at 14°C. At hatching, the thymus contained a small number of medium and large thymocytes interspersed among three different types of epithelial cells: (1) epthelial cells adjacent to the connective tissue capsule; (2) ramified dark epithelial cells with electron‐dense cytoplasm; and (3) pale electron‐lucent epithelial cells displaying secretory‐like features. All these cell types were anchored to one another by desmosomes and had apparently differentiated from the pharyngeal epithelium. At 4 days after hatching, the thymus enlarged, and numerous gaps occurred between the cell processes of contiguous epithelial cell processes of contiguous epithelial cells adjacent to the capsular connective tissue. In 21‐day‐old trout, thymic trabeculae developed carrying blood vessels, and a subcapsular zone became evident containing lymphoblasts and large subcapsular epithelial cells. In 30‐day‐old trout, an outer thymic zone developed consisting of spindle‐shaped epithelial cells which formed a dense network. At this stage, scattered cystic cells, which apparently differentiated from the pale epithelial cells, were present.