The thymus of the rainbow trout (Salmo gairdneri). Light and electron microscopic study.

The rainbow trout thymus is a paired organ, organized in 3 adjacent zones. The gland lying on a thick connective tissue layer, is covered by a thin epithelial capsule. Cellular components of thymus are essentially the thymocytes and the epithelial cells. Thymocytes occur mainly in the inner and outer zones but neither cortex nor medulla are clearly delimited. Septa represent a typical epithelial structure associated with thymocytes and blood vessels. Thymus is well vascularized and electron microscopy demonstrates a characteristic relationship between vascular system and lymphoid tissue.