Seasonal intrathymic erythropoietic activity in trout

Thymus glands of two salmonid species, Salmo trutta and Oncorhynchus mykiss, caught monthly throughout the year, were examined by light and transmission electron microscopy. Erythropoietic foci, consisting of both developing and mature erythroid cells, occurred in the subcapsular, inner, and outer thymic zones from April to November. We discuss the possible physiological significance of this seasonal erythropoietic activity, together with the role played by the thymic cell microenvironments and endocrine factors.