SEASONAL INTRATHYMIC ERYTHROPOIETIC ACTIVITY IN TROUT

Thymus glands of two salmonid species, Salmo trutta and Oncorhynchus m ykiss, caught monthly throughout the year, were examined by light and transmission electron microscopy. Erythropoietic foci, consisting of b oth 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 a ctivity, together with the role played by the thymic cell microenviron ments and endocrine factors.