Morphology of the lymphoid organs of channel catfish: Ontogeny of the thymus, anterior kidney, and spleen; histology and fine structure of the thymus; and thymic involution

The channel catfish (Ictalurus punctatus) has been used extensively as an animal model in immunologic studies at the molecular and cellular levels; however, basic studies of the ontogeny and morphology of the immune system in catfish have not previously been performed. The purpose of the present study was to characterize specific features of the channel catfish immune system by (1) describing, at the light microscopic level, the ontogeny of the thymus, anterior kidney, and spleen, (2) describing, at the light and electron microscopic levels, the cellular and structural characteristics of the thymus, and (3) determining the effect of a single event of acute stress on the morphology of the thymus. Catfish used in this study were hatched and reared in the laboratory.; The following conclusions were drawn from the present study. Temporal development of the lymphoid organs in catfish embryos and larvae was similar to that reported in other teleost species, that is, the thymus was the first lymphoid organ identified, and small lymphocytes were observed in the thymus and anterior kidney prior to being observed in the spleen. The lymphoid organs of 7-day-old catfish larvae had not acquired all of the morphologic characteristics of the lymphoid organs in older catfish, suggesting that 1-week-old catfish larvae may not be functionally immunocompetent. Also, a population of blast cells developed within the vitelline vein and gave rise to erythrocytes within the vitelline vein and intraembryonic vessels preceding the development of the thymic primordium and renal hemopoietic tissue, suggesting that the first site of origin of hemopoietic stem cells in the catfish embryo may be extraembryonic.; Similar to other vertebrate species, the thymuses of juvenile (9 to 10 month old) catfish consisted predominantly of a meshlike cytoreticulum whose spaces were filled with lymphocytes. Other cell types present included myoid and mucin-secreting cells.; Finally, it was not possible to define the effect of handling and transport stress on the thymuses of juvenile (9 to 10 month old) catfish exhibiting physiologic thymic hyperplasia because of concurrent stress-induced and physiologic thymic involution.