Rodlet cells in the thymus of the zebrafish Danio rerio (Hamilton, 1822)

The conspicuous rodlet cell (RC) of teleost fishes is a remarkable but still poorly understood phenomenon in the biology of this group. Since their discovery by Thelohan in 1892, a confusing discussion about their nature and physiological function has ensued with various interpretations of RCs as protozoan parasites, gland cells, or a specific type of immune cell. Here we present a study on RCs in the zebrafish Danio rerio (Hamilton, 1822) and the first observation of their occurrence in the thymus, based on light- and electron-microscopy. RCs were confirmed in thymus-surrounding connective tissue, the thymic epithelium and the parenchyma. Very few (late) immature stages of RCs could be identified, already displaying some rodlets and a developing fibrillar cell border but often also signs of a degenerating Golgi apparatus. Apoptotic RCs occurred mostly below the surface epithelium.The mature RCs ejected their rodlets not only at the surface of the epithelium but also into the extracellular environment of the thymic parenchyma. Coalescence of the rodlets into “multi-core” units seems to represent an organ-specific feature. No cellular immune reaction of reticulum cells or granulocytes against RCs and their cellular products was observed. RCs in tight contact with lymphocytes showed conspicuous formation of dense structures in the capsule-like cell membrane, but dense membrane appositions were also observed between RCs and erythrocytes in blood vessels. The possibility of immigration and defective development of RCs in the thymus is discussed.