Light- and electron microscopical observations on the caecilian spleen. A contribution to the evolution of lymphatic organs.

Red and white pulp were distinguished in the spleen of the caecilian species Ichthyophis paucisulcus and Afrocaecilia taitana. The red pulp was composed of endothelium-lined sinusoids and reticular connective tissue. Between the processes of the reticulum cells, accompanied by fine collagen fibrils, the following cell types were found: lymphocytes, macrophages (frequency containing fragments of erythrocytes), neutrophils, eosinophils, mast cells and/or basophils (metachromatic granules), thrombocytes, plasma cells, pigment cells as well as cells which presumably represent blast cells. Morphological evidence suggested the formation of thrombocytes in the red pulp. Besides sinusoids, ellipsoids and peculiar arteriolar vessels with a high endothelium and a loose layer of muscle cells were observed. Veins were concentrated in the splenic periphery. White pulp consisted of arterioles which were surrounded by a lymphocyte sheath. Follicles were not identified with certainty. Occasionally mitotic figures were associated with lymphocytes. On the basis of our findings, we suggest the following functions of the caecilian spleen: destruction of aged erythrocytes, formation of thrombo- and lymphocytes as well as of plasma cells and, to a marked lesser degree, of other blood cells.