MACROPHAGE-LYMPHOCYTE CELL CLUSTERS IN THE HYPOTHALAMIC VENTRICLE OF SOME ELASMOBRANCH FISH - ULTRASTRUCTURAL ANALYSIS AND POSSIBLE FUNCTIONAL-SIGNIFICANCE

Background: Previous studies have demonstrated the existence of lympho -haemopoietic tissue in the meninges and choroid plexuses of various p rimitive vertebrates, including the stingray Dasyatis akajei and in ea rly human embryos. In the present study, we extend these results analy zing macrophage-lymphocyte cell clusters found in the floor of the hyp othalamic ventricle of several specimens of elasmobranchs. Methods: Af ter aseptical isolation of the brain from several specimens of smooth dogfish Triakis scyllia, cloudy dogfish Scyliorhinus torazame, gummy s hark Mustelus manazo, and stingray Dasyatis akajei their hypothalamic regions were processed routinely by light, scanning, and transmission electron microscopy. Results: The study of serial histological section s demonstrated that the macrophage-lymphocyte cell clusters proceeded from the meningeal lymphohaemopoietic tissue, reaching the ventricular lumen along large blood vessels. In this tissue, macrophages, differe nt sized lymphocytes, lymphoblasts, granulocytes, monocytes, and devel oping and mature plasma cells were closely packed among a meshwork of fibroblastic reticular cell processes. It never invaded the brain pare nchyma. A cell layer of glial elements and a continuous basement membr ane interposed between the lymphoid tissue and the neural elements alt hough some macrophages had migrated across the ependymal cell layer. I n the ventricular lumen very irregular macrophages with long cell proc esses and containing abundant engulfed material of unknown origin form ed big cell clusters with neighboring lymphocytes, lymphoblasts, and p lasma cells, similar to those described during the immune response. Mo reover, electron lucent cells which resembled the antigen-presenting c ells of higher vertebrates established intimate surface cell contacts with the surrounding lymphocytes. In the third ventricle of several sp ecimens of gummy shark, Mustelus manazo, morphologically similar cell clusters appeared but these were not connected to the meningeal lympho -haemopoietic tissue. No intraventricular cell aggregates were found i n the stingray brain. Conclusions: Although we cannot rule out that th ese macrophage-lymphocyte cell clusters represent a permanent structur e in the elasmobranch brain they rather seem to be only established af ter specific stimulation for preventing the entrance of noxious, forei gn materials into the elasmobranch brain parenchyma. (C) 1995 Wiley-Li ss, Inc.