TISSUE DISTRIBUTION AND STRUCTURE OF BARRIER CELLS IN THE HEMATOPOIETIC AND LYMPHOID ORGANS OF SALMONIDS

Background: Barrier cells have been recognized as a discrete group of fibroblastic- or myofibroblastic-like cells located in the lymphoid an d hematopoietic organs of mammals. This paper reports the results of a morphological study of the main lymphoid organs of three salmonid spe cies, in which cells structurally similar to the mammalian barrier cel ls were observed in healthy animals. Methods: The spleen, kidney, and thymus of fingerlings of rainbow trout, Oncorhynchus mykiss, and Coho salmon, Oncorhynchus kisutch, and of adult brown trout, Salmo trutta f ario, were processed for electron microscopy study using various fixat ion methods. Semithin sections were used for the Periodic Acid-Schiff (PAS) staining technique, and for the demonstration of the endogenous peroxidase activity. Results: The kidney and spleen of all the species contained a variable, but usually low, number of electron-dense, elon gated, and branched cells, ultrastructurally similar to the mammalian barrier cells. They also occurred in the thymus of some brown trout an d Coho salmon, but not of rainbow trout. The electron density of this cell type was present after the various types of fixation procedures. They show numerous ribosomes, well-developed secretory organelles, ele ctron-clear vesicles, large granules, and microfilaments. In all the s almonid species, barriers cells were positive for PAS staining and for endogenous peroxidase activity. A small number of barrier cells were in mitosis. In the different organs barrier cells appeared as isolated cells, or forming syncytial networks. They were found lining the bloo d sinusoids of the splenic red pulp and of the renal hematopoietic tis sue, in the periellipsoidal sheaths, and closely associated with eryth ropoietic and plasmacytopoietic foci. Conclusions: Our results demonst rate the presence of cells closely resembling mammalian barrier cells in the hematopoietic and lymphoid organs of salmonids. The structure a nd tissue distribution of the salmonid barrier cells are discussed in relation to the functional roles described for this cell type in mamma ls. (C) 1996 Wiley-Liss, Inc.