Investigation of the structural and functional features of splenic ellipsoids in rainbow trout (Oncorhynchus mykiss)

The ultrastructure of ellipsoids in the spleen of rainbow trout (Oncorhynchus mykiss) is described. The endothelium of terminations of arterioles bulged into the lumen, and gaps between the endothelial cells were evident. A continuous basal lamina was not present, and there were extensive interdigitations between the endothelial cells and surrounding reticular cells. The interdigitating processes were rich in microfilaments. Intravenously injected colloidal carbon, approximately 0.03 μm in diameter, was held in the reticular matrix of the ellipsoidal wall and taken up by macrophages that extended cellular processes among the reticular and endothelial cells. The intravenous injection of fluorescent polystyrene microspheres of known diameter showed that microspheres with a diameter of 0.5 or 1.0 μm were localised in the red pulp, whereas microspheres with a diameter of 0.15 μm were retained in ellipsoidal walls. Thus, the terminations of splenic arterioles in rainbow trout were found to be consistent with descriptions of ellipsoids in other vertebrates in that they possessed a speciallised cuboidal endothelium, lacked a continuous basal lamina, were surrounded by a sheath of macrophages and reticular cells, and had a sheath of macrophages and reticular cells, and had a role in the selective filtration and retention of bloodborne particles.