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 (Oncor hynchus mykiss) is described. The endothelium of terminations of arter ioles bulged into the lumen, and gaps between the endothelial cells we re evident. A continuous basal lamina was not present, and there were extensive interdigitations between the endothelial cells and surroundi ng reticular cells. The interdigitating processes were rich in microfi laments. Intravenously injected colloidal carbon, approximately 0.03 m u m in diameter, was held in the reticular matrix of the ellipsoidal w all and taken up by macrophages that extended cellular processes among the reticular and endothelial cells. The intravenous injection of flu orescent polystyrene microspheres of known diameter showed that micros pheres with a diameter of 0.5 or 1.0 mu m were localised in the red pu lp, whereas microspheres with a diameter of 0.15 mu m were retained in ellipsoidal walls. Thus, the terminations of splenic arterioles in ra inbow trout were found to be consistent with descriptions of ellipsoid s in other vertebrates in that they possessed a specialised cuboidal e ndothelium, lacked a continuous basal lamina, were surrounded by a she ath of macrophages and reticular cells, and had a role in the selectiv e filtration and retention of bloodborne particles.