The reticulo-endothelial system of teleost fish with special reference to the plaice (Pleuronectes platessa L.)

Abstract

The reticule-endothelial system of plaice (Pleuronectes platessa] was defined using a variety of different sized particles, both inert e.g. colloidal carbon, and biological, e.g. yeast cells. From these experiments, it became obvious that the major sites of particle uptake were the kidney, spleen, and atrium, while in complete contrast to the mammalian situation, the liver played virtually no part, Kupffer cells being absent. On the basis of these findings, it was decided to investigate at an ultrastructural level the following tissues:- i. the heart, in an attempt to establish the means of uptake by the endocardial lining cells of the atrium. ii. the ellipsoids or Schweigger-Seidel sheaths, which were the first site of particle trapping in the spleen. iii. the liver, with special reference to the sinusoidal lining due to the apparent absence of Kupffer cells. iv. blood, in an attempt to establish the presence of monocytes, the precursors of many macrophages in mammals. The presence of monocytes was established and the ultrastructural features of the other leucocytes noted. Both monocytes and thrombocytes were able to endocytose intra-venously injected carbon particles. The absence of Kupffer cells was confirmed and the degree of carbon uptake by the fat-storing cells was not considered to be sufficient to be of much importance in natural disease conditions. The endothelial lining of the axial vessel of the ellipsoids was observed to be comprised of 2 cell types, one of which contained sufficient microfibrillar elements to be considered as possibly contractile. Carbon particles were able to migrate out of the axial vessel into the sheath, between endothelial cells. Here they were endocytosed by the large pale reticular cells which were thought to subsequently migrate out of the sheaths and into the melano-macrophage centres. Uptake by the endocardial lining cells was restricted to those of the atrium. Macropinocytosis and not phagocytosis, was the mechanism thought to be responsible for regulating particle uptake. The pathophysiological indications of all of these findings are discussed. Finally, kinetic studies were performed, involving the clearance of radio-labelled Salmonella gallinarum bacteria from the bloodstream of plaice, kept under various conditions. It was shown that clearance at low temperatures was effected but at a slower rate than normal due probably to a purely metabolic effect. Similarly, clearance at high temperatures was enhanced, as was the clearance from the bloodstream of fish previously exposed to the antigen.