Ultrastructural and histochemical investigation of the terminal capillaries in the spleen of the carp (Cyprinus carpio L.)

In the spleen of the carp arterial capillaries of a highly differentiated structure have been studied by light and electron microscopy. These capillaries share various structural characteristics with the sheathed capillaries (ellipsoids of Schweigger-Seidel) of higher vertebrates. The long arterial capillaries of the carp spleen are provided with cuboidal endothelial cells containing filaments approximately 7 nm in diameter. There is no basal lamina. The endothelial cells form various types of cell junctions, but there are also extensive areas without any junctions. Here, a free passage is possible between the capillary lumen and the subendothelial space. The capillaries possess a single-layered sheath of macrophages. Characteristically, the sheath macrophages posses long and slender cell processes forming a loose framework, the meshes of which are filled with lymphocytes and spindle cells. The sheath macrophages show a zone of ectoplasm rich in filaments. The also contain numerous phagolysosomes rich in hydrolytic enzymes, as identified histochemically. The sheath is sharply limited against the pulp by a thick layer of collagen fibers.