Ultrastructure of blood vessels in the head kidney of the carp, Cyprinus carpio.

The ultrastructural features of blood vessels were examined by transmission and scanning electron microscopy in the perfused and nonperfused head kidney of the carp. They consisted mainly of the sinusoidal capillaries, arterioles, veins and portal veins. The sinusoidal capillaries were composed of endothelial cells that frequently exhibited large pores and an indistinct basal lamina. Some endothelial cells were trabecular in shape and protruded into the lumen. The reticular cells frequently attached to the outside of the endothelial cells, and macrophages were located on the luminal and abluminal surfaces of the endothelial cells. The arterioles possessed continuous endothelial cells with a thick basal lamina, continuous smooth muscle layers and a thick adventitia with collagen fibres and reticular cells. The veins were characterised by fenestrated endothelial cells and thin connective tissue. The portal veins were composed of continuous electron-lucent endothelial cells and thick layers of reticular cells and collagen fibres. The differences between the endothelial cells in each blood vessel probably reflect the various functions of the carp head kidney. The ultrastructural features of the sinusoids and the portal veins resemble those of lymphatic vessels of mammals and appear to relate to macrophage and lymphoid cell migration and proliferation in the carp head kidney.