Electron microscopic study of possible sites of ultrafiltration in Lumbricus terrestris (Annelida, Oligochaeta)

Electron microscopic investigations of blood vessels of Lumbricus terrestris (L. terrestris) were conducted to show sites of filtration such as podocytes or irregular fenestrations. The endothelia of the blood vessels consist of myoendothelial cells, chloragocytes and podocytes. The podocytes form large archs over a considerable area of the vessels. Numerous small pedicels are presented on the lumen side and the gaps between adjacent pedicels are bridged by slit membranes. The podocytes are restricted to the front part of the ventral vessel. They are regarded as the structural basis for ultrafiltration. Additionally to the filtration site between ventral vessel and coelomic cavity, a second putative filtration site was found in the front part of the body between intestinal blood sinus and coelomic cavity. The endothelium of the sinus is formed by myoendothelial cells, pedicels and chloragocytes with footlike processes. In such areas the basement membrane is the only continous layer between blood vessel and coelomic cavity. The basement membrane of the blood vessels of L. terrestris is a characteristic association of three layers of different composition and thickness. Possible filtration sites in form of podocytes and irregular fenestrations could be localized at the border between the blood compartment and the coelomic compartment. Presumably the primary urine is formed by ultrafiltration of blood.