Filtration barriers in the coelomic sac of the crayfish, Procambarus clarkii

Abstract

The crayfish coelomic sac forms a protein-free urine by ultrafiltration. Ultrastructural studies of the coelomic sac wall reveal two layers which may represent filtration barriers to the hemolymph proteins. These are the fibrillar basement membrane and the epithelial slit diaphragm. The diaphragm in P. clarkii, as revealed by fixation with tannic acid, has a unique substructure unlike that previously described in the mammalian glomerulus. The diaphragm appears as two parallel, helical filaments joined to each other and the plasma membranes of adjacent foot processes. Ferritin, a molecule which resembles the monomer of crustacean hemocyanin in size and several other physical-chemical properties, was used as an ultrastructural tracer to determine which layer in the coelomic sac acts as the barrier to molecules the size of hemocyanin. The penetration of ferritin into the subepithelial region of the basement membrane and the absence of this tracer from the urinary space indicate that the slit diaphragm is an important barrier to ferritin and presumably hemocyanin.