Occurrence and distribution of sialic acid residues in developing rat glomerulus: investigations with the Limax flavus and the wheat germ agglutinin.

Evidence has accumulated suggesting that cell surface sialic acid is required for the development and maintenance of the specialized shape of glomerular podocytes in kidney. We have investigated the occurrence and distribution of plasma membrane sialic acid residues during glomerulus development in newborn rat kidney with special attention to the differentiating podocytes. Terminal sialic acid residues were detected with a high resolution cytochemical lectin technique applying the Limax flavus agglutinin in conjunction with the fetuin-gold complex. In addition, the wheat germ agglutinin/ovomucoid-gold technique was performed. Lectin binding, indicative of the presence of terminal sialic acid residues in glycoconjugates, was found in the plasma membrane of the cells of the nephrogenic cap, the earliest stage of glomerulus development. Label of similar intensity was found in the apical, lateral and basal plasma membrane of all cell types in later developmental stages such as renal vesicle, S-shaped body, capillary loop stage and maturing glomeruli. Considering the podocytes, already the presumptive podocytes clearly identifiable in the S-shaped body and early capillary loop stage exhibited binding sites for the Limax flavus and the wheat germ agglutinin along their plasma membrane domains irrespective of the position of the tight junctions and before foot process and slit diaphragm formation occurred. These results demonstrate that in the plasma membrane of all cell types during the different stages of glomerulus development sialylated glycoconjugates are present. They indicate that the general occurrence of sialic acid residues does not coincide with the development of foot processes and slit diaphragms of glomerular podocytes.