Polysialic acid and N-CAM localisation in embryonic rat kidney: mesenchymal and epithelial elements show different patterns of expression.

The expression of polysialic acid (PSA) and the neural cell adhesion molecule (N-CAM) during the embryonic development of rat kidney was investigated using immunocytochemistry and immunoblotting. A monoclonal antibody (mAb 735), which recognised only long chain PSA, and polyclonal antibodies specific for N-CAM were employed. At the earliest stages of metanephros formation, PSA and N-CAM immunostaining was found in both embryonic anlagen, namely the uretic bud and the metanephrogenic mesenchyme. Reactivity in uretic bud derivatives declined during embryonic development and was generally absent in the collecting duct system by embryonic day 18 (E18). Uninduced metanephrogenic mesenchyme was immunostained throughout development while induced mesenchymal cells showed greatly increased PSA and N-CAM immunoreactivity during their transformation into epithelium. This staining declined rapidly as nephrons differentiated. These processes were preceded by sorting of PSA and N-CAM to the basolateral plasma membrane. Similar N-CAM and PSA patterns were observed in mesonephros development. In adult kidney parenchyma both PSA and N-CAM were undetectable. Immunoblotting of samples of embryonic kidney with mAb 735 revealed a broad band ranging from 140 to greater than 200 x 10(3) Mr. N-CAM antibodies revealed reactivity in a band of 140 x 10(3) Mr after removal of PSA by endoneuraminidase treatment. Expression of N-CAM and PSA in both embryonic anlagen indicates that neither molecular species acts primarily as an inductive signal. These molecules were localised in areas where changes in cell adhesion during organogenesis might be important and thus may be involved in the grouping of developing cells.