ROLE OF APOPTOTIC AND NONAPOPTOTIC CELL-DEATH IN REMOVAL OF INTERCALATED CELLS FROM DEVELOPING RAT-KIDNEY

In the developing rat kidney, both type A and type B intercalated cell s are present throughout the medul lary collecting duct (MCD), as well as the papillary surface epithelium. After birth, intercalated cells gradually disappear from the papillary surface epithelium and the term inal MCD, and type B cells disappear from the entire MCD. The purpose of this study was to establish the mechanism(s) by which intercalated cells are deleted from the MCD during development. Kidneys from 14-, 1 6-, 18-, and 20-day-old fetuses and 1-, 3-, 7-, and 14-day-old pups we re preserved for light microscopic immunohistochemistry and electron m icroscopy. Intercalated cells were identified by immunostaining for H-adenosinetriphosphatase (H+-ATPase) and band 3 protein. Apoptosis was identified by nick end labeling of DNA fragments, staining with the v ital dye toluidine blue, and transmission electron microscopy. Two dis tinct mechanisms of elimination of intercalated cells were detected. C ells with apical labeling for H+-ATPase and basolateral labeling for b and 3 protein protruded into the lumen of the MCD as if they were bein g extruded from the epithelium, and many had lost contact with the bas ement membrane. Extrusion of cells with basolateral H+-ATPase or with no labeling for H+-ATPase was never observed. Apoptosis was observed i n the MCD from shortly before birth to 7 days after birth, gradually p rogressing from the papillary tip toward the outer medulla. Staining f or apoptosis was present in H+-ATPase-positive apoptotic bodies, locat ed in cells that were negative for H+-ATPase. Staining was also occasi onally observed in apoptotic cells with basolateral H+-ATPase but neve r in cells with apical H+-ATPase. Electron microscopy confirmed the pr esence of apoptotic intercalated cells in the MCD and demonstrated tha t apoptotic bodies were located in inner medullary collecting duct (IM CD) cells and principal cells. These results demonstrate that intercal ated cells are deleted from the MCD by two distinct mechanisms, one in volving simple extrusion from the epithelium and the other involving a poptosis and subsequent phagocytosis by neighboring principal cells or IMCD cells. Elimination by extrusion affects only type A intercalated cells, whereas deletion by apoptosis appears to occur only in type B intercalated cells.