The morphologic relationship of light and dark cells of the collecting tubule in potassium-depleted rats.

The luminal surface of collecting tubule cells in the inner stripe of the renal medulla in normal and potassium-depleted rats was studied by scanning electron microscopy. In normal rats the luminal surfaces were of two types. One cell type was sparsely covered with small bulbous microvilli and had either a single or double cilium. This type corresponds to the light cell seen in transmission electron microscopy. The second cell type was covered by prominent microplicae and represents the dark cell observed in transmission electron microscopy. In potassium-depleted animals, numerous cells with a morphologic appearance of intermediate forms were identified. By scanning electron microscopy, the luminal surface of these cells was covered by a mixed population of villi and microplicae in different stages of development and often showed cilia, which were previously considered to exist only on light cells. On the basis of these morphologic findings, we conclude that the dark and light cells are not different cell types but rather represent different forms of a single type of cell.