The ultrastructure of the rat renal medulla as observed after improved fixation methods.

The ultrastructure of the rat renal medulla was studied after improved fixation methods. Rat kidneys were fixed by vascular perfusion with 3% glutaraldehyde in a 0.08 M sodium cacodylate buffer. The total osmolality of the fixatives was varied by the addition of different amounts of sodium chloride. Good fixation was obtained when the osmolality of the fixative was adjusted to the particular level of the medulla to be studied; inner stripe of the outer zone, 700 mOsm/kg, outer level of the inner zone, 1 000 mOsm/kg, middle level of the inner zone, 1 300 mOsm/kg, and papillary tip, 1 800 mOsm/kg. The addition of 3% dextran to the fixative solutions was beneficial for the preservation of interstitial structures and intercellular relations. In the inner stripe of the outer zone and in the outer level of the inner zone, three types of interstitial cells were found. Type 1 was closely related to loops of Henle and capillaries. The cytoplasm contained lipid droplets and much rough endoplasmic reticulum. Type 2 was a rounded cell without lipid droplets, with a sparse endoplasmic reticulum but containing many free ribosomes and lysosomes. Type 3 was a pericyte related to the descending vasa recta. In the inner levels of the inner zone interstitial cells were of type 1 only. Capillaries and loops of Henle were usually surrounded by a single-layered basement membrane. In the middle level of the inner zone all thin limbs were of the simple or “ascending” type.