The fine structure of the rat renal papilla was studied after fixation with various fixative solutions and by using three modes of fixative application: immersion, vascular perfusion and injection of fixative into the renal pelvis. The morphology of the following was described: thin limbs of Henle's loop, collecting ducts, capillaries, interstitial cells, and interstitium. Several previously undescribed features were seen. Two types of capillaries were identified on the basis of their fine structure. Regions identified as limbs of Henle had at least two types of mural structure differing in degree of cellular interdigitation. In addition, numerous projections were seen extending from the basal cytoplasm of capillary endothelium and collecting duct epithelium through the basement membrane into the interstitium.It has long been assumed that the renal papilla is important in the production of concentrated urine (Marshall, '34). Though the exact mechanisms involved are still the subject of considerable debate, several groups of workers (Wirz and co-workers, '51; Wirz, '53; and others) hypothesized that the papilla was the site of a countercurrent multiplier, a counter-current exchanger and an osmotic exchanger. The loop of Henle is presumed to function as the counter-current multiplier by creating an increasing gradient of solute concentration from the cortex to the papillary tip. When the collecting duct (an osmotic exchanger) passes through the concentrated interstitium of this region, water leaves the tubular lumen resulting in the production of hypertonic urine. The capillaries form a counter-current exchange system allowing blood flow without abolition of the gradient. Micropuncture studies by Gottschalk and Mylle ('59) were consistent with this hypothesis. The majority of other physiological experiments are also consonant with the counter-current hypothesis, but some contradictory experiments have been reported '58; and Marsh and Solomon, '65).Although a few reports concerning selected aspects of papillary fine structure have been published (Rhodin, '58a,b, '62, '63; Sakaguchi and Suzuki, '58 study of all the structures in the papilla seems warranted as a basis for our morphological studies of this region in water diuresis and antidiuresis.
MATERIALS AND METHODSSixteen Sprague-Dawley and 14 Charles River male and female rats, weighing 175-300 gm were used in this experiment. The rats were allowed access to food and water at will until the time of sacrifice.The animals were anesthetized with pentobarbital and opened with a mid-ventral incision. Kidneys were fixed by three methods.( 1 ) The kidneys were removed and small (-1 mm) slices or pieces immersed in the fixative solutions for various time intervals. (2) At the time of laparotomy with the blood supply still intact, 5 ml of fixative solution were injected into the renal pelvis for a period of approximately five minutes with a needle and syringe. The fixative could be seen to move down the ureter into the bladder if the needle was in the renal pel...