The altered structure of renal papillary outflow tracts in obesity.

Weight gain is associated with an expanded renal medullary interstitium in humans and in animal models of obesity. In this study, the consequence of obesity and this expanded matrix on renal papillary structure was examined in 15 obese rabbits fed a high fat diet for 8-12 weeks compared to 21 rabbits fed a standard diet. When examined under a dissecting microscope, the tips of the renal papillae from formalin-fixed, methylene blue-stained kidneys showed patent ducts of Bellini in 5 of 5 instances from 2 lean rabbits, but in only 2 out of 12 ducts from 3 obese rabbits. The ostia of the remaining ducts were significantly distended (205 +/- 42 microns versus 56 +/- 8 microns) and occupied by lightly staining granular material. When examined with scanning electron microscopy, all ducts were patent in lean rabbits (6 ducts in 4 rabbits, averaging 104 +/- 12 microns across), whereas only 6 of 11 ducts were patent in papillae from 4 obese rabbits. Renal medullary parenchymal tissue appeared at the openings of the remaining 5 ducts of Bellini in the 4 rabbits. Not only were these 5 ducts significantly distended by the interstitial material (with openings averaging 248 +/- 56 microns across), but the associated collecting ducts were dilated relative to control (100 +/- 15 microns versus 75 +/- 7 microns). Since the ducts of Bellini are the only renal openings that are not corsetted by a fibrous capsule, the authors speculate that the expanded medullary interstitium and increased renal sinus lipid partially obstruct renal outflow and elevate renal interstitial hydrostatic pressure in obesity, causing a prolapse of parenchymal contents, further obstructing urine outflow and leading to distention of the collecting ducts and ducts of Bellini.