The middle and terminal thirds, IMCD 2 and IMCD 3, are the portions in the proximal and terminal halves of the papilla, respectively. 328,329,391 The outer third, IMCD 1, is the portion in the base of the inner medulla. 241,391 In the rabbit and rat, the papillary tip is located in the renal pelvis at or near the renal hilum, whereas in the mouse, the papilla extends beyond the hilum, turning and extending a short distance into the ureter.įor ultrastructural characterization in the rat, the inner medulla was subdivided arbitrarily into thirds, and the IMCD in each region designated accordingly. 241,390 In the rabbit, the cell height gradually increases from ~10 µm initially to ~50 µm near the papillary tip, whereas in the rat, the height increases from ~6 µm to ~15 µm at the papillary tip. The length of the papilla, the number of collecting duct fusions, and cell height vary among species. The IMCDt opens on the tip of the papilla at the area cribrosa (see Figs.

Cell height and tubule diameter increase distally, and the most distal portion, the ducts of Bellini, are composed of relatively tall, columnar epithelium. The IMCDs fuse successively as they descend in the inner medulla, such that few tubules remain at the papillary tip ( Fig. However, in some mice, the TALs arise in the proximal portion of the papilla, in which case the IMCD is entirely contained within the papilla. In rat and rabbit, the transition from thin to thick ascending limbs, and thus the beginning of the IMCD, occurs within the renal parenchyma proximal to the papilla, forming the base of the inner medulla. The IMCD extends from the boundary between the outer and inner medulla, defined by the transitions from thin to thick ascending limbs, to the papillary tip. Yu MB, BChir, in Brenner and Rector's The Kidney, 2020 Inner Medullary Collecting Duct

Adrenal medulla hormones