Medulla: Inner Zone

At the inner medulla near the renal papilla, the collecting tubules converge to form papillary ducts, which convey urine into the pelvicalyceal space. The papillary ducts are lined by a layer of columnar or high columnar epithelial cells; they have a structure similar to that lining the collecting tubules. In addition to the papillary ducts, only thin segments of Henle's loop approach this inner zone. They run down to this zone and return to the outer medulla. There are no histological differences between the descending and ascending thin segment of Henle's loop.

Among these ducts and tubules there is a great amount of connective tissue. It contains capillaries, collagen fibers, and interstitial cells. These are fibroblast-like cells, whose function, apart from their collagen production, remains unknown.

The peripheral margin of the inner zone close to the papilla is covered by transitional epithelium, which is composed of only two or three layers of epithelial cells. These cells have a large nucleus, pale-staining cytoplasm, and a clear intercellular border.