Renal papilla

The renal papilla (Apex of the renal pyramid) is the tapered, cone-shaped tip of the renal pyramid (inner medulla) that projects into the renal calyx. It represents the most distal portion of the renal medulla, where the inner medulla narrows to form a crest-like structure. The collecting ducts converge within the papilla and merge to form large terminal ducts known as the ducts of Bellini (papillary ducts), which open at the papillary tip through small orifices called foramina papillaria on the cribriform area. Urine draining through these ducts empties directly into the minor calyx of the renal pelvis and from there flows into the ureter.

Each kidney contains multiple papillae, which can be classified morphologically as simple papillae (draining a single renal lobe) or compound (fused) papillae (draining two or more lobes). Compound papillae are more commonly found in the upper pole of the kidney.

The papilla is lined by a specialized epithelium distinct from the urothelium that lines the renal pelvis it does not express uroplakins (urothelium-specific proteins). The papilla is particularly vulnerable to ischemic necrosis (renal papillary necrosis) due to its unique blood supply arrangement and the hypertonic interstitial environment required for the urinary concentrating mechanism. The papilla also harbors a population of label-retaining cells with stem cell characteristics, which may play a role in renal repair after injury.