Overview

Each papilla projects into the cup of a minor calyx. Its tip, the area cribrosa, is pierced by ≈10–25 openings of the papillary ducts (of Bellini) — the terminal, largest collecting ducts. Urine leaving the papilla has its final composition; no further modification occurs in the calyces, pelvis, ureter or bladder. Compound papillae (formed by fusion of adjacent pyramids, typical at the poles) have wide, open duct orifices that allow reflux of infected urine into the parenchyma (intrarenal reflux), explaining why polar scarring is common in reflux nephropathy.

The papillary tip is the most hypertonic (≈1200 mOsm/kg) and least well-perfused part of the kidney.

Location: Apex of each renal pyramid, projecting into a minor calyx.

Anatomy

Histology: Papillary ducts lined by tall columnar epithelium; surface covered by a simple cuboidal/columnar epithelium becoming urothelium on the calyceal side.

Function

Discharge final urine from papillary ducts into the minor calyx
Site of maximal interstitial osmolality

Clinical significance

Papillary necrosis

Ischaemic necrosis of the papillary tips (NSAIDs, diabetes, sickle cell trait/disease, obstruction, pyelonephritis); necrotic papillae slough into the calyces producing "ring shadows" and clubbed calyces on urography.

Randall plaques

Subepithelial calcium phosphate deposits at the papillary tip are thought to be the nidus on which calcium oxalate stones grow.

Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.