Macroscopic and microscopic organization
The renal medulla is formed by renal pyramids (pyramides renales), with a total count ranging from 8 to 12. The free tapering apex of each such pyramid forms a renal papilla (papilla renalis).
These structures project directly into the renal calyx, where the excretion of definitive urine is completed. Straight tubules and collecting ducts pass through the thickness of the medulla and at the level of the papillae, merging into terminal papillary ducts.
Features of histological structure
The histological composition of the renal papillae and the pyramids forming them is closely related to the terminal parts of the urinary tract in the renal parenchyma:
- Collecting ducts run perpendicular to the organ surface, descending from the medullary rays into the pyramids.
- At the apices of the pyramids, they unite into large papillary ducts.
- Papillary ducts open on the surface of the renal papilla directly into the lumen of the minor calyces.
- The surface of the papilla itself is covered with a special lining epithelium.
Epithelial lining of the papillae
Upon microscopic examination, the cellular composition is a key diagnostic element of the renal papilla zone. The surface of the papilla projecting into the renal calyx is lined with transitional epithelium. The same lining is characteristic of the adjacent wall of the calyx itself, which ensures tissue resistance to constant contact with urine and changes in the volume of the cavities.
The pyramid points downwards: the apex is the papilla, and the outer surface features transitional epithelium facing the calyx.
Class and exam questions
What epithelium lines the surface of the renal papilla?
The surface of the renal papilla is lined with transitional epithelium, which also covers the adjacent renal calyx.
Where do the ducts at the apex of the renal pyramid open?
Papillary ducts, formed by the fusion of collecting ducts, open on the surface of the renal papilla into the lumen of the minor calyces.
How many renal papillae are formed in a single kidney?
The renal medulla is formed by 8–12 renal pyramids, the apices of which form a corresponding number of renal papillae.
What else to study on this topic
- Structure and topography of the renal cortex and medulla
- Classification of nephrons by their localization (cortical, intermediate, juxtamedullary)
- Morphofunctional organization of the loop of Henle
- System of collecting ducts and papillary ducts
- Features of the blood supply to the renal medulla
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