A renal column is an inward extension of the renal cortex between neighboring renal pyramids. It is part of the kidney's living tissue, continuous with the outer cortex, rather than an empty drainage passage. The renal medulla is organized into pyramids, with cortical tissue extending between them.[ref]
A longitudinal section provides a useful identification exercise. First locate two adjacent renal pyramids by following their striations toward their renal papilla outlets. Then trace the intervening tissue outward into the renal cortex. That continuity identifies a renal column; the hollow receiving spaces beside the papillary tips belong instead to the renal calyx system.[ref]
Longitudinal section through Kidney by the Manipal Museum of Anatomy and Pathology. specimenEXC002: columns between pyramids
This distinction explains why location alone can mislead: tissue lying deep between two pyramids is not necessarily medulla. The term describes how cortical tissue is arranged in three dimensions. A section's apparently separate cortical islands may simply be different cuts through one connected anatomical region.
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