= Renal cortex
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The <renal cortex> is the outer region of functional <kidney> <tissue>, surrounding the <renal medulla>. It contains the <renal corpuscles>, each with a <glomerulus>, and many convoluted portions of the <renal tubules>. Cortical extensions between neighboring <renal pyramids> form <renal columns>. The cortex is thus a substantial region containing multiple structures, not merely the kidney's thin external covering.<Kidneys by the National Cancer Institute>[internal anatomy: cortex, medulla and renal columns]{ref}
A practical <histology> comparison uses a section containing both regions. Look for the rounded <renal corpuscle> profiles among the tubular profiles: their occurrence identifies cortical tissue. In contrast, the medullary field emphasizes more parallel passages and lacks these corpuscles. Straight tubular segments can also extend through cortical tissue, so one straight tube is insufficient to classify the entire region as medulla.<Kidney histology laboratory by Columbia University>[slide50: cortical labyrinth, corpuscles and medullary rays]{ref}
This example separates a region from a functional unit. A <nephron> has its <renal corpuscle> in the <renal cortex>, while its <Loop of Henle> extends to a depth that varies among nephrons. Following one nephron therefore crosses anatomical regions; drawing a cortex–medulla boundary does not divide the organ into two independent sets of complete nephrons.
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