What is the histologic signature of the proximal tubule in the kidney?

• A. Tall, eosinophilic cells with a prominent brush border and abundant mitochondria
• B. Flattened cells with scant cytoplasm
• C. Cuboidal cells with microvilli but no brush border
• D. Squamous cells with a thin cytoplasm

Answer: (A)

The proximal tubule is built for active reabsorption, so its lining shows tall, eosinophilic cells with a prominent apical brush border and abundant mitochondria. The tall shape reflects a robust epithelial layer, while the brush border—dense microvilli on the apical surface—increases surface area for reclaiming water, ions, glucose, and amino acids from the filtrate. The numerous mitochondria power the active transport processes at the basolateral and apical membranes, which is why the cytoplasm stains pink with many mitochondria.

So, this combination—tall eosinophilic cells, a distinct apical brush border, and lots of mitochondria—best identifies the proximal tubule. Descriptions of flattened or cuboidal cells without a brush border, or squamous epithelium, do not match the key features of the proximal tubule and fit other renal segments or non-renal tissues.