Which characteristic of proximal tubule cells supports their high reabsorptive capacity?

• A. Short, flat cells with reduced mitochondria
• B. Tall eosinophilic cells with a brush border and abundant mitochondria
• C. Cuboidal cells with few mitochondria and no microvilli
• D. Squamous cells with minimal cytoplasm

Answer: (B)

The ability to reabsorb most of what is filtered in the kidney relies on cells that have both a large absorptive surface and plenty of energy to drive transport. Proximal tubule cells fit this need with tall, eosinophilic epithelia that display a prominent brush border of microvilli on the apical surface. This microvillous border dramatically expands surface area for membrane transporters that reclaim ions, nutrients, and water. In addition, these cells are packed with mitochondria, supplying the ATP required for active transport, especially Na+/K+-ATPase pumps and secondary transport mechanisms that move solutes from the filtrate back into the bloodstream. The combination of extensive apical surface area and abundant energy makes them exceptionally capable of high reabsorptive work. If cells lacked microvilli or mitochondria, their reabsorptive capacity would be much lower because there would be less surface area for transport and less ATP to power it.