Expansion
Proximal tubule has a brush border; distal tubule does not
Expansion
| Segment | Appearance |
|---|---|
| Proximal convoluted tubule | Cuboidal, brush border of microvilli, intensely eosinophilic (many mitochondria), indistinct lumen, fewer nuclei per cross section |
| Thin limb of Henle | Simple squamous, resembling a capillary but without red cells |
| Thick ascending limb | Cuboidal, no brush border, moderately eosinophilic |
| Distal convoluted tubule | Cuboidal, no brush border, clear wide lumen, more nuclei visible |
| Collecting duct | Columnar, distinct cell borders, pale; principal and intercalated cells |
The brush border is the single most useful discriminator, and it reflects function: the proximal tubule reabsorbs about two-thirds of the filtrate, so it needs maximal surface area and abundant mitochondria to power the sodium-potassium ATPase.
The distinct lateral cell borders of the collecting duct are equally characteristic and are not seen in the tubules.
Medullary rays contain straight tubules and collecting ducts running towards the papilla; the cortical labyrinth contains the convoluted segments and corpuscles.
In acute tubular necrosis, the proximal tubular cells lose their brush border and slough into the lumen, forming the muddy brown granular casts seen in the urine.