## Pathological Hallmark of Acute Tubular Necrosis **Key Point:** The defining lesion in ATN is necrosis and desquamation of tubular epithelial cells, particularly in the proximal tubule (S3 segment) and thick ascending limb of the loop of Henle. ### Structural Changes in ATN | Feature | Finding | |---------|----------| | **Brush border** | Loss or flattening | | **Cell necrosis** | Focal to extensive | | **Cell detachment** | Epithelial cells slough into tubular lumen | | **Basement membrane** | Typically **intact** (key distinguishing feature) | | **Interstitium** | Minimal inflammation (early stage) | **High-Yield:** The preservation of the basement membrane in ATN is crucial—it allows for complete recovery of tubular function when the insult is removed, unlike in chronic tubular injury where the basement membrane is disrupted. **Clinical Pearl:** The loss of brush border leads to impaired active transport and concentrating ability, explaining the clinical presentation of polyuric acute kidney injury with inability to concentrate urine. ### Why the Basement Membrane Matters The intact basement membrane serves as a scaffold for epithelial regeneration. This is why ATN is potentially **reversible**, whereas diseases with basement membrane disruption (e.g., FSGS, MPGN) often progress to chronic kidney disease.
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