Alveolar septum

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Cryptococcosis of lung in patient with AIDS. Mucicarmine stain. Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of Cryptococcus neoformans. The inner layer of the yeast capsule stains red.

The alveolar septum separates adjacent alveoli in lung tissue. The minimal components of an alveolar septum consist of the basement membranes of alveolar-lining epithelium (mostly type I pneumocytes) and capillary endothelium. Thicker alveolar septa may also contain elastic fibers, type I collagen, interstitial cells, smooth muscle cells, mast cells, lymphocytes and also monocytes.


  • Robbins and Cotran, Pathologic Basis of Disease, 7th Ed. pp 712–713.