Alveolar septum
![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.](/Images/godic/202412/20/Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores5851.jpg")
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.