Alveolar septum

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.