Type I alveolar epithelium constitutes most of the alveolar surface
area, and conducts most of the lung water trafficking through aqueporin
channels necessary to keep alveoli from flooding. These epithelial
cells overlie a matrix of mesenchyme-containing pulmonary fibroblasts
within collagen, actin, and elastin fibers. Together with pulmonary capillaries,
they form the alveolar septum, which attenuates and flattens during
infancy, increasing surface area for gas exchange. Type II alveolar
epithelial cells are metabolically active, synthesizing, secreting,
and metabolizing pulmonary surfactant that allows alveoli to remain
gas filled at the end of exhalation. These cells also appear to serve
as a progenitor pool from which other type II cells and type I cells
derive during development, repair, and regeneration.7 They
secrete cytokines such as vascular endothelial growth factor (VEGF),
which induce proliferation in neighboring capillary endothelial
cells to allow extension of capillary tubes into alveolar septa
during alveologenesis. Along with alveolar macrophages and bronchiolar
epithelium, type II cells secrete proinflammatory cytokines and
leukocyte chemokines in response to inflammatory stimuli.