Alveolar Type I Epithelial Cells: The Forgotten Cells in Fetal Lung Development and Lung Injury.
The alveolar surface of the lung is covered by large flat type I epithelial cells. Even
though type I cells represent only around 10% of the cells present in the alveolus; they cover much of the surface area in the developed lung. Given their thinness and proximity to the capillary endothelium; it is well accepted that type I cells play an important role in gas exchange.
In addition, these cells are important to maintain adequate fluid balance in the alveolus via
the tight junctions, ion transport channels and aquaporin-5. Recent studies also indicate that type I cells participate in innate immunity; they express toll-like receptor 4 and produce proinflammatory cytokines.
Studies from T1α knockout mice indicate that alveolar type I cells may be
critical for normal lung development. T1α, a lung type I cell differentiation gene, is
developmentally regulated and expressed only in type I cells.
T1α knockout mice died at birth of respiratory failure. Histologic analysis show fewer alveolar type I cells and decreased alveoli. All together, these investigations suggest a critical role for type I cells in gas exchange, alveolar fluid
hemostasis, immunity and fetal lung development.
The typical flat morphology of type I cells begin to appear in the late canalicular period and increase in number during the saccular and alveolar stages of lung development.
Pulm Res Respir Med Open J. 2016; 2(4): e6-e9. doi: 10.17140/PRRMOJ-2-e003
