What is the function of the Type I alveolar cells? Type II?

Question

What is the function of the Type I alveolar cells? Type II?

Answer 1

Alveoli are lined by simple squamous epithelial cells (called Type I pneumocytes or Type I alveolar cells) and round Type II pneumocytes (also called septal cells or greater alveolar cells or Type II alveolar cells).

Most (95%) of the alveolar surface is covered with exceedingly thin squamous Type I pneumocytes, which are interconnected by tight junctions to reduce leakage of tissue fluid into the airs paces.  They contain many pinocytotic vesicles.

The remainder of the alveolar lining is made up of cuboidal Type II pneumocytes.  These cells outnumber Type I cells, form tight junctions with Type I cells and divide to give rise to both Type I and Type II cells.  Type II cells are characterized by lamellar bodies, which contain pulmonary surfactant, which is synthesized in the RER and is composed of a variety of lipids and proteins. Surfactant is released from Type II cells by exocytosis and coats the entire alveolar surface.  It functions to reduce surface tension and hence decrease the work required to inflate the alveolus. Prematurely born infants can suffer from fetal Respiratory Distress Syndrome if the Type II cells have not matured enough to produce surfactant.  These infants are treated with synthetic surfactant and steroids to stimulate Type II cell maturation.