Alveolar epithelial cells surface area

The respiratory part of the airways is also shielded by a ciliated epithelium. Along this region the ciliated cells are interspersed by Clara cells which are discussed as progenitor cells of the respiratory epithelium [8,9]. This epithelium has to be separated from the alveolar epithelium, which is a non-ciliated epithelium and formed by alveolar epithelial cells type I and type II. The major surface area of the alveoli is covered by type I cells. Nevertheless, the cuboidal alveolar type II cells represent about 90 % of the alveolar epithelial cells. 

In the separation of alveolar air space and blood circulation, the alveolar epithelium is a more restrictive paracellular barrier than the capillary endothelium. About one-third of the alveolar epithelial cells are type I cells, but these cells make up approximately 95% of the cellular surface area. The remaining two-thirds of the alveolar epithelial cells that comprise the remaining 5% of the cellular surface area are the surfactant-producing cuboidal type II cells.43,45,52 Type I cells have thin cytoplasmic extensions and exhibit a large number of plasmalemmal invaginations called caveolae, which may play a role in macromolecular and protein transport across the blood-air barrier of the lung.44,45,58,59... 

The alveolar type II cells are spherical pneumocytes which comprise only 4% of the alveolar surface area, yet they constitute 60 % of alveolar epithelial cells and 10-15 % of all lung cells. Four major functions have been attributed to alveolar type II cells ... 

In the human, the alveoli are about 200-300 im in diameter and are lined by very thin alveolar epithelial cells. The most prominent of these cells are the type I (also called type A) alveolar epithelial cells, which are only about 0.1 (Am in thickness and cover a wide area. The second most prominent of these cells are the type II cells, which are roughly cuboidal. The type II cells, which are eapable of division, are thought to serve as precursors of type I cells during lung growth or repair. The type II cells also produce surfactant, a surface tensionlowering material that lines the alveoli and reduces the tendency of the very thin-... 

The alveoli are lined by two main types of epithelial cells. T e I cells (squamous pneu-monocytes) have flattened nuclei and thin but very extensive cytoplasm covering most of the alveolar wall. Because this cell has a very large surface area it is very susceptible to injury. 

Figure 3.10 The structure of the mammalian respiratory system (A) trachea, (B) bronchiole, (C) alveolar sac with blood supply, (D) arrangement of blood vessels around alveoli, (E) arrangement of cells and airspaces in alveoli showing the large surface area available for absorption, (F) cellular structure of alveolus showing the close association between (G) the endothelial cell of the capillary (H) with erythrocytes and (I) the epithelial cell of the alveolar sac. The luminal side of the epithelial cell is bathed in fluid, which also facilitates absorption and gaseous exchange. Source From Ref. 1.

The alveolar epithelium is made up of type I cells or pneumonocytes, which cover most of the alveolar surface. These prevent fluid loss and form the thin gas-exchange barrier. Type II cells are cuboidal and are twice as numerous as the type I cell but cover only about 7% of the surface area. They are metaboli-cally active and responsible for both epithelial cell renewal and synthesizing surfactant, a phospholipid that reduces surface tension forces in the lung (50). Airway epithelium is differentiated by 12 weeks gestation, whereas alveolar epithelium differentiates from 23 weeks of gestation (51). 

Alveolar epithelium consists of three types of cells. Roughly 98 % of the alveolar surface is covered by squamous (type I) epithelial cells, from which thin sheets of cytoplasm extend to cover large areas of surface of even several alveoli (Haies et al. 1981). Type II epithelial cells, though more numerous but compact, occupy only 2 % of the alveolar surface. Type III cells, also called alveolar brush cells, are rare. 

The lung is the largest epithelial surfaee in the human body, the alveolar surface area being estimated at 70 m. It is highly complex and constitutes a three dimensional structure of unique cells lining a series of bifurcating... 

---