Alveolar surface cells

Vitamin C (VC, L-ascorbic acid) is known to be essential for many enzymatic reactions. Sodium-dependent VC transporters (SVCT), SVCT1 and SVCT2, were recently identified and reported to be localised in the apical cell membrane of AECs in the lung of adult rats. These results suggest that SVCT proteins could transport the reduced form of VC from the airway/alveolar surface liquid into respiratory epithelial cells [106],... 

The situation is, however, different in the alveolar region of the lung where the respiratory gas exchange takes place. Its thin squamous epithelium is covered by the so-called alveolar surface liquid (ASL). Its outermost surface is covered by a mixture of phospholipids and proteins with a low surface tension, also often referred to as lung surfactant. For this surfactant layer only, Scarpelli et al. [74] reported a thickness between 7 and 70 nm in the human lung. For the thickness of an additional water layer in between the apical surface of alveolar epithelial cells and the surfactant film no conclusive data are available. Hence, the total thickness of the complete ASL layer is actually unknown, but is certainly thinner than 1 gm. 

The alveolar epithelium consists of so-called Type I and Type II cells. Type I cells cover over 90% of the alveolar surface, have a large surface, and are thin. Type II cells are larger in numbers but are small. Therefore, they cover only about 7% of the surface of the alveoli. Type II cells produce the phospholipids that make up the surfactant layer. 

T e II cells (granular pneumonocytes) are distributed throughout the alveoli between T e I cells. Although they are more numerous than T e I cells, they are cuboidal in shape and occupy far less of the alveolar surface area. The prime function of this cell is the production of pulmonary surfactant, and it is generally less susceptible to injury than the Type I cell. 

Despite the fact that most of the alveolar surface is composited of alveolar epithelium, three primary types of cells are present in the alveoli type I alveolar cells, type II alveolar cells, and alveolar macrophages. Type I alveolar cells are also referred to as squamous pulmonary epithelial cells and are the continuous lining of the alveolar sac. Type II alveolar cells are also referred to as septal cells. Type II alveolar cells secrete the alveolar fluid that is necessary to keep the surface moist and to maintain surface tension of the alveolar fluid surface tension is necessary to keep the alveoli from collapsing. Alveolar fluid is a suitable environment for proteins when compared to the low pH and high protease levels associated with the intestine... 

At the distal respiratory site, the alveolar epithelial cell layer is much flatter (0.1 -0.5 pm) and composed of two major cell types, squamous type I and agranular type II pneumocytes. Type I pneumocytes are non-phagocytic and highly flattened cells with broad and thin extensions. They occupy -95 % of the alveolar luminal surface, although they are less numerous than type II cells. The remaining surface is occupied by type II pneumocytes, which have blunt microvilli and contain multivesicular bodies [3, 11]. 

The pulmonary alveolar epithelium is comprised of two morphologically distinct cells, type I and type II cells. Type I cells are extremely large, squamous cells that make up 95% of the alveolar surface. Type II cells are smaller cuboidal cells that secrete and recycle surfactant and cover the remaining 5% of the alveolar surface. Mechanical distention of fetal lung tissue has been shown to stimulate expression of the type I cell phenotype and inhibit expression of the type II phenotype. Lumenal mechanical stim-... 

Type I cells comprise 8-11% of the structural cells found in the alveolar region and yet cover 90-95% of the alveolar surface. Their major function is to allow gases to equilibrate across the air-blood barrier and to prevent leakage of fluids across the alveolar wall into the lumen. The type I epithelium is particularly sensitive to damage from a variety of inhaled toxicants due to their large surface area. Moreover, their repair capacity is limited because they have few organelles associated with energy production and macromolecular synthesis. 

These studies of AS multilayer films as well as of lipid bilayers give reason to propose a new hypothesis of the structure of the alveolar surface. According to it the continuous lipid monolayer of the alveolar surface is in contact with the multilayer or with the membrane of epithelial cells situated under it, i.e. at the contact sites as well as between the individual... 

Primary infection is initiated by the alveolar implantation of organisms in droplet nuclei that are small enough (1 to 5 mm) to escape the ciliary epithelial cells of the upper respiratory tract and reach the alveolar surface. Once implanted, the organisms multiply and are ingested by pulmonary macrophages, where they are killed, or, they continue to multiply. With bacterial multiplication, the macrophages eventually rupture, releasing many bacilli. 

Type I cells are alveolar lining cells that are involved with the transfer of substances from the alveolus through the wall to the blood. Type II cells are alveolar cells with two functions oxidative enzymes for lung metabolism, and the production and secretion of the surfactant coating the alveolar surface. 

The surface of the cells lining the tracheobronchial tree and the surface of most of the cells lining the nasopharyngeal region are covered with a layer of relatively thick mucous material in the alveolar region, cells are lined with a thin film of fluid. The aqueous environment provided by these surface liquids favors at least partial dissolution and eventually absorption of water-soluble particles, especially those present as liquid droplets. Various defense mechanisms may help to remove less soluble particles from their site of deposition. 

Pulmonary drug delivery is also applicable using liposomes, because they are absorbed through the thin layer of alveolar epithelial cells (vast surface for an adult is 43-102 m ) (12-14) and transported into the systemic circulation (15-17). However, the inhaled objects can be eliminated by macrophages from the alveoli surface when they are bigger than 260 nm (18). 

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... 

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