Apical surface

Airway cross-sections have the nominal anatomy shown in Fig. 5.16. Airway surface liquid (AST), primarily composed of mucus gel and water, surrounds the airway lumen with a thickness thought to vary from 5 to 10 mm. AST lies on the apical surface of airway epithelial cells (mostly columnar ciliated epithelium). This layer of cells, roughly two to three cells thick in proximal airways and eventually thinning to a single cell thickness in distal airways, rests along a basement membrane on its basal surface. Connective tissue (collagen fibers, basement membranes, elastin, and water) lies between the basement membrane and airway smooth muscle. Edema occurs when the volume of water within the connective tissue increases considerably. Interspersed within the smooth muscle are respiratory supply vessels (capillaries, arteriovenous anastomoses), nerves, and lymphatic vessels. 

Figure 6, Polarized epithelial cells in culture. Epithelial cells in culture possess an apical surface with microvilli that faces the tissue culture medium (equivalent to the lumenal side of the cells in vivo), and a basolateral surface that faces the tissue culture dish (equivalent to the blood side of the cells in vivo).

Enterocytes in the proximal duodenum are responsible for absorption of iron. Incoming iron in the Fe " state is reduced to Fe " by a ferrireductase present on the surface of enterocytes. Vitamin C in food also favors reduction of ferric iron to ferrous iron. The transfer of iron from the apical surfaces of enterocytes into their interiors is performed by a proton-coupled divalent metal transporter (DMTl). This protein is not specific for iron, as it can transport a wide variety of divalent cations. 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

The membrane surface facing the lumen is called the apical surface, and the membrane surface on the side facing blood is called the basolateral surface. The intestinal cells are joined at the tight junctions [63,75]. These junctions have pores that can allow small molecules (MW < 200 Da) to diffuse through in aqueous solution. In the jejunum, the pores are 7-9 A in size. In the ileum the junctions are tighter, and pores are 3-4 A in size (i.e., dimensions of mannitol) [63]. 

The apical surface is loaded with more than 20 different digestive enzymes and proteins the protein lipid ratio is high 1.7 1 [63]. The half-life of these proteins is 6-12 h, whereas the epithelial cells last 2-3 days. So the cell must replace these constituents without depolarizing itself. The cytoskeleton may play a role... 

Interestingly, homologues of some of the PAR proteins have been found in other metazoans where they also have a polarized distribution. For instance, in Drosophila, bagooka encodes a PAR-3 homologue that localizes to the apical surface of epithelia cells and neuroblasts moreover, bagooka is required for... 

The superficial cells are irregular arrays of polygonal cells with a diameter of 40-60 pm and a thickness of 2-6 pm each. These cells, the most differentiated cells of the epithelium, possess microvilli in their apical surfaces, which are covered with a glycocalyx. It is, however, controversial whether mucus exists on their surface [58,59], As cell division occurs in the basal cells of the cornea, the daughter cells move toward the surface while becoming more differentiated. As the daughter cells migrate toward the outermost layer, the superficial cells are... 

In the enterocyte as it enters the absorptive zone near to the villus tips, dietary iron is absorbed either directly as Fe(II) after reduction in the gastrointestinal tract by reductants like ascorbate, or after reduction of Fe(III) by the apical membrane ferrireductase Dcytb, via the divalent transporter Nramp2 (DCT1). Alternatively, haem is taken up at the apical surface, perhaps via a receptor, and is degraded by haem oxygenase to release Fe(II) into the same intracellular pool. The setting of IRPs (which are assumed to act as iron biosensors) determines the amount of iron that is retained within the enterocyte as ferritin, and that which is transferred to the circulation. This latter process is presumed to involve IREG 1 (ferroportin) and the GPI-linked hephaestin at the basolateral membrane with incorporation of iron into apotransferrin. (b) A representation of iron absorption in HFE-related haemochromatosis. 

More recently, P-gp has been shown to be expressed in normal human tissues. In the gut, the expression is highly localized to the apical surfaces of the gut wall epithelium [30], where it is well placed to intercept its substrates and deposit them back into the lumen of the gut in a counter-absorptive manner. 

P-glycoprotein is not only expressed in tumor cells, but also in cells of several healthy tissues. In liver it was detected in the biliary canalicular surface of hepato-cytes and the apical surface of small biliary ductules. In the small intestine and colon, it is localized in the apical surface of columnar epithelial cells, and in kidneys it is found in the brush border membrane of proximal tubules. Moreover, it is detectable on the apical surface of small ductules in the pancreas and on the surface of cells in the medulla and cortex of adrenals [2]. 

The OSN is a bipolar cell that extends a single dendrite to the apical surface of the epithelium. From the dendritic... 

Hair cells are neuroepithelial cells their large basolat-eral surface includes synaptic contacts with afferent and efferent nerve fibers, while the mechanically sensitive hair bundle is located on their apical surface. The hair bundle is an ensemble of 30-300 actin-filled stereocilia and a single axonemal kinocilium (Fig. 51-2). The kinocilium,... 

The GPI-anchored proteins are related to various cellular functions. For example, they participate in the protein sorting to the apical surface of polarized cells and clathrin-independent endocytosis (see Section III,... 

For AIC conditions, the apical surface of the epithelial cell layer is exposed to air after the nasal cells reached confluence on the Transwell insert, while the basolateral side is fed with culture fluid. Figure 9.3 shows TEER changes in epithelial cell layers cultured up to 20 days in LCC versus AIC methods [46], In AIC condition (initiated from day 3 after seeding), TEER peaked on day 5 and maintained above the TEER values observed for LCC counterparts. By contrast, TEER observed for LCC conditions peaked on day 2 and declined toward zero by day 15. These data indicate that human nasal epithelial cells at an air interface culture exhibit better electrophysiological characteristics than those cultured by the conventional liquid-covered conditions. 

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. 

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