Columnar cells 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. 

Cilia are thin cylindrical hair-like structures with a cross-sectional radius of 0.1 gm projecting from the apical epithelial surface of ciliated columnar cells. Ciliary length is thought to correspond to periciliary fluid depth and range from approximately 7 gm in proximal airways to roughly 5 gm in more distal airways.- Each ciliated epithelial cell supports approximately 200 cilia at a density of eight cilia/gm. Short microvilli, possibly associated with secretory functions, are interspersed among the cilia. 

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 highly vascularized respiratory epithelium is composed of five major cell types ciliated cells, nonciliated cells, columnar cells, goblet cells, and basal cells. Low numbers of neurosecretory cells are present in the basement membrane [17]. Approximately 20% of the total number of cells in the lower turbinate area is ciliated with fine projections ( 100 per cell) on the apical cell surface. Cilia are used to transport the mucus toward the nasopharynx. These long (4-6 fxm) and thin projections are mobile and beat with a frequency of 1,000 strokes per min. Ciliated and nonciliated columnar cells are populated with about 300 microvilli per cell, which help in enlarging the surface area. 

The airway surface is formed by columnar ciliated and nonciliated cells interspersed with mucus-secreting goblet cells. The surface area of the epithelium is increased by the presence of numerous microvilli. The epithelial cells are held together at their apical surface by tight... 

The stomach has a surface epithelium composed of a single layer of columnar cells with few apical microvilli. The epithelial lining of the small intestine consists of a single layer of columnar cells with densely packed microvilli to promote absorption. The epithelium in the large intestine is similar to that in the small intestine except for the absence of villi in the large intestine. 

A monolayer of multiple cell types comprises the intestinal epithelium. Entero-cytes, the absorptive cells, are the main focus in drug delivery issues. In addition to enterocytes, there are undifferentiated crypt cells, mucus-secreting goblet cells, and M cells, which sample the gut lumen for the lymphatic system.6 However, it is the columnar villar enterocyte that forms the primary barrier to absorption. An extensive network of microvilli on the apical surface of villar enterocytes provides a vast amount of surface area for absorption of nutrients, pharmaceuticals, and xenobiotics.6,7... 

A FIGURE 6-5 The principal types of cell Junctions that connect the columnar epithelial cells lining the small intestine, (a) Schematic cutaway drawing of intestinal epithelial cells. The basal surface of the cells rests on a basal lamina, and the apical surface is packed with fingerlike microvilli that project into the intestinal lumen. Tight Junctions, lying Just under the microvilli, prevent the diffusion of many substances between the intestinal lumen and the blood through the extracellular space... 

In the nasal cavity of the rat, the nonciliated columnar cells have an extensive accumulation of smooth endoplasmic reticulum in the apical cytoplasm (Monteiro-Riviere and Popp 1984, Popp and Monteiro-Riviere 1985) which in some cases was so dense as to exclude other organelles such as mitochondria. The nonciliated columnar epitheUal cell was identified on the surfaces of the conchae and the lateral nasal wall but not identified on the septum. 

P-gp is constitutively expressed in nearly all barrier tissues. Techniques involving Northern blots (37) or Western blots with monoclonal antibodies such as C219 (38) and MRK 16 (39) have been used extensively to determine the tissue distribution of P-gp. It is expressed in adrenal cortex, kidney, liver, intestine, and pancreas endothelial cells at blood-tissue barriers, namely, the CNS, the testis, and in the papillary dermis (3,4,38,40,41). P-gp displays specific subcellular localization in cells with a polarized excretion or absorption function. More specifically, P-gp is found at the apical (AP) canalicular surface of hepatocytes, in the AP membrane of the columnar epithelial cells of colon and jejunum, and the AP brush border of the renal proximal tubule epithelium (3,4,40 1-2). In endothelial cells, P-gp is located in the luminal membrane (4,43). 

A FIGURE 6-4 Principal types of epithelium. The apical and basolateral surfaces of epithelial cells exhibit distinctive characteristics, (a) Simple columnar epithelia consist of elongated cells, including mucus-secreting cells (in the lining of the stomach and cervical tract) and absorptive cells (in the lining of the small intestine), (b) Simple squamous epithelia, composed of thin cells, line the blood vessels (endothelial cells/endothelium) and many body cavities, (c) Transitional epithelia, composed of several layers of cells with different shapes, line certain cavities subject to expansion and contraction (e.g., the urinary bladder). 

Intestinal absorptive cells are simple columnar, highly polarized cells (Fig. 4). The luminal plasma membrane is modified to form numerous fingerlike projections, the microvilli, which markedly increase the total absorptive surface of the cell. Directly beneath the microvillous border is an area relatively devoid of cellular organelles, but containing numerous filaments extending from the microvilli into the cytoplasm. This region is termed the terminal web zone of the cell. Below the terminal web area is the apical cytoplasm, which extends to the level of the nucleus and Golgi complex. The apical cytoplasm, in the fasted... 

---