Surface epithelium

Four major tissue layers, from the lumen outward, form the large intestine the mucosa, submucosa, muscularis externa, and serosa (Fig. 88-2). Complete replacement of surface epithelial cells occurs approximately weekly, with the total number of epithelial cells remaining constant in normal colonic tissue. As patients age, abnormal cells accumulate on the surface epithelium and protrude into the stream of fecal matter their contact with fecal mutagens can lead to further cell mutations and eventual adenoma formation.4... 

Structurally, the large intestine is similar to the small intestine, although the luminal surface epithelium of the former lacks villi. The muscularis mucosa, as in the small intestine, consists of inner circular and outer longitudinal layers. Figure 6 illustrates a photomicrograph and diagrammatic sketches of this region. 

Pflugfelder, S.C., et al. 2000. Detection of sialomucin complex (MUC4) in human ocular surface epithelium and tear fluid. Invest Ophthalmol Vis Sci 41 1316. 

The superficial two to three cell layers of the corneal and conjunctival epithelium are the main barrier for the permeation of topically applied compounds. In this rate-limiting cell layer, the transcellular permeation is dictated by the lipophilicity of the cell membrane whereas the paracellular permeation is limited by the paracellular pore size and density. Vesicular penetration (e.g., receptor- or endocytosis-mediated) of macromolecules across surface epithelium is possible [33], However, the proposed mechanism is energy consuming (e.g., incorporation into pinocytotic vesicles and phagosomes) and thus more feasible in cell lines with abundant intracellular energy sources like corneal endothelium and RPE [34-37]. 

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. 

Fig. 5. Immunohistochemical expression of (a) hK14 by the ovarian surface epithelium (polyclonal antibody), (b) hK14 by a cystadenocarcinoma of the ovary (polyclonal antibody),...

Protective-, this is found in the oral cavity, pharynx, esophagus and anal canal. The surface epithelium is stratified squamous and may be keratinized (see Section 1.3.1). 

Side effects associated with use of topical tretinoin ointment include transient hyperemia, irritation, or burning. Ocular pharmacokinetic studies in rabbits show low levels of drug in the aqueous humor after topical application of [ H]-tretinoin, with major tissue uptake in the surface epithelium and the iris. Because retinoic acid has poor stability in the presence of light and oxygen and is insoluble in water, its formulations and clinical use are limited. Although it is capable of reversing squamous metaplasia and keratinization, these ocular surface changes are seen only in severe, not mild to moderate, cases of dry eye. 

Thus, along with the secretion of surface epithelium mucus and its mechanical shedding, there must be a constant digestion of mucus by a mucolytic enzyme present in the mucus itself (G26, G33, G34). However, the mechanism of removal of mucus from gastric mucosa with formation of its degradation product, detectable in the gastric juice as "dissolved mucoproteose (G27) needs clarification. 

Probable cellular derivation Mucoid neck cells Surface epithelium... 

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