Epithelium cell layer

In general, when molecules transport through the epithelium cell layer directly into the mesenteric blood draining the small intestine, the total mass transfer resistance may be described by the sum of resistance for barriers (aqueous boundary layer in front of the membrane, the membrane itself, and the aqueous boundary layer in blood side) in series ... 

Permeability and Associated Assays Orally administered drugs need to cross the intestinal epithelium cell layer to reach the systemic circulation. Therefore, membrane permeability is a major determining factor of intestinal absorption and oral bioavailability for drug molecules. Several in vitro systems mimicking the epithelium cell layer are routinely used in ADME profiling to assess membrane permeability of NCEs. The parallel artificial membrane permeability assay (PAMPA) uses a dual chamber sandwich plate separated by an artificial lipid membrane to simulate the epithelium layer (Figure 6.6A) [75]. Compounds are... 

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. 

A similar process of differentiation has been demonstrated in the basal cell layer of the squamous epithelium of the skin which seems to contain the stem cells... 

Thus far, a wide array of useful cell culture models of the corneal epithelium has been established. Many of these cell culture models focus on toxicity testing and ocular irritation, but some cell layer models for drug permeation studies are also available. Indispensable for successful drug penetration testing is a cell layer that exhibits a tight epithelial barrier. This latter requirement of tight barrier properties disqualifies some of the models that were established as substitutes for the Draize test. At least two cell lines are available for pharmaceutical studies and some newer models may qualify as a useful tool, once they are characterized for their barrier properties. 

Figure 14.1 Schematic diagram of the blood-retinal barrier (BRB). The retinal cell layers seen histologically consist of retinal pigment epithelium (RPE) photoreceptor outer segments (POS) outer limiting membrane (OLM) outer nuclear layer (ONL) outer plexiform layer (OPL) inner nuclear layer (INL) inner plexiform layer (IPL) ganglion cell layer (GCL) nerve fiber layer (NFL) inner limiting membrane (ILM).

The epithehal cell layer forms the major barrier to absorption of drug molecules. In the large airways stratified epithehum occurs, whereas in the alveoh the epithelium is only one cell layer thick. 

Epithelial, epithelium The layers of cells and connecting tissues that form the covering of internal and external body surfaces, including the lining of small vessels and cavities. 

No exposure-related clinical signs or lesions of systemic toxicity and no oncogenic responses were observed in rats exposed by inhalation at concentrations of 0, 15, 45, or 135ppm 6 hours/day, 5 days/week, for 24 consecutive months." Dose-related changes occurred in the anterior portion of the olfactory epithelium and consisted of atrophy of the neurogenic epithelial cells followed by progressive hyperplasia of the reserve cells and ultimately loss of the upper epithelial cell layer. Opacity and neovascularization of the cornea were also observed in methyl acrylate-exposed animals. 

Extensive destruction of the olfactory epithelium was observed in male Fischer 344 rats exposed to 200 ppm [780 mg/m ] methyl bromide for 6 h per day for five days. By day 3, despite continued exposure, there was replacement of the olfactory epithelium by a squamous-cell layer, followed by progressive reorganization toward the normal architecture, and by week 10,75-80% of the epithelium appeared histologically normal. Olfactory epithelial-cell replication was maximal on day 3 of exposure, with a labelling index of 14.7% compared with 0.7% in the controls (Hurtt et al., 1988). Degeneration and subsequent regeneration were also observed in an inhalation experiment w ith Fischer 344 rats exposed to 175 ppm [680 mg/m ] 6 h twice, separated by a 28-day interval (Bolon et al., 1991). 

Squamous epithelium A layer of flattened epithelial cells. 

The thickness of the buccal epithelium varies from 10 to about 50 cell layers in different regions because of serrations in connective tissue. In fact, the thickness of buccal mucosa has been observed to be 580 pm, the hard palate 310 pm, the epidermis 120 pm, and the floor of mouth mucosa 190 pm. 

The vaginal fluid is also characterized by a certain enzymatic activity due to bacterial flora and enzymes present in the secretions and in the epithelium. The external and basal cell layers of the vagina express the majority of the enzymatic activity [2,5,9]. 

After administration, either drug penetration into the deep cell layers to obtain a local effect or drug permeation across the epithelium to reach the systemic circulation can occur. 

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