Epithelial lining

Cantin, A.M., Hubbard, R.C. and Crystal, R.G. (1989). Glutathione deficiency in the epithelial lining fluid of the lower respiratory tract of patients with idiopathic pulmonary fibrosis. Am. Rev. Resp. Dis. 139, 370-372. 

Salmonella are motile, non-lactose-fermenting, gram-negative bacilli. In salmonellosis, the organisms penetrate the epithelial lining to the lamina propia with production of diffuse inflammation. The distal ileum and colon are sites of infection. 

Soluble aerosols deposited on the epithelial linings of the respiratory tract are absorbed and metabolized in the same way as soluble gases and vapors. 

It should be noted that the permeability per surface unit of alveolar epithelium per se is not particularly high. The significant absorption found for various substances after pulmonary administration is rather explained by a number of beneficial factors such as the large surface area of the alveoli, the low volume of the epithelial lining fluid, the relatively thin diffusion layer, the absence of mucociliary clearance from the alveoli as well as the limited enzymatic activity in the lining fluids. 

Proteases occurring in the epithelial lining fluids are another source of variability in protein absorption from the lung. In the epithelial lining fluids proteases and peptidases do occur. Inhibition of proteases and peptidases with substances such as bacitracin or aprotinin might improve the bioavailability of proteins. For example bacitracin was shown to increase insulin bioavailability by a factor of 6.8 [3]. 

Tandon et al. (1975) applied daily dermal applications of 132.5 mg thorium/kg body weight/day (15 nCi/kg/day = 555 Bq/kg/day), 265 mg thorium/kg body weight/day (29 nCi/kg/day = 1073 Bq/kg/day), or 529 mg thorium/kg body weight/day (58 nCi/kg/day = 2146 Bq/kg/day) to the lateroabdominal and scrotal areas of rats for 15 days. The thorium was administered to skin (hair was clipped) as thorium nitrate, and the area remained uncovered for the duration of treatment. Mild hyperkeratinization of the lateroabdominal skin was found at all exposure levels. At the highest exposure level, mild acanthosis and thickening of the epithelial lining of the lateroabdominal skin were seen. At this level, mild acanthosis, swollen collagen fibers, and foamy dermis were found in the scrotal skin. The value of 15 nCi/kg/day is a less serious LOAEL, and the exposure level of 58 nCi/kg/day is a serious LOAEL for dermal effects in the rat. These values are reported in Table 2-3. 

In rodents and humans, ethylene dibromide is metabolized both by cytochrome P450 and GST enzymes the latter seem to be responsible for DNA adduct formation. In rodents, covalently bound radioactivity has been detected in the epithelial lining of a number of organs. 

As mentioned above, the villi of the small intestine (Figure 1.2) house a dynamic, self-renewing population of the epithelial cells that includes absorptive cells (enterocytes), secretory cells, and endocrine cells. The thin lining (height 25 p,M height of the microvilli is 1.5 pM) of the columnar enterocytes is the only barrier between the intestinal lumen and the muscularis mucosa, which represents, in this context, the entire body interior. The entire epithelial lining of the intestine replaces itself every 3-5 d [128], It is the enterocyte and its neighboring cells where absorption processes occur and it will therefore be the focus of the mechanistic discussions below. 

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