Gastro-intestinal epithelium

The permeability of various mammalian tissues. In recent years, a great deal of work has been done on the comparative permeability of mammalian tissues. It has been found that the mechanism of absorption and distribution of foreign organic substances is much simpler than those of natural cell substrates and constituents. The following structures offer a Type i lipid barrier to the passage of many foreign molecules the gastro-intestinal epithelium, the renal tubular epithelium, the blood-brain barrier, and the blood-cerebrospinal fluid barrier (Schanker, 1961). 

Adminstration via the oral route has been, and still is, the most popular and convenient route for patient therapeutics. However, even though it is the most convenient route, it is not the simplest route, as the barriers of the gastro-intestinal (GI) tract are in many cases difficult to circumvent. The main barriers of the GI tract to systemic delivery are the environment in the stomach and intestinal lumen, the presence of different enzymes, the physical barrier of the epithelium and the liver extraction. These barriers are of functional importance for the organism in controlling intake of water, electrolytes and food constituents and still remain a complete barrier to harmful organisms such as bacteria, viruses and toxic compounds. 

The thymus is considered a key organ of the immune system in jawed vertebrates. It is thought to have evolved in early fish species as a thickening in the epithelium of the pharyngeal area of the gastro-intestinal tract (Bowden et al, 2005), and is identifiable in the Chondrichthyes and the... 

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