The Extrinsic Gastrointestinal.
The gastrointestinal mucosa forms a barrier between the body and a lumenal environment which not only contains nutrients, but is laden with potentially hostile microorganisms and toxins. Damage to the gastrointestinal barrier due to ischemia and reperfusion injury is a common and serious condition. Ischemia occurs when blood flow is insufficient to deliver an amount of oxygen and nutrients necessary for maintenance of cell integrity. In vertebrates, mucus (/mjuːkəs/ MYOO-kəss; adjectival form : mucous) is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from cells found in mucous glands. In the human female reproductive system, cervical mucus prevents infection. The consistency of cervical mucus varies depending on the stage of a woman's menstrual cycle. At ovulation cervical mucus is clear, runny, and conducive to sperm; post-ovulation, mucus becomes thicker and is more likely to block sperm. The increased focus on mucus as a barrier to drug delivery during the past years has resulted in the very recent publication of a number of manuscripts describing systems to meet the requirement for relevant mucus-containing in vitro models. Figure 2. The steric (left) and interactive (right) barrier properties of mucus. The mucus forms a filter, which can prevent the diffusion across the gel according to size (left) or surface properties (right) of the diffusing compound. The Extrinsic Gastrointestinal Barrier. Mucus and Bicarbonate. The entire gastrointestinal epithelium is coated with mucus, which is synthesized by cells that form part of the epithelium. Mucus serves an important role in mitigating shear stresses on the epithelium and contributes to barrier function in several ways. Damage to the gastrointestinal barrier due to ischemia and reperfusion injury is a common and serious condition. Ischemia occurs when blood flow is insufficient to deliver an amount of oxygen and nutrients necessary for maintenance of cell integrity. 1 The intrinsic barrier is composed of the epithelial cells lining the digestive tube and the tight junctions that tie them together. 2  The extrinsic barrier consists of secretions and other influences that are not physically part of the epithelium, but which affect the epithelial cells and maintain their barrier function. Damage to the gastrointestinal barrier due to ischemia and reperfusion injury is a common and serious condition. Ischemia occurs when blood flow is insufficient to deliver an amount of oxygen and nutrients necessary for maintenance of cell integrity. Viscoelastic mucus lines all mucosal surfaces of the body and forms a potential barrier to mucosal drug delivery. Mucus is mainly composed of water and mucins; high molecular weight glycoproteins forming an entangled network. Figure 2. The steric (left) and interactive (right) barrier properties of mucus. The mucus forms a filter, which can prevent the diffusion across the gel according to size (left) or surface properties (right) of the diffusing compound. The mucus lining of the gastrointestinal tract epithelium is recognized as a barrier to efficient oral drug delivery. Recently, a new in vitro model for assessment of drug permeation across intestinal mucosa was established by applying a biosimilar mucus matrix to the surface of Caco-2 cell monolayers. As PIM disrupted the Caco-2 cell monolayers, a cell-free mucus barrier model was implemented in the studies. Both the biosimilar mucus and the PIM reduced the permeation of the selected peptide drugs to varying degrees illustrating the interactive properties of both mucus matrices. Therefore, the availability of an in vitro model incorporating a mucus barrier is of key importance when evaluating the mucosal drug permeation of oral drug candidates or delivery systems. A biosimilar mucus matrix with in vivo relevant composition and rheological properties has recently been described [10] and [11] . As PIM disrupted the Caco-2 cell monolayers, a cell-free mucus barrier model was implemented in the studies. Both the biosimilar mucus and the PIM reduced the permeation of the selected peptide drugs to varying degrees illustrating the interactive properties of both mucus matrices. For successful oral drug delivery, mucus is a barrier that needs to be overcome or a lining that can be exploited for improving the bioavailability of a drug, for example by increased mucoadhesion of the drug delivery system. Figure 2. The steric (left) and interactive (right) barrier properties of mucus. The mucus forms a filter, which can prevent the diffusion across the gel according to size (left) or surface properties (right) of the diffusing compound.
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/stomach/gibarrier.html https://en.wikipedia.org/wiki/Mucus http://onlinelibrary.wiley.com/doi/10.1111/bcpt.12342/full http://www.vivo.colostate.edu/hbooks/pathphys/digestion/stomach/gibarrier.html http://www.vivo.colostate.edu/hbooks/pathphys/digestion/stomach/gibarrier.html http://onlinelibrary.wiley.com/doi/10.1111/bcpt.12342/full http://www.sciencedirect.com/science/article/pii/S093964111500017X http://www.sciencedirect.com/science/article/pii/S093964111500017X http://onlinelibrary.wiley.com/doi/10.1111/bcpt.12342/full
what type of barrier is mucus