Endothelial layer barrier

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

Atherosclerosis is accepted as a common mechanism underlying all CVDs [2], Atherosclerosis is a disease of large and medium-sized arteries. It affects all three coats of the arterial wall in its more advanced stages. The arterial wall consists of three layers intima, media, and adventitia. The most inner luminal part of the intima is a monolayer of endothelial cells lining the whole wall. The intact endothelial layer is a selective barrier for plasma lipids and also has antitrombotic properties [3,7]. The pathogenesis of atherosclerosis can be divided into three main stages. 

Hypoxia (low oxygen tension), which can be induced by a blood phase transport limitation, can lead to a breakdown of the endothelial transport barrier either by a direct effect on the endothelial layer or by an indirect mechanism in which hypoxia up-regulates the production of hyperpermeabilizing cytokines from other cells in the arterial wall. A number of recent studies have shown that hypoxia increases macro-molecular transport across endothelial monolayers in culture due to metabolic stress [40-42]. These studies describe direct effects on the endothelial layer since other cells present in the vessel wall were not present in the cell culture systems. 

The normal arterial wall consists of the intima, media, and adventitia, as illustrated in Fig. 4—3A. The endothelium is located in the intima and consists of a layer of endothelial cells that line the lumen of the artery and form a selective barrier between the vessel wall and blood contents. The internal elastic lamina separates the intima and media, where vascular smooth muscle cells are found. The vascular adventitia comprises the artery s outer layer. Atherosclerotic lesions form in the subendothelial space between the endothelial cells and internal elastic lamina. 

Figure 11.1 Ultrastructure of the human lung alveolar barrier. The tissue specimen is obtained via lung resection surgery. (A) Section through a septal wall of an alveolus. The wall is lined by a thin cellular layer formed by alveolar epithelial type I cells (ATI). Connective tissues (ct) separate ATI cells from the capillary endothelium (en) within which an erythrocyte (er) and granulocyte (gc) can be seen. The minimal distance between the alveolar airspace (ai) and erythrocyte is about 800-900 nm. The endothelial nucleus is denoted as n. (B) Details of the lung alveolar epithelial and endothelial barriers. Numerous caveolae (arrows) are seen in the apical and basal plasma membranes of an ATI cell as well as endothelial cell (en) membranes. Caveolae may partake transport of some solutes (e.g., albumin). (C) ATII cells (ATII) are often localised in the comers of alveoli where septal walls branch off. (D) ATII cells are characterised by numerous multilamellar bodies (mlb) which contain components of surfactant. A mitochondrion is denoted as mi.

Added to these two layers is an additonal barrier formed by the brain itself. Foot-like projections of brain cells called astrocytes encase the collagenous basement membrane forming a glial sheath. These cells are believed to "induce" the formation of the tight junction of the endothelium. In addition the endothelial blood cells and astrocytes contain enzymes which can alter an invading... 

Barriers to pulmonary absorption of proteins and peptides include respiratory mucus, mucociliary clearance, pulmonary enzymes/proteases, alveolar lining layer, alveolar epithelium, basement membrane, macrophages and other cells [3, 18]. The molecular weight cutoff of tight junctions for alveolar type I cells is 0.6 nm, while endothelial junctions allow the passage of larger molecules (4-6 nm). In order to reach the bloodstream in the endothelial vasculature, proteins and peptides must cross this alveolar epithelium, the capillary endothelium, and the intervening extracellular matrix. 

The distribution of drugs depends on both the physicochemical properties of the drug molecules and the composition of tissue membranes. These factors can either result in a uniform or uneven distribution of dmgs into the various body compartments and fluids. In the extreme, distribution may tend toward an accumulation of drugs in particular tissues or to an almost complete exclusion of the drag from a particular compartment in a defined length of time. One unique compartment that has to be considered in this respect is the brain, which is separated from the capillary system of the blood by the blood-brain barrier, whose membrane has a special structure. It consists of a cerebral capillary network formed by a capillary endothelium that consists of a cell layer with continuous compact intercellular junctions. It has no pores, but special cells, astrocytes, which support the stability of the tissues, are situated at the bases of the endothelial membrane separating the brain and CSF from the blood. The astrocytes form an envelope around the capillaries. 

From Figure 1.3 it can be seen that in order to reach the underlying blood capillaries to be absorbed, the drug must pass through at least two epithelial membrane barriers (the apical and basolateral epithelial cell membranes) and also the endothelial membrane of the capillaries. In some cases, for example in stratified epithelia such as that found in the skin and buccal mucosa, the epithelial barrier comprises a number of cell layers rather than a single epithelial cell. Thus the effective barrier to drag absorption is not diffusion across a single membrane as described above, but diffusion across the entire epithelial and endothelial barrier, which may comprise several membranes and cells in series. 

Biodistribution of plasmid to either extracellular or intracellular targets is dependent on the structure of capillary walls, (patho)physiological conditions, the rate of blood and lymph supply, the physicochemical properties of plasmid and its carrier molecules. The fate of plasmid after in vivo administration is illustrated in Figure 14.4. The blood capillary walls are comprised of four layers, namely plasma-endothelial interface, endothelium, basal lamina, and adventia. Macromolecules can cross the endothelial barrier ... 

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