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Freeze Fracture in Lung Research
Published in Joan Gil, Models of Lung Disease, 2020
The morphological characterization of intercellular junctions between endothelial and epithelial cells of the lung has relied on the freeze fracture technique. At present, it affords the only means of visualizing these specialized membrane structures en face, in addition to providing useful structural correlates for functional measurements. A variety of intercellular junctions are present between endothelial and epithelial cells of the lung; these include gap junctions (macula communicans), tight junctions (zonulae occludentes), zonula adherans, and desmosomes. Only the first two of these will be discussed.
Immune function of epithelial cells
Published in Phillip D. Smith, Richard S. Blumberg, Thomas T. MacDonald, Principles of Mucosal Immunology, 2020
Richard S. Blumberg, Wayne Lencer, Arthur Kaser, Jerrold R. Turner
Intercellular junctions between adjacent epithelial cells are of course required for barrier function. Intercellular junctions are located along the lateral surfaces of the epithelial cells. They are composed of the most apically oriented tight junctions, and the subjacent adherens junction and desmosome (Figure 5.2). While the latter are required for physical integrity of the epithelial monolayer, movement across tight junctions is the rate-limiting barrier for solute and particle transport. Tight junctions are physiologically dynamic and regulate the access of small solutes, water, and macromolecules into the paracellular space. Movement across tight junctions can be considered to occur via two routes, the pore pathway and the transcellular pathway. The pore pathway is a high-capacity route that is exquisitely size selective, excluding molecules with Stokes radii greater than ∼5 Å, and are also charge selective. Intestinal tight junctions allow cations to cross via the pore pathway at rates up to 10-fold greater than anions. While this cation selectivity is critical for normal physiologic function and can be reversed to anion selectivity by modifying tight junction protein expression, it is far less than that exhibited by transmembrane ion channels. Nevertheless, the paracellular channels that define the pore pathway are actively gated, i.e., they open and close, in a manner similar to transmembrane ion channels.
The cardiovascular system
Published in C. Simon Herrington, Muir's Textbook of Pathology, 2020
Mary N Sheppard, C. Simon Herrington
The microcirculation is the capillaries, the arterioles that supply them, and the venules that drain the blood from the capillary bed. A capillary consists of a single endothelial cell encircling a lumen that only just admits the passage of red blood cells. Intercellular junctions join adjacent endothelial cells. The microcirculation is adapted to each organ and tissue. Thus, the liver sinusoids and kidney have a highly permeable fenestrated endothelium, whereas the capillaries in the brain are watertight and contribute to the blood–brain barrier. Capillary endothelial cells are surrounded by pericytes, which support them, synthesize basement membrane, and can differentiate into a variety of cell types including vascular smooth muscle cells. Capillaries act as a semipermeable membrane. They retain most of the protein but permit free exchange of fluid.
Structure–activity relationship of a peptide permeation enhancer
Published in Tissue Barriers, 2023
Drug development is often hampered by the physicochemical properties of the drug candidate. The selection of excipients may lead to an optimization of key pharmacokinetic parameters such as absorption and distribution.1 Since the majority of approved drugs are administered orally,2 sufficient intestinal absorption and resulting bioavailability is decisive for drug efficacy and safety. At epithelia, drug absorption can occur by the transcellular or paracellular pathway.3 It has been estimated that the intercellular (paracellular) space in the intestinal epithelium is about 4 Å.4 In addition, the passage of molecules through the paracellular space is regulated by networks of proteins forming different types of intercellular junctions. The most important of these connections are tight junctions (TJs),5 located at the apical side of the epithelial cells.
Correlation between Keratoconus and Pollution
Published in Ophthalmic Epidemiology, 2021
Tristan Jurkiewicz, Anne-Sophie Marty
The cornea is a complex barrier composed of three layers and the tear film which plays a role in stopping the external particles. These different layers have different physico-chemical properties and act as a barrier to block the entry of drugs or molecules into the ocular globe. The epithelium is a stratified cell membrane with tight junctions between cells. It is the most important barrier to block the penetration of molecules or particles. Then there is the corneal stroma, which is a widely hydrated tissue composed of a layer of collagen fibers with dispersed cells. And finally an endothelium composed of a monolayer of cells with large intercellular junctions. The cornea benefits from several barriers: mechanical (tight junctions of the epithelium) and chemical (hydrophobic epithelium and lipophobic stroma). The epithelium is the main barrier limiting absorption, as the stroma and endothelium offer very low resistance.36,37
Cell-cell junctions: structure and regulation in physiology and pathology
Published in Tissue Barriers, 2021
Mir S. Adil, S. Priya Narayanan, Payaningal R. Somanath
Intercellular junctions are structurally and biochemically differentiated regions of the plasma membrane through which adjacent cells interact in a specific manner. These structures were originally identified and named according to their morphology and purported function.29 To retain barrier function and to prevent the invasion of pathogens and their rapid systemic spread, cell junctions need to be kept tight and repaired quickly after vessel rupture.30 There are three functional categories of cell junction: anchoring junctions; tight, or occluding, junctions, and gap (GJ), or permeable, junctions Figure 1.17,31,32 The AJs and desmosomes provide essential adhesive and mechanical properties that contribute to barrier function but do not seal the paracellular space,33 the TJs hold cells together and form a near leakproof intercellular seal by fusion of adjacent cell membranes34 since interactions between cells are important for the assembly and maintenance of three-dimensional tissues.35 The latter is a selectively permeable barrier that generally represents the rate-limiting step of paracellular transport.33 Many cell types also possess GJs, which allow small molecules to pass from one cell to the next through channels.34