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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
The other pathway for movement across epithelial barriers is to go around the cell, through the intercellular junctions that join adjacent cells together. This is termed the “paracellular pathway.” Movement of solutes and water through the paracellular pathway can only be passive. There is no way to harness cell energy directly to this process, and paracellular transport is completely dependent on concentration gradients across the barrier epithelium, or indirectly harnessed to osmotic or electrical gradients established by other activities of the epithelial cell.
Magnesium homeostasis
Published in Kupetsky A. Erine, Magnesium, 2019
Ravi Sunderkrishnan, Maria P. Martinez Cantarin
The intestinal cells process magnesium both via a paracellular transport and in an active transcellular manner, similar to the nephrons. Paracellular transport depends on electrical voltage between the intestinal epithelial lumen and the blood. The active transcellular absorption takes place via transient receptor potential melastatin-6 (TRPM6), which facilitates the apical entry of the Mg+2 cation from the lumen. This transporter is present in the intestinal cells and the distal convoluted tubule (DCT).13
Importance of the Microcirculation to Intestinal Secretion
Published in T. S. Gaginella, Regulatory Mechanisms — in — Gastrointestinal Function, 2017
Active transport of glucose and amino acids increases epithelial conductance through a specific effect of increasing tight junctional permeability.18This process requires mucosal nutrients, Na, and metabolic energy, and is mediated through activation of Na-coupled organic transport and cytoskeletal structures. This response is dependent on adequate oxygenation, and therefore ischemia may reduce mucosal permeability through tight junctions.142Dilations form within the tight junctions, and their resistance to paracellular transport decreases. This process increases the absorption of nutrients past the tight junctions through increased solvent drag. The solvent drag is driven by the active transport into, and consequent osmotic pressure in, the lateral spaces. Pressure-driven secretion across the epithelium would be increased when tight junction dilations occur, but the concurrent active absorption may mask this secretion.143
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
Formulation of acyclovir-loaded solid lipid nanoparticles: 2. Brain targeting and pharmacokinetic study
Published in Pharmaceutical Development and Technology, 2019
Sanaa A. El-Gizawy, Gamal M. El-Maghraby, Asmaa A. Hedaya
The blood-brain barrier (BBB) is the major obstacle for the delivery of many drugs to the brain. The physicochemical and structural characters of the drugs, such as the molecular weight, hydrogen bonding, and the lipophilicity are the major parameters that affecting the passage of the drugs across the BBB (Qin et al. 2010; Matsumoto et al. 2016). There are several mechanisms for drug transport across the BBB. Paracellular transport is for hydrophilic substances. Transcellular passive diffusion is a mechanism for low molecular weight lipophilic drugs transport (van Rooy et al. 2011; Matsumoto et al. 2016). Most drugs enter the brain in their free form via the transcellular pathway. However, SLNs are larger and enter the brain via endocytosis. Endocytosis at the BBB is either receptor binding or adsorptive-mediated endocytosis (AME). It was observed an enhanced brain uptake due to the effect of tween 80 in LDL-mediated endocytosis and P-gp inhibition at the BBB (Prabhakar et al. 2013). AME is originated by the binding of cationic elements (such as nanoparticles coated with chitosan) to negative charges on the BBB membrane (Lu 2012). Another mechanism for the transport of drugs through the BBB is the carrier-mediated transcytosis. Only drugs that have similar characters of the endogenous carrier ligands will be transported (van Rooy et al. 2011).
Gingival epithelial barrier: regulation by beneficial and harmful microbes
Published in Tissue Barriers, 2019
Naoki Takahashi, Benso Sulijaya, Miki Yamada-Hara, Takahiro Tsuzuno, Koichi Tabeta, Kazuhisa Yamazaki
A primarily structural bond between epithelial cells is created by junctional molecules, including tight junctions, adherens junctions, and gap junctions8,26 (Figure 1). Tight junctions are responsible for paracellular transport of ions, water, and solutes due to their semipermeable structure.28 Several proteins are found in the tight junctions, such as occludin,29 claudins,30 and zonula occludens (ZO) protein ZO-1, ZO-2, and ZO-3.30–32 Occludin has been detected in the gingival epithelium’s surface layer, whereas claudin-1 was found in the uppermost layer.26 Claudins have barrier properties, which directly regulate gate function at paracellular tight junction channels.30 Adherens junctions play a vital role in controlling the junctional complex activity.8 Adherens junctions are cell-to-cell adhesion sites where the actin-based cytoskeleton and cytoplasmic components are constructed, also known as the classic cadherins function.33 The intercellular communication in gap junctions is involved in homeostasis, regeneration, and developmental processes.34 Furthermore, gap junctions regulate the reciprocal exchange of metabolites and ions of molecular weight ≤1 kDa, such as cyclic adenosine monophosphate and Ca,35+ between adjacent cells. The form of this junction is a head-to-head docking of hexameric structures called connexons, and membrane proteins called connexins.36