Claudins: A large family of transmembrane proteins found in TIGHT JUNCTIONS. They take part in the formation of paracellular barriers and pores that regulate paracellular permeability.Claudin-3: A ubiquitously-expressed claudin subtype that acts as a general barrier-forming protein in TIGHT JUNCTIONS. Elevated expression of claudin-3 is found in a variety of tumor cell types, suggesting its role as a therapeutic target for specific ANTINEOPLASTIC AGENTS.Claudin-4: A claudin subtype that takes part in maintaining the barrier-forming property of TIGHT JUNCTIONS. Claudin-4 is found associated with CLAUDIN-8 in the KIDNEY COLLECTING DUCT where it may play a role in paracellular chloride ion reabsorption.Tight Junctions: Cell-cell junctions that seal adjacent epithelial cells together, preventing the passage of most dissolved molecules from one side of the epithelial sheet to the other. (Alberts et al., Molecular Biology of the Cell, 2nd ed, p22)Claudin-1: An integral membrane protein that is localized to TIGHT JUNCTIONS, where it plays a role in controlling the paracellular permeability of polarized cells. Mutations in the gene for claudin-1 are associated with Neonatal Ichthyosis-Sclerosing Cholangitis (NISCH) Syndrome.Occludin: A MARVEL domain protein that plays an important role in the formation and regulation of the TIGHT JUNCTION paracellular permeability barrier.Claudin-5: A claudin subtype that is found localized to TIGHT JUNCTIONS in VASCULAR ENDOTHELIAL CELLS. The protein was initially identified as one of several proteins which are deleted in VELOCARDIOFACIAL SYNDROME and may play an important role in maintaining the integrity of the BLOOD-BRAIN BARRIER.Membrane Proteins: Proteins which are found in membranes including cellular and intracellular membranes. They consist of two types, peripheral and integral proteins. They include most membrane-associated enzymes, antigenic proteins, transport proteins, and drug, hormone, and lectin receptors.Zonula Occludens-2 Protein: A zonula occludens protein subtype found in epithelial cell junctions. Several isoforms of zonula occludens-2 protein exist due to use of alternative promoter regions and alternative mRNA splicings.Claudin-2: A claudin subtype that is associated with the formation of cation-selective channels and increased epithelial permeability. It is localized to the TIGHT JUNCTIONS of the PROXIMAL KIDNEY TUBULE and INTESTINAL EPITHELIUM.MARVEL Domain Containing 2 Protein: A tight junction-associated MARVEL protein that may play a role in separating the endolymphatic and perilymphatic spaces of the ORGAN OF CORTI. Defects in the gene that codes for MARVELD2 protein are a cause of deafness autosomal recessive type 49.Zonula Occludens-1 Protein: A 195-kDa zonula occludens protein that is distinguished by the presence of a ZU5 domain at the C-terminal of the molecule.Zonula Occludens Proteins: A family of proteins that play a role in TIGHT JUNCTION formation by binding to and anchoring proteins to the ACTIN CYTOSKELETON.Clostridium perfringens: The most common etiologic agent of GAS GANGRENE. It is differentiable into several distinct types based on the distribution of twelve different toxins.Permeability: Property of membranes and other structures to permit passage of light, heat, gases, liquids, metabolites, and mineral ions.Epithelial Cells: Cells that line the inner and outer surfaces of the body by forming cellular layers (EPITHELIUM) or masses. Epithelial cells lining the SKIN; the MOUTH; the NOSE; and the ANAL CANAL derive from ectoderm; those lining the RESPIRATORY SYSTEM and the DIGESTIVE SYSTEM derive from endoderm; others (CARDIOVASCULAR SYSTEM and LYMPHATIC SYSTEM) derive from mesoderm. Epithelial cells can be classified mainly by cell shape and function into squamous, glandular and transitional epithelial cells.Tight Junction Proteins: Proteins that take part in the formation or structure of TIGHT JUNCTIONS.Nephrons: The functional units of the kidney, consisting of the glomerulus and the attached tubule.Loop of Henle: The U-shaped portion of the renal tubule in the KIDNEY MEDULLA, consisting of a descending limb and an ascending limb. It is situated between the PROXIMAL KIDNEY TUBULE and the DISTAL KIDNEY TUBULE.Dogs: The domestic dog, Canis familiaris, comprising about 400 breeds, of the carnivore family CANIDAE. They are worldwide in distribution and live in association with people. (Walker's Mammals of the World, 5th ed, p1065)Madin Darby Canine Kidney Cells: An epithelial cell line derived from a kidney of a normal adult female dog.Electric Impedance: The resistance to the flow of either alternating or direct electrical current.Cell Membrane Permeability: A quality of cell membranes which permits the passage of solvents and solutes into and out of cells.Junctional Adhesion Molecules: A family of membrane glycoproteins localized to TIGHT JUNCTIONS that contain two extracellular Ig-like domains, a single transmembrane segment, and a cytoplasmic tail of variable length.Keratosis, Actinic: White or pink lesions on the arms, hands, face, or scalp that arise from sun-induced DNA DAMAGE to KERATINOCYTES in exposed areas. They are considered precursor lesions to superficial SQUAMOUS CELL CARCINOMA.Freeze Fracturing: Preparation for electron microscopy of minute replicas of exposed surfaces of the cell which have been ruptured in the frozen state. The specimen is frozen, then cleaved under high vacuum at the same temperature. The exposed surface is shadowed with carbon and platinum and coated with carbon to obtain a carbon replica.