It completely covers, or lines, all external body surfaces as well as nearly all internal body surfaces. Several internal organs, such as the liver, kidneys, and lungs as well as other internal structures, like some glands, are primarily made up of epithelial tissue. Epithelial tissue provides both a protective barrier from the environment for other tissues and organs but also acts as an interface with the outside world.
The intrinsic cascade which has less in vivo significance in normal physiological circumstances than the extrinsic cascade is initiated when contact is made between blood and exposed negatively charged surfaces.
The extrinsic pathway is initiated upon vascular injury which leads to exposure of tissue factor, TF also identified as factor IIIa subendothelial cell-surface glycoprotein that binds phospholipid.
The green dotted arrow represents a point of cross-over between the extrinsic and intrinsic pathways. The two pathways converge at the activation of factor X to Xa. Active factor Xa hydrolyzes and activates prothrombin to thrombin.
Factor XIIIa also termed transglutaminase cross-links fibrin polymers solidifying the clot. The kinins are released from both high molecular weight kininogen HMWK and low molecular weight kininogen LMWK as a result of activation of either tissue kallikrein or plasma kallikrein.
The kallekreins themselves exist in inactive pre-forms. The kinins are involved in many physiological and pathological processes including regulation of blood pressure and flow via modulation of the renin-angiotensin pathwayblood coagulation, cellular proliferation and growth, angiogenesis, apoptosisand inflammation.
Kinin action on endothelial cells leads to vasodilation, increased vascular permeability, release of tissue plasminogen activator tPAproduction of nitric oxide NOand the mobilization of arachidonic acid, primarily resulting in prostacyclin PGI2 production by endothelial cells.
Although the activities of the kallikrein-kinin system are involved in numerous processes, this section will deal only with their function in blood coagulation.
With respect to hemostasis the most important kinin is bradykinin which is released from HMWK. The two forms of prekallikrein, plasma and tissue, are derived from distinct genes on different chromosomes.
The plasma kallikrein gene symbol KLKB1 is on chromosome 4q34—q35 and the tissue kallikrein gene symbol KLK1 located on chromosome 19q When plasma prekallikrein is activated to kallikrein it cleaves HMWK in a two-step process that ultimately releases bradykinin.
Bradykinin is a 9-amino acid vasoactive peptide that induces vasodilation and increases vascular permeability. Activated tissue kallikrein cleaves lysyl-bradykinin also called kallidin from LMWK. Lysyl-bradykinin is bradykinin with a lysine residue at the N-terminus making it a amino acid vasoactive peptide.
Its activities are essentially identical to those of bradykinin. Exons 1 to 9 encode the heavy chain of both kininogens. Exon 10 encodes bradykinin as well as the light chain of HMWK. Exon 11 encodes the light chain of LMWK. The heavy and light chain nomenclature refers to the disulfide-bonded structure of each kininogen after their activation, which results from kallikrein cleavage.
The protein circulates in the plasma as single-chain polypeptide with a molecular weight of 88— kDa dependent upon the level of glycosylation.
The heavy chain is 64 kDa and contains domains 1, 2, and 3 whereas the light chain is 45—56 kDa and comprises domains 5 and 6.
The heavy and light chains are linked together through domain 4 which also contains the bradykinin sequence. Domain 1 contains a low affinity calcium-binding site. Domain 3 also has platelet and endothelial cell-binding activity. Domain 5 has sequences for heparin binding, cell-binding sites, and antiangiogenic properties.
The binding of HMWK to negatively charged surfaces occurs through a histidine region of the light chain which is in domain 5. Domain 6 contains the prekallikrein and factor XI-binding sites.A single cell layer of epithelial tissue is called simple epithelial tissue, while stratified epithelial tissue is an epithelial tissue that is more than one cell layer thick.
Epithelium (/ ˌ ɛ p ɪ ˈ θ iː l i ə m /) is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.
Epithelial Tissue Function Epithelial tissue covers the outside of the body and lines organs, vessels (blood and lymph), and cavities. Epithelial cells form the thin layer of cells known as the endothelium, which is contiguous with the inner tissue lining of organs such as the brain, lungs, skin, and heart.
Tissue definition, an aggregate of similar cells and cell products forming a definite kind of structural material with a specific function, in a multicellular organism. See more. Epithelium (/ ˌ ɛ p ɪ ˈ θ iː l i ə m /) is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous attheheels.comlial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.
An example is the epidermis, the . Epithelial Tissue. Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity. Two forms occur in the human body: Covering and lining epithelium– forms the outer layer of the skin; lines open cavities of the digestive and respiratory systems; covers the walls of organs of the closed ventral body cavity.; Glandular epithelium– .