Extracellular Matrix (ECM) PDF

Extracellular matrix 1&2 By UCM Histology Unit 1  Learning outcome 1. Outline the components of extracellular matrix. 2. Describe types & functions of the ground substance. 3. Classify types of collagen fibers based on the function of each t...

Extracellular matrix 1&2 By UCM Histology Unit 1  Learning outcome 1. Outline the components of extracellular matrix. 2. Describe types & functions of the ground substance. 3. Classify types of collagen fibers based on the function of each type. 4. Describe the biosynthesis of collagen fibers. 5. Describe the structure and biochemical characteristics of elastic fibers. 6. Compare and contrast the components of the basal lamina & basement membrane. 2  Extracellular matrix (ECM) consists of: 1. Fibrous component: ✓ Collagen fibers ✓ Elastic fibers 2. Amorphous component = Ground substance: 1. Glycosaminoglycans (GAGs) 2. Proteoglycans 3. Adhesive glycoproteins 3. Basement membrane. 3  General Functions of the ECM 1. Provides mechanical support 2. Affect metabolic functions 3. Affect proliferation and shape of cells 4. Provides pathways for cellular migration 5. Affect embryonic development 4  Specific functions of the ECM components 1) Fibrous protein: Collagen, gives strength to extracellular matrices Elastin, provides resiliency (elasticity). 2) Glycosaminoglycans (GAGs) and proteoglycans: ✓ give connective tissues a gel-like nature. 3) Adhesive glycoproteins: ✓Form a direct bridge between cells and the fibrous proteins of connective tissues. 5  Amorphous component (Ground substance) of ECM 1- Glycosaminoglycans (GAGs) Structure: Long, inflexible, unbranched polysaccharides chains of repeating disaccharide units Types: 1. Nonsulfated GAGs: eg. Hyaluronic acid 2. Sulfated GAGs: eg Chondroitin sulfate Keratan sulfate Dermatan sulfate Heparan sulfate 6  Glycosaminoglycans (GAGs) Function: GAGs are negatively charged, attract cations, especially Na+ (by electrostatic (=ionic) bonds) & attract extracellular fluid (intense hydrophilic) 7  2- Proteoglycans Structure: Sulfated GAGs attached to a protein core that bind hyaluronic acid. Sulfated GAGs Protein core hyaluronic acid 8  Proteoglycans Examples: ▪Aggrecan: ✓ keratan & chondroitin sulfate attached to protein core that bind hyaluronic acid to form aggregates. ✓is the major proteoglycan in cartilage ▪Perlecan:(heparan sulfate + protein core) in basal lamina 9  Proteoglycans Functions: 1. Resist compression and act as shock absorbers 2. Retard the rapid movement of microorganisms and metastatic cells through the ECM by occupying a large volume. 3. Filtration of macromolecules 4. Binding sites for growth factors 5. Secreted proteoglycans promote cell adhesion 10  3- Adhesive Glycoproteins The ability of cells to adhere to ECM components is mediated to a great extent by the adhesive glycoproteins. Function: These adhesive large macromolecules Adhesive are molecular glue which: Glycoproteins 1. usually binds to: ✓ Integrins of cell membrane (Transmembrane linker proteins) Integrins ✓ Collagen fibers, and ✓ Proteoglycans. 2. Provide cells with signals required for the development and repair of tissues. Most adhesive glycoproteins that interact with cells bind to integrins of the cell membrane. 11  Adhesive Glycoproteins Types : The major adhesive molecules are fibronectin & laminin 1. Fibronectin: Function: Adhesion of cells to the ECM. Has large dimer that has binding sites for various ECM components (e.g type IV collagen, hyaluronic acid) & for integrins Synthesis: Fibronectin is mainly produced by fibroblasts. hyaluronic acid Proteoglycan Fibronectin integrin Cell 12  Glycoproteins 2. Laminin: Function: It is limited to the basal lamina. Very large glycoprotein having binding sites for type IV collagen, heparan sulfate, entactin, & integrins 3. Entactin: Binds laminin to type IV collagen meshwork in basal lamina 13 NB ❑ECM is the major constituent of connective tissues, ❑Other tissues (epithelium, muscle & nervous) consist mainly of cells. ❑ In ordinary connective tissues the ground substance: ❖includes: - glycosaminoglycans, - proteoglycans, and - glycoproteins ❖is highly hydrated so it: ✓ has the properties of a semi-fluid gel. ✓ permits diffusion of the following between blood & cells: - gases, nutrients & metabolites - hormones & growth factors - microbes 14  Structural Fibers (Fibrous component of ECM) 1- Collagen 25% of all proteins in the body 21 types in humans Best understood are types I, II, III, IV and VII Named numerically in order of discovery 15  Collagen Collagen families according to function A. Fibrillar collagens: ▪ Form “fibrils” and display the 67-nm banding e.g., types I, II and III B. Sheet-forming collagens: ▪ Polymerize into “sheets” rather than fibrils e.g., types IV and X C. Linking collagens: ▪ link fibrils and sheet-forming collagens to other structures e.g., types VII, IX and XII 16  Type I collagen %: The most common type (90% of all collagens) Synthesis: by fibroblasts, and other cells Function: provides tensile strength; inelastic Type II collagen Site: Exists as fibrils only in hyaline and elastic cartilage Type III collagen Structure: Arranged as delicate networks “reticular fibers” More carbohydrate than type I collagen Sites : 1.Support for capillaries, small nerves, muscle cells 2.Component of basement membranes 3.Main stromal fibers in myeloid & lymphoid tissues, & glands 17  Type IV collagen Sites : net-like sheets in basal lamina 18  Type VII collagen Sites “Anchoring fibrils” in the reticular lamina of the basement membrane beneath epidermis of thick skin Function: Link type IV collagen of basal lamina to anchoring plaques in CT  Type X collagen Sites : epiphyseal plate of growing bones Type IX and XII collagens Sites “fibril-associated collagens with interrupted helices” Function Help regulate the orientation and functions of the fibrillar collagens [ type IX binds to type II & type XII binds to type I ] Type XII Collagens Type I Collagens fibril 20  Biosynthesis & Assembly of Type I Collagens Composed of long, stiff tropocollagen molecules Each molecule is formed of 3 polypeptide chains (called  chains) wound around one another in a rope-like superhelix to form a triple-stranded helix. Tropocollagen Collagens are rich in amino acids: ✓Glycine & proline Important in formation of the triple-stranded helix ✓ Hydroxyproline & hydroxylysine which are not commonly found in other proteins. 21 7. Cleavage of registration peptides by procollagen peptidase to form tropocollagen 22  Collagen fibril Collagen fibril 9. Side-by side cross- linking of collagen Collagen Tropocollagen fibrils to form fibril fibers is mediated by FACIT collagen & proteoglycans Tropocollagen 10. fibers aggregate Molecules polymerize staggered one fifth their length with further to form type a gap (hole zone or lacunar region ) 23 between the C-terminal of one & N-terminal of the next I collagen Bundles  EM of collagen Heavy metal stains used in electron microscopy deposit in the gap regions. Consequently, viewed in the EM, collagen displays alternating dark and light bands; the dark bands represent the gap regions filled with heavy metal, and the light bands represent overlap regions, where the heavy metal cannot be deposited. 24 By EM 25 By EM 26  2- Elastic Fibers Characters: Elastic fibers in the ECM gives tissues ability to recoil after transient stretch. Elastic fibers may be stretched up to one and a half of their resting length without breaking. When the force is released, elastic fibers return to their resting length. 27  Elastic Fibers Structure: ❑ The main component of elastic fibers is elastin, a highly hydrophobic, nonglycosylated protein which, like collagen, is rich in proline & glycine but, unlike collagen, contains little hydroxyproline & no hydroxylysine. Synthesis: ❑ Elastin is secreted as tropoelastin, which is similar to the formation of and secretion of tropocollagen during collagen synthesis. 28  Elastic Fibers Locations of elastic fibers: In tissues requiring elasticity in addition to tensile strength in order to function: 1. Ordinary connective tissue (e.g., dermis of skin), 2. Elastic ligaments (e.g., ligamentum nuchae), 3. Elastic cartilages (pinna, auditory tube, epiglottis). 4. Muscle layers of blood vessels except capillaries. 5. lungs 6. Vocal cords. 29  Basal Lamina Definition: Basal laminae are flexible thin (40-120 nm thick) mats of specialized ECM. Sites: Basal lamina: ✓ Underlie all epithelial cell sheets and tubes & separates these cells from the underlying connective tissue. ✓ Surround individual muscle cells, fat cells, & Schwann cells (here the basal lamina is the external lamina) & separates these cells from the surrounding connective tissue. 30  Basal Lamina By EM: Most basal laminae consist of two distinct layers: A-Lamina rara: ▪ [electron-lucent layer] ▪ Adjacent to the basal plasma membrane of the cells that rest on the lamina [typically epithelial cells] B-Lamina densa: ▪ (electron-dense layer). ▪ Composed primarily of a meshwork of type IV collagen. 31  Basal Lamina Synthesis ❑ Basal lamina is synthesized by the cells that rest on it. Structure: ✓ It is a tough mat of type IV collagen with specific additional molecules on each face that help bind it to the adjacent cells or matrix. ✓ Although composition of basal laminae varies from tissue to tissue , all basal laminae contain: ▪Type IV collagen ▪Proteoglycans (primarily heparan sulfates) and ▪ Glycoproteins (laminin and entactin). 32  Laminin is present mainly on the plasma membrane side of the lamina densa, where it helps bind epithelial cells to the lamina. while fibronectin binds the matrix macromolecules and connective tissue cells on the opposite side. Function of Basal Lamina: Basal laminae have structural and filtering roles. They are able to determine cell polarity, influence cell metabolism, organize the proteins in adjacent plasma membranes, induce cell differentiation, and serve as specific "highways" for cell migration. 33  Basement membrane The term basement membrane is used Basemen to specify a PAS-positive layer, visible membrane with light microscope, beneath epithelia & in kidney glomerulus & lung alveoli. The basement membrane is therefore thicker and is usually formed by the fusion of either two basal laminae (i.e., fused basal lamina) or a basal lamina & a reticular lamina. 34  Basement membrane The use of these terms is not agreed upon by all; they are frequently used indiscriminately, causing confusion. The term basal lamina will be used to denote the lamina densa and the variable presence of the lamina rara, structures seen with the electron microscope. When referring to the thicker structures seen with the light microscope, the term basement membrane will be used. 35 36