Bone Lecture 1 2024-2025 PDF

Bone (lecture 1) Dr. Mariati Abdul Rahman Dept. of Clinical Oral Biology, Faculty of Dentistry  References 1. Biochemistry and Oral Biology by Cole A.S. and Eastoe J.E. 2nd edition, Wright 1988 2. Oral Bioscience by Ferguson D.B. 1st edition, Churchill Li...

Bone (lecture 1) Dr. Mariati Abdul Rahman Dept. of Clinical Oral Biology, Faculty of Dentistry  References 1. Biochemistry and Oral Biology by Cole A.S. and Eastoe J.E. 2nd edition, Wright 1988 2. Oral Bioscience by Ferguson D.B. 1st edition, Churchill Livingstone 1999. 3. Medical Biochemistry 2nd edition, Baynes J.W. and Dominiczak M.H. Elsevier Mosby 2005  Objectives: ⚫ At the end of the lecture, students must be able to describe: ⚫ Bone composition and function. ⚫ Bone mineralisation.  Bone: Composition and function ⚫ What are four types of tissue? ⚫ What is connective tissue? Tissues that supports/binds/separate a more specialised tissue/organ. ⚫ Bones are mineralised connective tissue. ⚫ Components of bone: ⚫ Extracellular matrix – proteoglycan, glycoprotein, phospholipid and H2O. ⚫ Cells – fibroblast, osteoblast, osteoclasts and osteocytes. ⚫ Fibre proteins – Collagen, Reticulin.  The extracellular matrix in calcified tissue ⚫ In connective tissue, generally the extracellular matrix forms a gel holding the extracellular fluid. ⚫ The matrix contains proteins & proteoglycans which mediates attachment between different cells & between extracellular fibers & cells. ⚫ Molecules of the matrix also have signaling functions & can influence cell shape, movement & multiplication by binding to cell- surface proteins – integrins. ⚫ ECM of calcified tissue allows crystallization of calcium salts to Proteoglycans- proteins with occur. glycosaminoglycans Chemical composition of hard tissue  Bone (Osseous tissue) ⚫ Specialised connective tissue (mineralised). ⚫ In constant process of modification. ⚫ Two important roles – Structural ⚫ Has the capacity to adapt to changing loads on the skeleton. – Chemical ⚫ Helps to maintain a mineral homeostasis (stable state/in equilibrium) by regulating concentrations of Ca2+ , H+ and (HPO4)2-. ⚫ Protects internal organs ⚫ Facilitate movements (in conjunction with muscles) ⚫ Also involved in cell formation, calcium metabolism & mineral storage. ⚫ Compact on the outside, cancellous (spongy) inside.  Bone composition ⚫ 25% organic matter. – Type 1 collagen – Non collagen protein e.g acidic glycoproteins, proteoglycan, lipids. – 5% cells – Osteoprogenitor/preosteoblast – Osteoblasts – Osteoclasts – Osteocytes. ⚫ About 65 % inorganic matter. – Primarily in the form of calcium hydroxyapatite. – Mineralised connective tissue. ⚫ 10% water  Collagen ⚫ Collagen actually describes a family of 20 proteins encoded by at least 30 genes. ⚫ All collagen contain at least one region with triple helix conformation. ⚫ In triple helix, 3 alpha chains are wound around each other & linked by hydrogen bonds into a right handed helixes.  Collagen After secretion from the fibroblasts, the collagen forms a variety of molecules e.g for fibrillar collagen – type I, II, III, & V. Other collagens types which do not form the fibrils are arranged in a meshwork (interlaced structure). (type IV found in the basement membranes) or may link collagen fibrils with other molecules in the matrix (type IX, XII & XIV) ⚫ Collagen proteins: – Collagen type I : Major protein (90%), composed of 2 1 chains & one 2 chain, making up the triple helix. – Collagen type V : minor. ⚫ Non collagen proteins – Acidic glycoproteins – Phosphoproteins, osteocalcins, GLA proteins – Proteoglycans : chondroitin sulfate  Glycoproteins ⚫ Glycoproteins – adhesive molecules- binds together other matrix components to each other or with tissue cells. – Fibronectin – most studied glycoprotein, part of its amino acids bind to integrins, other part bind to other proteins in the matrix.  Glycoproteins Other glycoproteins: laminin, thrombospondin.  Non collagen proteins ⚫ One group of glycoproteins – phosphoproteins which exhibits strong calcium-binding properties & can bind to collagen. ⚫ Such proteins might become the seeds or may be able to do so when linked to collagen. ⚫ In supersaturated solutions of calcium & phosphate, many of these proteins inhibits crystal formation but when they attached to immobilized layer of collagen they promote crystal formation. Non collagen proteins ⚫ Gamma- carboxyglutamate- containing (GLA) proteins in bones. ⚫ 2 types : Osteocalcins & matrix GLA proteins. ⚫ Osteocalcins – very small proteins that make up most of GLA proteins in bone. ⚫ Although osteocalcin binds poorly to calcium, they can induce some mineral formation in calcium phosphate solutions & binds to hydroxyapatite. ⚫ Matrix GLA proteins – normal function – regulate bone growth & calcification.  Other acidic non- collagen proteins ⚫ Contains aspartic, glutamic & sialic acid so they becomes acidic in nature. ⚫ Osteonectin - major non-collagenous proteins of bone. – Binds strongly to hydroxyapatite & has non- specific binding to collagen. – Associated with bone formation & remodeling. ⚫ Bone acidic sialoprotein I or osteopontin – bone regeneration and remodelling.  Proteoglycans ⚫ Family of molecules consisting a protein core with one or more glycosaminoglycan chains attached. ⚫ The glycosaminoglycans include hyaluronic acid, chondroitin sulphate, dermatan sulphate, as well as very large hyaluronan.  Proteoglycans ⚫ Many matrix proteoglycans are small molecules ( decorin, biglycan & fibromodulin) but others form large aggregates – aggrecan in cartilages. ⚫ Function: 1. to withstand compressional forces through hydration and swelling pressure to the tissue. 2. can initiate collagen fiber formation by providing a nucleus for the molecule, later however they retard fiber formation & stabilize a collagen fiber network. ⚫ These properties suggest that they may be controlling factors in the organization of collagen network.  Bone cells – Osteoprogenitor/preosteoblast – Osteoblasts – Osteoclasts – Osteocytes A section through the bone to show the osteocytes embedded in the bone, osteoblasts on the bone margin and the multinucleate octeoclasts.  Bone matrix = collagen fibrils + BONE CELLS mineral crystals Active Osteoblasts Bone canal vessels Osteoclast Periosteum Resting cells Osteocyte Ca 2+  Mineralised tissue Group of Mineralised tissue Inorganic Organic organisms component component Molluscs Exoskeleton CaCO3 Chitin Arthropods (Calsite) Protein Hydroxyapatite Chitin Vertebrates Epithelia Hydroxyapatite Keratins claws, nails, feathers Tooth Hydroxyapatite Amelogenins Enamel, dentine Collagen Cementum Skeleton Hydroxyapatite Collagen Bone Cartilage  Bone mineralization ⚫ the process by which the organic bone matrix becomes filled with calcium phosphate nanocrystals, occurs in a mineralization specific, highly ordered process. ⚫ Mineralization occurs in the calcification extracellular matrix. ⚫ In most bones, mineralization occurs both within collagen fibrils and external to the collagen fibrils. ⚫ Enamel has no collagen, mineralization occurs within the extracellular organic matrix.  Inorganic materials ⚫ With the exception of calcite otoliths in the middle ear, all mammalian calcified tissues are made of calcium phosphate salts. ⚫ Mostly : Crystalline hydroxyapatite ⚫ (Ca10[PO4]6[OH]2) ⚫ There are at least 7 other crystalline calcium phosphates reported in calcified tissue. ⚫ Human bone - 65% carbonated apatite, 20% magnesium whitlockite &15% non-carbonate containing apatite. ⚫ Dental enamel is a mixture of carbonated & non-carbonated apatite with a small proportion of magnesium whitlockite ⚫ Dentine – contains 20% of non-carbonated apatite & about 30% of magnesium whitlockite. ⚫ Key crystal is apatite. Biological apatite (Greek- to deceive) ⚫ Biological apatite- inorganic components of mineralised tissue in vertebrates in the form of extremely small crystals. ⚫ A significant proportion of non-crystallline calcium phoshate may also be present. ⚫ The crystalline minerals in bones and teeth- generally regarded as imperfect calcium hydroxyapatite. ⚫ Calcium fluorapatite- abundant and ubiquitous (found everywhere). Unit cell of fluorapatite ⚫ One unit cell of calcium fluorapatite – – 10 calcium ions, 6 phosphate ions and 2 fluoride ions. ⚫ One unit cell of calcium hydroxyapatite- – 2 hydroxyl ions instead of fluoride ions. ⚫ One crystal consists of hundreds or even thousands of unit cells. ⚫ Formula: Ca10(PO4)6X2. ⚫ Does not exist on its own. The relative positions of the ions in part of a crystal of fluorapatite as they would appear when viewed along the c axis.  Variable composition of biological apatite ⚫ Variations from the ideal ratio value of 10:6. ⚫ Some ions which can substitute for calcium, phosphate or hydroxyl ions. Ca 2+ PO4 3- OH- Mg 2+ CO3 2- Cl- Na 2+ HCO3- F- Sr 2+ HPO4 2- CO3 2-  Contribution of biological apatite to electrolyte and acid-base balance. ⚫ Skeletal mineral has a large surface area. ⚫ Enable the mineral to act as a reservoir. ⚫ Sudden changes in electrolyte concentrations in serum or extracelluler fluid (ECF). ⚫ Bone mineral also important in balancing body fluids. Under increased acidity, some bone minerals will dissolves. 2PO4 3- + 3H+ HPO4 2- + H2PO4 - Bone represents a vast reserve of phosphate ions which can function as part of the phosphate buffer system.  Process of calcification ⚫ Laying down of fibres and released of matrix vesicles(mv) by specialised types of cells (osteoblast, odontoblast and cementoblast. ⚫ In mv- deposition of calcium salts will start. ⚫ Tiny amorphous clusters of salts grows rapidly to crystals of limited size. ⚫ The crystals multiply in numbers and can sometimes break the mv.  As minerals are deposited, water is lost and the film of water becomes so thin, no further influx of mineral ions. Crystals ceased to grow. (lost of minerals is also prevented) Osteoblasts become embedded in the mineralised bone- osteocytes.  Process of calcification ⚫ Specialised matrix in bone- osteoid ⚫ Specialised matrix in cementum- cementoid ⚫ Specialised matrix in dentine- predentine ⚫ In enamel, no obvious specialised matrix.  Problems of calcification? ⚫ If calcium and phosphate ion concentrations are adequate for crystallisation, why doesn’t it occur throughout the body? ⚫ If calcium and phosphate ion concentrations are not adequate for crystallisation, why should it occur at all? And surprisingly at specific location? Thank you