Organization of bone Claudia Stanescu, Ph.D. Office Hours in Gittings 108 Tue 10-11am and Thurs 1-2pm or by appointmentObjectives: 1. List the components of the matrix of bone and describe how each component contributes to the bone's strength. 2. Describe the differences between compact and spongy bone in terms of location, function, and composition. 3. Describe the microscopic structure of compact bone (the osteon) and understand the purpose of its organization.Composition of bone Connective tissue Extracellular Matrix Cells Fibers Ground Substance Osteogenic cells Osteoblasts Osteocytes Osteoclasts Organic component Collagen Fibers Contains organic and inorganic components (Noncollagenous proteins)Extracellular matrix • Organic components (secreted by osteoblasts) – Ground substance (glycosaminoglycans – GAG) • Glycoproteins = polysaccharide + protein – Chondroitin sulfate and Hyaluronic acid • Negatively charged – trap water – Collagen fibers (type I) • Fibrous protein arranged in helical form • Very resistant to pulling forces • Provides flexibility and a framework for deposition of calcium crystalsT. A. Einhorn, Bone metabolism and metabolic bone disease. In Orthopaedic Knowledge Update 4 Home Study Syllabus (J. W. Frymoyer, ed.). Am Acad Orthop Surg., Rosemont, 69–88 (1994). Type I Collagen http://www.ncbi.nlm.nih.gov/books/NBK26810/Extracellular matrix • Inorganic components – Water • Attracted to ground substance • Makes up 25% of the extracellular matrix – Hydroxyapatite Ca10(PO4)6(OH)2 • Ca3(PO4)2 + Ca(OH)2 (Calcium phosphate and calcium hydroxide) + other minerals (calcium carbonate) and some ions (Mg, F, K)Organization of the organic/inorganic components • Salts (hydroxyapatite) are deposited within collagen fibers • As the hydroxyapatite condenses, other inorganic salts and ions precipitate in the matrix to fill in the spaces between fibersOrganization of mineral 1) The collagen fiber contains mineralized fibrils 2) The fibrils contain small mineral plates bound by helical noncollagenous proteins (collagen cross links) Principles of bone biology, 3rd ed, Seeman, Modeling and remodeling, 2008.Collagen and minerals provide strength to the matrix • Collagen – provides flexibility • Minerals – provide firmness Experiments: • Soak bone in weak acid (vinegar) – remove minerals from bone  bone becomes rubbery/flexible • Apply proteolytic enzymes (denature protein) – remove collagen from bone  bone becomes brittle/crumbly Bone needs both collagen and minerals for maximum strengthSkeletal disorders Rickets (inorganic component deficient) – Calcium deficiency due to lack of vitamin D leads to flexible bones (bowed legs) Scurvy (organic component deficient) – Problem with collagen synthesis due to vitamin C deficiency leads to brittle bones that can fracture easily3D arrangement of matrix Spongy bone vs. Compact bone Both have the same chemical composition but different organizationSpongy bone • Organization – Irregular lattice of thin plates called trabecullae – Osteocytes housed in lacunae Space for red bone marrow Trabeculae Osteocyte Osteoclast Osteoblasts aligned along trabeculae of new bone Lacuna Lamellae CanaliculiLow-power scanning electron microscope image of normal bone architecture in the 3rd lumbar vertebra of a 30 year old woman marrow and other cells have been removed to reveal thick, interconnected plates of boneSpongy bone • Location – Epiphyses of long bones – Surrounding marrow cavities – Flat, short, irregular bones • Functions – Withstand forces from many directions – trabecullae arranged along lines of stress – Lightens the skeleton (not as strong as compact bone) – Contains red marrow for hemopoiesisCompact bone • Organization – Solid network of bone organized in concentric ring structures called osteons (see next slide) • Location – External layer of all bones – Diaphysis of long bones • Function – Gives long bones ability to withstand forces along longitudinal axisOsteon = functional unit of compact boneGreenspans’s Basic and Clinical endocrinolgy 8th edition. Metabolic Bone Disease; Dolores Shoback, MD, Deborah Sellmeyer, MD, & Daniel D. Bikle, MD, PhD. 2007 Osteon organizationOsteocytes in lacunae and canaliculi Cytoplasmic extension (dendrite) of osteocyte Lacuna (space) Canaliculi (little canals)Osteocyte dendrites in canaliculi Osteoporosis, 3rd ed, Marcus, Feldman, Nelson, Rosen. Osteocytes.