Bone 2007

Functions of Bone Rigid skeleton supports the body Rigid skeleton supports the body Provides sites for attachment of muscles and organs Provides sites for attachment of muscles and organs Protective cover for soft organs Protective cover for soft organs Hemopoiesis (marrow cavities) Hemopoiesis (marrow cavities) Reservoir for body’s calcium supply Reservoir for body’s calcium supply Calcium used for muscle contraction, blood clotting, cell membrane permeability, transmission of nerve impulses Calcium used for muscle contraction, blood clotting, cell membrane permeability, transmission of nerve impulses

Bone Connective tissue with a mineralized matrix (calcium phosphate) Connective tissue with a mineralized matrix (calcium phosphate) Matrix is dense and rigid, contains fibers of mainly Type I collagen. Matrix is dense and rigid, contains fibers of mainly Type I collagen. Both the collagen and the ground substance become mineralized in bone. Both the collagen and the ground substance become mineralized in bone.

Bone Matrix Calcified matrix of fibers (Type I collagen) and ground substance (proteoglycans with chondroitin sulfate and keratan sulfate side chains, glycoproteins) Calcified matrix of fibers (Type I collagen) and ground substance (proteoglycans with chondroitin sulfate and keratan sulfate side chains, glycoproteins) Organic component (osteoid)= type I collagen, highly cross- linked Organic component (osteoid)= type I collagen, highly cross- linked Inorganic (mineral) component = crystals of calcium hydroxyapatite (calcium and phosphorus) Inorganic (mineral) component = crystals of calcium hydroxyapatite (calcium and phosphorus) Hard – nutrients and metabolites cannot diffuse through the calcified matrix as done in cartilage. Hard – nutrients and metabolites cannot diffuse through the calcified matrix as done in cartilage. Highly vascularized: Highly vascularized: Canaliculi –small channels or canals between lacunae that allow osteocytes to exchange wastes and bring metabolites in; osteocytes send out cytoplasmic processes into the canaliculi and contact neighboring cells through gap junctions. Canaliculi –small channels or canals between lacunae that allow osteocytes to exchange wastes and bring metabolites in; osteocytes send out cytoplasmic processes into the canaliculi and contact neighboring cells through gap junctions.

Structure of Bones Periosteum – covers bone’s external surfaces and consists of an outer layer of dense, irregular collagenous (fibrous) CT and an inner layer of osteoprogenitor and osteoblast cells Periosteum – covers bone’s external surfaces and consists of an outer layer of dense, irregular collagenous (fibrous) CT and an inner layer of osteoprogenitor and osteoblast cells Marrow – in central cavity of bone, hemopoietic tissue, red or yellow Marrow – in central cavity of bone, hemopoietic tissue, red or yellow Diaphysis – shaft Diaphysis – shaft Epiphysis – articular ends Epiphysis – articular ends

Epiphysis of an adult long bone Compact bone Spongy (cancellous) bone Compact bone

Spongy vs. Compact Bone Spongy – composed of spicules or trabeculae of bone, forms network around bone marrow. Also known as cancellous bone. Spongy – composed of spicules or trabeculae of bone, forms network around bone marrow. Also known as cancellous bone. Compact – dense bone, no cavities; found along the outer surface of shafts of long bone. Has lamellar structure of circumferential lamellae (on surfaces) and Haversian and interstitial lamellae. Compact – dense bone, no cavities; found along the outer surface of shafts of long bone. Has lamellar structure of circumferential lamellae (on surfaces) and Haversian and interstitial lamellae. Haversian canal is space running through osteon with neurovascular bundle, osteoblasts and osteoprogenitor cells. Haversian canals of adjacent osteons connected by Volkmann’s canals.

Compact Bone – Low Power highly vascularized Haversian canal Volkmann’s canal

Ground Bone – higher power canaliculus lacuna

Spongy (cancellous) Bone

Bone Marrow

Bone Cells – 4 types Osteogenic cells (osteoprogenitor cells) – undifferentiated stem cells, during development give rise to osteoblasts, line inner layer periosteum, haversian canals and endosteum. Osteogenic cells (osteoprogenitor cells) – undifferentiated stem cells, during development give rise to osteoblasts, line inner layer periosteum, haversian canals and endosteum. Osteoblasts – immature bone cells; must secrete bone matrix to become mature. Osteoblasts – immature bone cells; must secrete bone matrix to become mature. Osteocytes – mature bone cells; trapped within lacunae of bone matrix they secrete around themselves. Osteocytes – mature bone cells; trapped within lacunae of bone matrix they secrete around themselves. Osteoclasts – large, multinucleate cells; found at the surfaces of bone where resorption and remodeling is taking place; motile cells; function to resorb bone in response to blood calcium levels and hormone stimuli. Osteoclasts – large, multinucleate cells; found at the surfaces of bone where resorption and remodeling is taking place; motile cells; function to resorb bone in response to blood calcium levels and hormone stimuli. Howships lacunae – depressions made by osteoclasts as they etch away the bone.

Bones are dynamic structures! Continually being resorbed and remodeled by osteoclasts Continually being resorbed and remodeled by osteoclasts Hormones regulate amount of calcium in the blood Hormones regulate amount of calcium in the blood Parathyroid hormone – increases blood calcium levels; stimulates activity of osteoclasts and increases their numbers. Parathyroid hormone – increases blood calcium levels; stimulates activity of osteoclasts and increases their numbers. Calcitonin (from thyroid gland) – decreases blood calcium levels; reduces the numbers and activity of osteoclasts. Calcitonin (from thyroid gland) – decreases blood calcium levels; reduces the numbers and activity of osteoclasts.

Bone Formation – 2 types Intramembranous ossification – formation of bone directly from mesenchyme (embryonic connective tissue) with no cartilage precursor Intramembranous ossification – formation of bone directly from mesenchyme (embryonic connective tissue) with no cartilage precursor Endochondral – formation of bone on cartilage scaffolding; bone eventually replaces cartilage. Endochondral – formation of bone on cartilage scaffolding; bone eventually replaces cartilage.

Intramembranous Ossification Occurs in flat bones of skull, mandible, maxilla. Occurs in flat bones of skull, mandible, maxilla. Osteoblasts develop from connective tissue mesenchyme cells Osteoblasts develop from connective tissue mesenchyme cells Osteoblasts secrete bone matrix. Osteoblasts secrete bone matrix. Form a network of bone spicules or trabeculae Form a network of bone spicules or trabeculae

From: Gartner and Hiatt, 2001 Intramembranous Ossification

From: Gartner and Hiatt, 2001 Intramembranous Ossification osteoblasts osteocytes

From: Gartner and Hiatt, 2001 Intramembranous Ossification

Endochondral Ossification Occurs in long and short bones, vertebrae Occurs in long and short bones, vertebrae Bone is proceeded by temporary hyaline cartilage model that serves as a structural scaffold. Bone is proceeded by temporary hyaline cartilage model that serves as a structural scaffold. Bone tissue REPLACES the cartilage Bone tissue REPLACES the cartilage Cartilage begins to calcify and causes chondrocytes to hypertrophy. Cartilage begins to calcify and causes chondrocytes to hypertrophy. Cartilage calcifies so diffusion of nutrients to chondrocytes stops and they die. Cartilage calcifies so diffusion of nutrients to chondrocytes stops and they die. Fragmented pieces of calcified cartilage serve as the framework to deposit the bone. Fragmented pieces of calcified cartilage serve as the framework to deposit the bone. Osetoprogenitor cells and blood vessels from surrounding connective tissue penetrate and invade degenerating cartilage model. Osetoprogenitor cells and blood vessels from surrounding connective tissue penetrate and invade degenerating cartilage model. The area/center of ossification expands, replacing cartilage with bone. The area/center of ossification expands, replacing cartilage with bone. Cartilage of diaphysis replaced by bone except for at epiphyseal plate. Cartilage of diaphysis replaced by bone except for at epiphyseal plate. Primary and secondary centers of ossification. Primary and secondary centers of ossification.

From: Gartner and Hiatt, 2001 Endochondral Ossification

From: Gartner and Hiatt, 2001 Endochondral Ossification

Multiplying chondrocytes and hypertrophy

Primary vs. Secondary Bone Primary – immature or “woven” bon Primary – immature or “woven” bon First bone formed in fetal development or bone repair First bone formed in fetal development or bone repair Abundant osteocytes Abundant osteocytes Irregular bundles of collagen (Type I) Irregular bundles of collagen (Type I) Secondary bone – mature or lamellar bone Secondary bone – mature or lamellar bone Composed of concentric or parallel lamellae; osteocytes arranged in rows. Composed of concentric or parallel lamellae; osteocytes arranged in rows. Canaliculi connect neighboring lacunae Canaliculi connect neighboring lacunae Haversian canal system (osteon) Haversian canal system (osteon) Cylinders of lamellae concentrically arranged around a central Haversian canal Cylinders of lamellae concentrically arranged around a central Haversian canal Volkmann’s canals – connect one osteon to another; oriented perpendicular to Haversian canals. Volkmann’s canals – connect one osteon to another; oriented perpendicular to Haversian canals.

Methods to Make Slides of Bone Decalcified bone sections Decalcified bone sections Acid solution removes calcium salts and softens bone. Acid solution removes calcium salts and softens bone. Bone can then be embedded and sectioned. Bone can then be embedded and sectioned. Osteocytes are distorted. Osteocytes are distorted. Ground bone sections Ground bone sections Saw bone in slices. Saw bone in slices. Grind piece down until thin enough to put on a slide. Grind piece down until thin enough to put on a slide. Cells are destroyed and lacunae and canaliculi filled with bone debris. Cells are destroyed and lacunae and canaliculi filled with bone debris.