BoneBone

20072007

Functions of BoneFunctions of Bone

Rigid skeleton supports the bodyRigid skeleton supports the body Provides sites for attachment of Provides sites for attachment of

muscles and organsmuscles and organs Protective cover for soft organsProtective cover for soft organs Hemopoiesis (marrow cavities)Hemopoiesis (marrow cavities) Reservoir for body’s calcium supplyReservoir for body’s calcium supply

Calcium used for muscle contraction, Calcium used for muscle contraction, blood clotting, cell membrane blood clotting, cell membrane permeability, transmission of nerve permeability, transmission of nerve impulsesimpulses

BoneBone

Connective tissue with a mineralized Connective tissue with a mineralized matrix (calcium phosphate)matrix (calcium phosphate)

Matrix is dense and rigid, contains Matrix is dense and rigid, contains fibers of mainly Type I collagen.fibers of mainly Type I collagen.

Both the collagen and the ground Both the collagen and the ground substance become mineralized in substance become mineralized in bone.bone.

Bone MatrixBone Matrix Calcified matrix of fibers (Type I collagen) and Calcified matrix of fibers (Type I collagen) and

ground substance (proteoglycans with chondroitin ground substance (proteoglycans with chondroitin sulfate and keratan sulfate side chains, sulfate and keratan sulfate side chains, glycoproteins)glycoproteins)

Organic component (osteoid)= type I collagen, Organic component (osteoid)= type I collagen, highly cross-linkedhighly cross-linked

Inorganic (mineral) component = crystals of Inorganic (mineral) component = crystals of calcium hydroxyapatite (calcium and phosphorus)calcium hydroxyapatite (calcium and phosphorus)

Hard – nutrients and metabolites cannot diffuse Hard – nutrients and metabolites cannot diffuse through the calcified matrix as done in cartilage.through the calcified matrix as done in cartilage.

Highly vascularized: Highly vascularized: Canaliculi –small channels or canals between lacunae that Canaliculi –small channels or canals between lacunae that

allow osteocytes to exchange wastes and bring metabolites allow osteocytes to exchange wastes and bring metabolites in; osteocytes send out cytoplasmic processes into the in; osteocytes send out cytoplasmic processes into the canaliculi and contact neighboring cells through gap canaliculi and contact neighboring cells through gap junctions.junctions.

Structure of BonesStructure of Bones

Periosteum – covers bone’s external Periosteum – covers bone’s external surfaces and consists of an outer surfaces and consists of an outer layer of dense, irregular collagenous layer of dense, irregular collagenous (fibrous) CT and an inner layer of (fibrous) CT and an inner layer of osteoprogenitor and osteoblast cellsosteoprogenitor and osteoblast cells

Marrow – in central cavity of bone, Marrow – in central cavity of bone, hemopoietic tissue, red or yellowhemopoietic tissue, red or yellow

Diaphysis – shaftDiaphysis – shaft Epiphysis – articular endsEpiphysis – articular ends

Epiphysis of an adult long bone

Compact boneSpongy (cancellous) bone

Compact bone

Spongy vs. Compact Spongy vs. Compact BoneBone

Spongy – composed of spicules or trabeculae of Spongy – composed of spicules or trabeculae of bone, forms network around bone marrow. bone, forms network around bone marrow. Also known as cancellous bone.Also known as cancellous bone.

Compact – dense bone, no cavities; found along Compact – dense bone, no cavities; found along the outer surface of shafts of long bone. Has the outer surface of shafts of long bone. Has lamellar structure of circumferential lamellae lamellar structure of circumferential lamellae (on surfaces) and Haversian and interstitial (on surfaces) and Haversian and interstitial lamellae.lamellae.Haversian canal is space running through Haversian canal is space running through osteon with neurovascular bundle, osteoblasts osteon with neurovascular bundle, osteoblasts and osteoprogenitor cells. Haversian canals of and osteoprogenitor cells. Haversian canals of adjacent osteons connected by Volkmann’s adjacent osteons connected by Volkmann’s canals.canals.

Compact Bone – Low PowerCompact Bone – Low Powerhighly vascularizedhighly vascularized

Haversian canal

Volkmann’s canal

Ground Bone – higher powerGround Bone – higher powercanaliculuscanaliculus

lacuna

Spongy (cancellous) Spongy (cancellous) BoneBone

Bone MarrowBone Marrow

Bone Cells – 4 typesBone Cells – 4 types Osteogenic cells (osteoprogenitor cells) – Osteogenic cells (osteoprogenitor cells) –

undifferentiated stem cells, during development give undifferentiated stem cells, during development give rise to osteoblasts, line inner layer periosteum, rise to osteoblasts, line inner layer periosteum, haversian canals and endosteum.haversian canals and endosteum.

Osteoblasts – immature bone cells; must secrete Osteoblasts – immature bone cells; must secrete bone matrix to become mature.bone matrix to become mature.

Osteocytes – mature bone cells; trapped within Osteocytes – mature bone cells; trapped within lacunae of bone matrix they secrete around lacunae of bone matrix they secrete around themselves.themselves.

Osteoclasts – large, multinucleate cells; found at the Osteoclasts – large, multinucleate cells; found at the surfaces of bone where resorption and remodeling is surfaces of bone where resorption and remodeling is taking place; motile cells; function to resorb bone in taking place; motile cells; function to resorb bone in response to blood calcium levels and hormone response to blood calcium levels and hormone stimuli.stimuli.Howships lacunae – depressions made by osteoclasts Howships lacunae – depressions made by osteoclasts as they etch away the bone.as they etch away the bone.

Bones are dynamic Bones are dynamic structures!structures!

Continually being resorbed and Continually being resorbed and remodeled by osteoclastsremodeled by osteoclasts

Hormones regulate amount of Hormones regulate amount of calcium in the bloodcalcium in the blood Parathyroid hormone – increases blood Parathyroid hormone – increases blood

calcium levels; stimulates activity of calcium levels; stimulates activity of osteoclasts and increases their osteoclasts and increases their numbers.numbers.

Calcitonin (from thyroid gland) – Calcitonin (from thyroid gland) – decreases blood calcium levels; reduces decreases blood calcium levels; reduces the numbers and activity of osteoclasts.the numbers and activity of osteoclasts.

Bone Formation – 2 typesBone Formation – 2 types

Intramembranous ossification – Intramembranous ossification – formation of bone directly from formation of bone directly from mesenchyme (embryonic connective mesenchyme (embryonic connective tissue) with no cartilage precursortissue) with no cartilage precursor

Endochondral – formation of bone on Endochondral – formation of bone on cartilage scaffolding; bone cartilage scaffolding; bone eventually replaces cartilage.eventually replaces cartilage.

Intramembranous Intramembranous OssificationOssification

Occurs in flat bones of skull, Occurs in flat bones of skull, mandible, maxilla.mandible, maxilla.

Osteoblasts develop from connective Osteoblasts develop from connective tissue mesenchyme cellstissue mesenchyme cells

Osteoblasts secrete bone matrix.Osteoblasts secrete bone matrix. Form a network of bone spicules or Form a network of bone spicules or

trabeculaetrabeculae

From: Gartner and Hiatt, 2001

IntramembranousOssification

From: Gartner and Hiatt, 2001

Intramembranous Ossification

osteoblasts

osteocytes

From: Gartner and Hiatt, 2001

Intramembranous Ossification

Endochondral Endochondral OssificationOssification

Occurs in long and short bones, vertebraeOccurs in long and short bones, vertebrae Bone is proceeded by temporary hyaline cartilage model that Bone is proceeded by temporary hyaline cartilage model that

serves as a structural scaffold.serves as a structural scaffold. Bone tissue REPLACES the cartilageBone tissue REPLACES the cartilage Cartilage begins to calcify and causes chondrocytes to Cartilage begins to calcify and causes chondrocytes to

hypertrophy.hypertrophy. Cartilage calcifies so diffusion of nutrients to chondrocytes Cartilage calcifies so diffusion of nutrients to chondrocytes

stops and they die.stops and they die. Fragmented pieces of calcified cartilage serve as the framework Fragmented pieces of calcified cartilage serve as the framework

to deposit the bone.to deposit the bone. Osetoprogenitor cells and blood vessels from surrounding Osetoprogenitor cells and blood vessels from surrounding

connective tissue penetrate and invade degenerating cartilage connective tissue penetrate and invade degenerating cartilage model.model.

The area/center of ossification expands, replacing cartilage with The area/center of ossification expands, replacing cartilage with bone.bone.

Cartilage of diaphysis replaced by bone except for at epiphyseal Cartilage of diaphysis replaced by bone except for at epiphyseal plate.plate.

Primary and secondary centers of ossification.Primary and secondary centers of ossification.

From: Gartner and Hiatt, 2001

Endochondral Ossification

Endochondral Ossification

From: Gartner and Hiatt, 2001

Endochondral Ossification

Multiplying chondrocytes Multiplying chondrocytes and hypertrophyand hypertrophy

Primary vs. Secondary Primary vs. Secondary BoneBone

Primary – immature or “woven” bonPrimary – immature or “woven” bon First bone formed in fetal development or bone First bone formed in fetal development or bone

repairrepair Abundant osteocytesAbundant osteocytes Irregular bundles of collagen (Type I)Irregular bundles of collagen (Type I)

Secondary bone – mature or lamellar boneSecondary bone – mature or lamellar bone Composed of concentric or parallel lamellae; Composed of concentric or parallel lamellae;

osteocytes arranged in rows.osteocytes arranged in rows. Canaliculi connect neighboring lacunaeCanaliculi connect neighboring lacunae Haversian canal system (osteon)Haversian canal system (osteon)

Cylinders of lamellae concentrically arranged around a Cylinders of lamellae concentrically arranged around a central Haversian canalcentral Haversian canal

Volkmann’s canals – connect one osteon to Volkmann’s canals – connect one osteon to another; oriented perpendicular to Haversian another; oriented perpendicular to Haversian canals.canals.

Methods to Make Slides Methods to Make Slides of Boneof Bone

Decalcified bone sectionsDecalcified bone sections Acid solution removes calcium salts and softens Acid solution removes calcium salts and softens

bone.bone. Bone can then be embedded and sectioned.Bone can then be embedded and sectioned. Osteocytes are distorted.Osteocytes are distorted.

Ground bone sectionsGround bone sections Saw bone in slices.Saw bone in slices. Grind piece down until thin enough to put on a Grind piece down until thin enough to put on a

slide.slide. Cells are destroyed and lacunae and canaliculi Cells are destroyed and lacunae and canaliculi

filled with bone debris.filled with bone debris.