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Chapter 6Skeletal System:

Bones and Bone Tissue

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Functions of the Skeletal System• Support. Bone is hard and rigid; cartilage is flexible

yet strong. Cartilage in nose, external ear, thoracic cage and trachea. Ligaments- bone to bone

• Protection. Skull around brain; ribs, sternum, vertebrae protect organs of thoracic cavity

• Movement. Produced by muscles on bones, via tendons. Ligaments allow some movement between bones but prevent excessive movement (joint: formed where two or more bones come together------allows movement between bones)

• Storage. Ca and P. Stored then released as needed. Fat stored in marrow cavities

• Blood cell production. Bone marrow that gives rise to blood cells and platelets

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Components of Skeletal System• Bone

• Cartilage: three types– Hyaline– Fibrocartilage– Elastic

• Tendons and ligaments (Tendons: muscle to bone

Ligament: bone to bone)

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Hyaline Cartilage• Consists of specialized cells that produce matrix

– Chondroblasts: form matrix

– Chondrocytes: surrounded by matrix; are lacunae

• Matrix. Collagen fibers for strength, proteoglycans for resiliency

• Perichondrium. Double-layered C.T. sheath. Covers cartilage except at articulations

– Inner. More delicate, has fewer fibers, contains chondroblasts

– Outer. Blood vessels and nerves penetrate. No blood vessels in cartilage itself

• Articular cartilage. Covers bones at joints; has no perichondrium

• Growth

– Appositional. New chondrocytes and new matrix at the periphery (chondroblasts in perichondrium lay down new matrix & new

chondrocytes)

– Interstitial. Chondrocytes within the tissue divide and add more matrix between the cells.

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Bone Histology• Bone matrix. Like reinforced concrete. Steel bars

is collagen fibers, cement is hydroxyapatite– Organic: collagen and proteoglycans (35%)– Inorganic: hydroxyapatite. CaPO4 crystals (65%)

• If mineral removed, bone is too bendable• If collagen removed, bone is too brittle

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Bone Histology• Bone cells

– Osteoblasts– Osteocytes– Osteoclasts– Stem cells or osteochondral progenitor cells

• Woven bone: collagen fibers randomly oriented• Lamellar bone: mature bone in sheets• Cancellous bone (spongy): contain trabeculae

-less bone matrix & more space • Compact bone: dense

-more bone matrix & less space

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Bone Cells• Osteoblasts

– Formation of bone through ossification or osteogenesis. Collagen produced by E.R. and golgi. Released by exocytosis. Precursors of hydroxyapetite stored in vesicles, then released by exocytosis.

– Ossification: formation of bone by osteoblasts. It occurs by appositional growth on surface of previously existing bone. Osteoblasts communicate through gap junctions. Bone matrix produced by osteoblasts covers the older bone & surrounds osteoblasts

results in new layer of bone.

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Bone Cells• Osteocytes. Mature bone cells

once surrounded by matrix. They are relatively inactive, but can make small amounts of matrix to maintain it.– Lacunae: spaces occupied by

osteocyte cell body– Canaliculi: canals occupied by

osteocyte cell processes– Nutrients diffuse through tiny

amount of liquid surrounding cell and filling lacunae and canaliculi. Then can transfer nutrients from one cell to the next through gap junctions.

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Bone Cells• Osteoclasts. Responsible for resorption of bone

(breakdown of

bone) – Ruffled border (projections): where cell membrane

borders bone and resorption is taking place. – H ions pumped across membrane, acid forms, eats

away bone. – Release enzymes that digest the bone.– Derived from monocytes (which are formed from stem

cells in red bone marrow)– Multinucleated

• Stem Cells. Mesenchyme (Osteochondral Progenitor Cells) become chondroblasts or osteoblasts. (Remember: osteocytes derived from osteoblasts)

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Woven and Lamellar Bone

• Woven bone. Collagen fibers randomly oriented. - first formed during fetal development or repair of fracture- after formation, osteoclasts break down woven bone & osteoblasts

build new matrix---------------this process called remodeling

• Remodeling– Removing old bone and adding new

– Woven bone is remodeled into lamellar bone

• Lamellar bone– Mature bone in sheets called lamellae. Fibers are

oriented in one direction in each layer, but in different directions in different layers for strength.

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Cancellous (Spongy) Bone

• Trabeculae: interconnecting rods or plates of bone. Like scaffolding. – Spaces filled with marrow & blood vessels. – Covered with endosteum.– Oriented along stress lines– Trabeculae consist of several lamellae (thin sheets) w/ osteocytes

located in lacunae between them

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Compact Bone• Central or Haversian canals: parallel

to long axis

• Lamellae: thin sheets/layers concentric (circles), circumferential (perimeter, periphery), interstitial (between osteon)

• Osteon or Haversian system: central canal (also called haversian canals------contains blood vessels that run parallel to long axis of bone), associated concentric lamellae and osteocytes

• Perforating or Volkmann’s canal: perpendicular to long axis. Both perforating and central canals contain blood vessels. Central canals receive blood vessels from perforating canals & nutrients in blood enter central canals, pass into canaliculi, move through osteocytes to osteocytes (by gap

junctions). Waste removed in reverse direction.

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Bone Shapes

• Long

– Ex. Upper and lower limbs

• Short

– Ex. Carpals and tarsals

• Flat

– Ex. Ribs, sternum, skull, scapulae

• Irregular

– Ex. Vertebrae, facial

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Structure of a Long Bone• Diaphysis

– Shaft– Compact bone

• Epiphysis– End of the bone– Cancellous bone

• Epiphyseal plate: growth plate (separates epiphysis from diaphysis)

– Hyaline cartilage; present until growth stops

• Epiphyseal line: bone stops growing in length (epiphyseal plate becomes epiphyseal line)

• Medullary cavity (space): In children medullary cavity is red marrow, gradually changes to yellow in limb bones and skull (except for epiphyses of long bones). Rest of skeleton is red.

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Structure of a Long Bone, cont.• Periosteum

– Outer is fibrous (contains blood vessels & nerves)

– Inner is single layer of bone cells including osteoblasts, osteoclasts and osteochondral progenitor cells

– Tendons & ligaments attach to bone & become continuous with fibers of periosteum.

– Sharpey’s fibers: some collagen fibers of tendons or ligaments penetrate the periosteum and into the bone. Strengthen attachment of tendon or ligaments to bone.

• Endosteum. Similar to periosteum, but more cellular. Lines all internal surfaces of all cavities including spaces in cancellous bone.

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Structure of Flat, Short, and Irregular Bones

• Flat Bones– No diaphyses, epiphyses– Sandwich of cancellous

between compact bone

• Short and Irregular Bone– Compact bone that surrounds

cancellous bone center; similar to structure of epiphyses of long bones (bec. have processes that possess epiphyseal growth plates

– No diaphyses and not elongated

• Some flat and irregular bones of skull have sinuses (air filled spaces) lined by mucous membranes.

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Bone Development

• Intramembranous ossification– Takes place in connective tissue membrane

• Endochondral ossification– Takes place in cartilage

• Both methods of ossification– Produce woven bone that is then remodeled– After remodeling, formation cannot be

distinguished as one or other