Bone Structure and Function Quiz

Bone Tissue

• Osteology: study of osseous structures
• Functions:
  • Support
  • Protection
  • Movement
  • Mineral homeostasis
  • Hemopoiesis: blood cell formation
  • Storage of adipose tissue: yellow marrow

Long Bone Anatomy

• Diaphysis: long shaft of bone
• Epiphysis: ends of bone
• Epiphyseal plate: layer of hyaline cartilage allowing diaphysis growth in length
• Metaphysis: between epiphysis and diaphysis
• Articular cartilage: thin layer of hyaline cartilage covering the epiphysis where the bone forms a joint with another bone. Reduces friction and absorbs shock at freely movable joints.

Periosteum

• Bone covering (pain sensitive)
• Composed of outer fibrous layer & inner osteogenic layer (cells)
• Enables bone to grow in thickness
• Protects the bone
• Assists in fracture repair
• Nourishes bone tissue
• Attachment point for ligaments and tendons
• Sharpey's fibers: periosteum attaches to underlying bone through thick bundles of collagen fibers.

Medullary Cavity

• Hollow chamber in bone
• Yellow marrow is adipose.
• Red marrow produces blood cells

Endosteum

• Thin layer lining the medullary cavity

Long Bone

• Spongy bone: inside of the bone
• Compact bone: outside of the bone
• Medullary cavity: hollow chamber in the center of the diaphysis

Different Types of Cells in Bone Tissue

• Osteoprogenitor cells: derived from mesenchyme, unspecialized stem cells. Undergo mitosis and develop into osteoblasts. Found on the inner surface of periosteum and endosteum.
• Osteoblasts: bone-forming cells, found on bone surface, no ability to mitotically divide, secrete collagen and organic components, initiate calcification. Surrounding themselves with extra cellular matrix, they become trapped in their secretions and become osteocytes.
• Osteocytes: mature bone cells, derived from osteoblasts. Do not secrete matrix material. Cellular duties include nutrient and waste exchange with the blood. Do not undergo cell division.
• Osteoclasts: huge cells derived from the fusion of as many as 50 monocytes, concentrated in the endosteum. Bone resorbing cells. Plasma membrane on the side facing the bone surface is deeply folded into a ruffled border. Release powerful lysosomal enzymes and acids that digest protein and mineral components of the underlying bone. Growth, maintenance, and bone repair.

Compact Bone

• External layer, called lamellar bone, formed from elongated tubules called lamellae. Contains few spaces, majority of all long bones, protection and strength (wt. bearing), concentric ring structure.
• Blood vessels and nerves penetrate periosteum through horizontal openings called perforating (Volkmann's) canals and connect periosteum, medullary cavity and central canal.
• Central (Haversian) canals run longitudinally, around canals are concentric lamellae, ring of calcified extracellular matrix. Osteocytes occupy lacunae ("little lakes") which are between lamellae. Radiating from the lacunae are channels called canaliculi (finger-like processes of osteocytes)
• Lacunae are connected to each other by canaliculi and provide many routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes.
• Osteon contains: central canal, surrounding lamellae, lacunae, osteocytes, and canaliculi.

Spongy Bone (Cancellous Bone)

• Internal layer, latticework of bone tissue (haphazard arrangement). Made of trabeculae ("little beams"), lamellae arranged in irregular lattice (thin columns). Filled with red and yellow bone marrow. Osteocytes get nutrients directly from circulating blood. Short, flat, and irregular bone is made up of spongy bone.

• Spongy bone tissue is light, reducing overall weight so that it moves readily when pulled by a skeletal muscle.

• Trabeculae of spongy bone tissue support and protect red bone marrow.

• Spongy bone tissue in hip bones, ribs, sternum, vertebrae, and long bone ends stores red bone marrow, thus hemopoiesis (Blood cell production) occurs in adults

Calcification/Mineralization

• Calcium phosphate combines with calcium hydroxide to form crystals of hydroxyapatite [Ca10(PO4)6(OH)2].
• As crystals form, they combine with other mineral salts (e.g., calcium carbonate, magnesium, fluoride, potassium, and sulfate)
• Crystallization hardens the tissue

Ossification/Osteogenesis

• The process of bone formation occurs in four principal situations:
  1. Initial formation of bones in an embryo and fetus
  2. Growth of bones during infancy, childhood, and adolescence until adult sizes are reached
  3. Remodeling of bone (replacement of old bone by new bone tissue throughout life)
  4. Repair of fractures (breaks in bones) throughout life
• Ossification (osteogenesis) begins around the 6th-7th week of embryonic life. At this time the embryonic skeleton is made of fibrous membranes and hyaline cartilage.

Intramembranous Ossification

• Bone develops from a fibrous membrane.
• Flat bones of skull, mandible, clavicles.
• Mesenchymal cells become vascularized and become osteoprogenitor cells and then osteoblasts.
• Organic matrix of bone is secreted, osteocytes are formed, calcium and mineral salts are deposited and bone tissue hardens, trabeculae develop and spongy bone is formed. Red marrow fills spaces.

Endochondral Ossification

• Replacement of hyaline cartilage with bone. Most bones are formed this way (e.g., long bones)
• Mesenchymal cells differentiate into chondroblasts (immature cartilage cells) which produce hyaline cartilage.
• Chondrocytes (mature) mitotically divide, increasing in length (interstitial growth).
• Growth of cartilage in thickness (appositional growth) due to new matrix deposition by chondroblasts within the perichondrium
• Hypertrophy, swelling, and bursting of cartilage cells
• pH changes, initiates calcification, cartilage cells die. Lacunae now empty.

Skeletal Cartilage

• Chondrocytes: cartilage-producing cells
• Lacunae: small cavities where chondrocytes are encased
• Extracellular matrix: jelly-like ground substance
• Perichondrium: layer of dense irregular connective tissue surrounding the cartilage
• No blood vessels or nerves

Fractures

• Any bone break.
  • Blood clot forms around break (fracture hematoma).
  • Inflammatory process begins
  • Blood capillaries grow into clot
  • Phagocytes and osteoclasts remove damaged tissue
  • Procallus (soft callus) forms: invaded by osteoprogenitor cells and fibroblasts.
  • Collagen and fibrocartilage turn procallus to fibrocartilaginous callus
  • Broken ends of bone are bridged by callus
  • Osteoprogenitor cells are replaced by osteoblasts, and spongy bone is formed.
  • Bony (hard) callus is formed.
  • Callus is resorbed by osteoclasts and compact bone replaces spongy bone.
  • Remodeling: shaft is reconstructed to resemble original unbroken bone.
    • Closed reduction: bone ends coaxed back into place by manipulation
    • Open reduction: surgery, bone ends secured together with pins or wires.

Types of Fractures

• Simple/Closed: bone breaks cleanly, but does not penetrate skin
• Compound/Open: broken ends of bone protrude through tissue and skin.
• Comminuted: bone fragments into many pieces.
• Compression: bone is crushed (due to porous bone)
• Depressed: broken bone is pressed inward (e.g., in skull)
• Colles': posterior displacement of distal end of radius (extension)
• Smith's: anterior displacement of distal end of radius (flexion)
• Transverse: break occurs across the long axis of a bone.
• Impacted: broken bone ends are forced into each other.
• Spiral: ragged break as a result of excessive twisting of bone.
• Epiphyseal: break occurring along epiphyseal line or plate.
• Greenstick: bone breaks incompletely.
• Pott's: malleolus of tibia and fibula break

Bone Remodeling

• Bone continuously renews itself
• Never metabolically at rest
• Enables calcium to be pulled from bone when blood levels are low.
• Osteoclasts responsible for matrix destruction
• Produce lysosomal enzymes and acids
• Spongy bone replaced every 3-4 years, compact bone every 10 years
• Blood calcium levels signal release of either parathyroid hormone (PTH) or calcitonin
  • PTH: stimulates osteoclast activity and bone resorption.
  • Calcitonin: inhibits bone resorption and calcium salts deposition

Bone Growth

• Epiphyseal plate: 4 zones of bone growth:
  1. Zone of resting cartilage
  2. Zone of proliferating cartilage
  3. Zone of hypertrophic (maturing) cartilage
  4. Zone of calcified cartilage.
• Bone width: increase in diameter through appositional growth
  • Osteoblasts from periosteum secrete matrix and build bone on surface.
  • Osteoclasts from endosteum resorb bone