Bones and Joints

• Bone Function

  • Support: Structural framework for the body, provides attachment for tendon
  • Protection: Protect internal organs from injury
  • Assistance in movement: When muscles contract they pull on the bones
  • Mineral Homeostasis: Store mineral e.g. calcium, phosphorus
  • Blood cell production: Red bone marrow produces blood cells
• Bone structure

  • Diaphysis: Shaft or body
  • Epiphysis: Distal and proximal ends of the bone
  • Metaphysis: Region in mature bone where epiphysis joins diaphysis
  • Epiphyseal plate: The point where cartilage is replaced by bone – layer of hyaline cartilage
  • Articular cartilage: Layer of hyaline cartilage covering the epiphysis
  • Periosteum: Membrane surrounding the outside of bone
  • Medullary cavity: Space containing yellow bone marrow
  • Endosteum: Membrane that contains osteoblasts and lines the medullary cavity
• Types of bones

  • Axial skeleton: Skull, vertebrae and thoracic cage
  • Appendicular skeleton: 126 bones of limbs and their girdles
• Bones are named based on their shape

  • Long: Tubular shaft (diaphysis) and 2 expanded ends (epiphysis)
  • Short: Cuboid – named according to shape
  • Flat
  • Irregular
• Bone markings

  • The external surface of bone has distinct bone markings that tell us how muscles, tendons and ligaments attach to bone and how vessels run through the bone
• Types of bone cells

  • Osteoblasts: Bone building cells
  • Osteocytes: Mature bone cells that maintain bone tissue
  • Osteoclasts: Huge multinucleated cells that reabsorb and remodel bone within the endosteum
• Compact bone

  • Arranged in units called osteons or Haversian systems
  • Contain lamellae, lacunae, osteocytes, canaliculi and central canals
• Osteoblasts

  1. Synthesize and secrete matrix minerals

  2. Initiate calcification by the deposition of calcium and other mineral deposits
• Osteocytes


  Mature bone cells that maintain bone tissue
• Osteoclasts


  Reabsorb and remodel bone within the endosteum
• Compact bone components

  1. Central canal (Haversian): Contains blood vessels and nerves

  2. Lamellae: Concentric rings of hard calcified matrix

  3. Lacunae: Small spaces between lamellae that contain osteocytes

  4. Canaliculi: Provide routes for nutrients and oxygen to diffuse to osteocytes
• Bone structure

  • Hard calcified matrix
  • Surround the central canals
  • Lacunae
  • Canaliculi
• Spongy bone

  • Consists of lamellae arranged in an irregular lattice of thin columns called trabeculae
  • Microscope spaces between trabeculae are filled with red bone marrow, which produces blood cells
• Bone Formation - Intramembranous ossification

  1. The formation of bone directly on or within fibrous connective tissue membranes

  2. Formed by condensed mesenchyme cells (centre of ossification) which differentiate into osteoblasts

  3. Osteoblast lay bone matrix around themselves, to become osteocytes and calcification occurs

  4. Bone matrix develops into trabeculae to form spongy bone, spongy bone on the outside is eventually replaced by compact bone
• Growth and repair of cartilage - Interstitial growth

  1. Cartilage expands from within

  2. Cartilage increases in size due to the division of existing chondrocytes and the deposition of matrix

  3. Occurs while cartilage is young and pliable, during childhood and adolescence
• Growth and repair of cartilage - Appositional growth


  1. Activity of cells in the perichondrium leads to growth in width

  2. Fibroblasts within perichondrium divide into chondroblasts

  3. Chondroblasts lay matrix around themselves and become chondrocytes

  4. Growth starts later than interstitial growth and continues through adolescence
• Bone Formation - Endochondral ossification


  1. The formation of bone within hyaline cartilage

  2. Development of cartilage model

  3. Mesenchyme cells differentiate into chondroblasts which produce a hyaline cartilage ‘model’ of bone

  4. Chondroblasts surround themselves in matrix to become chondrocytes, with a thin membrane of perichondrium around the cartilage model

  5. Growth of cartilage model

  6. The cartilage model grows in length by the continual cell division of chondrocytes and the deposition of cartilage matrix (interstitial growth)

  7. Growth in thickness is due to deposition of matrix to the periphery of the model

  8. New chondroblasts in the perichondrium lay down matrix (appositional growth)
• Primary ossification centre

  1. A nutrient artery penetrates the cartilage model in the middle, stimulates the differentiation of osteoblasts which lay down matrix in the perichondrium

  2. These lay down a thin layer of compact bone beneath the perichondrium

  3. The membrane is now known as the periosteum

  4. Periosteal capillaries grow into disintegrating calcified cartilage

  5. These vessels plus associated osteoblast, osteoclasts and red bone marrow cells = periosteal bud

  6. Capillaries stimulate growth of primary ossification centre

  7. Osteoblasts deposit bone matrix (replace cartilage) form spongy bone

  8. The diaphysis’s spongy bone is then replaced by compact bone

  9. A cavity develops in the core of the model (medullary cavity)

  10. Primary ossification proceeds inward from the external surface of bone
• Secondary ossification centre

  Blood vessels enter the epiphysis to initiate ossification
• Osteoblasts deposit bone matrix


  Replace cartilage and form spongy bone
• The diaphysis’s spongy bone


  Is then replaced by compact bone
• A cavity develops in the core of the model

  Medullary cavity
• Primary ossification

  Proceeds inward from the external surface of bone
• Secondary ossification centre

  1. Blood vessels enter the epiphysis to initiate ossification

  2. Usually occurs around the time of birth

  3. Bone formation is similar to primary ossification

  4. Except spongy bone remains in the interior of the epiphysis

  5. Secondary ossification proceeds outwards from the centre
• Post natal bone growth

  1. Growth in length (long bones)

  2. Long bones grow in length by interstitial growth of the epiphyseal plate cartilage

  3. Cartilage cells in the plate form tall columns that allow fast growth

  4. The epiphyseal plate is divided into 5 zones: Resting zone, Proliferation zone, Hypertrophic zone, Calcification zone, Ossification zone

  5. During growth, epiphyseal plate remains the same thickness due to the rate of cartilage growth in the proliferation zone balancing the replacement by bone in the ossification zone

  6. Growth in length is accompanied by almost continuous remodeling of the epiphyseal ends to maintain the proportion between the diaphysis and epiphyses

  7. Epiphyseal plate closure occurs when chondroblasts of the epiphyseal plate stop dividing, the plate becomes thinner and eventually the diaphysis and epiphysis fuse (epiphyseal line), happens at about 18 years in females and 21 years in males

  8. Growth in width (thickness): Bones increase in thickness due to appositional growth, osteoblasts in the periosteum secrete matrix on the external bone surface, while osteoclasts reabsorb bone on the endosteal surface, normally there is slightly more building than reabsorbing
• Joints

  1. Also called an articulation or arthrosis

  2. Point of contact between bone-bone, or bone-cartilage

  3. Permit movement without damaging each other

  4. Offer mechanical support
• Joint classification

  • STRUCTURAL (Fibrous, Cartilaginous, Synovial)
  • FUNCTIONAL (amount of movement)
  • BIOMECHANICAL (simple/complex)
  • ANATOMICAL (wrist, shoulder, hip, articulations of hand, etc)