Skeletal

• The skeletal system has four components: bones, cartilage, tendons, and ligaments
• Skeletal System


  • Organ protection and body support
  • Body movement
  • Mineral storage
  • Blood cell production
• The average adult has 206 bones, but the actual number varies between people and decreases with age as bones become fused
• Axial Skeleton


  Consists of the bones of the head and the trunk; forms the upright axis of the body
• Bones of the Axial Skeleton

  • Cranium (skull)
  • Mandible (jawbone)
  • Sternum (breastbone)
  • Ribs
  • Vertebral column
  • Sacrum
  • Coccyx (tailbone)
• Skull/Cranium and Mandible


  • The skull, or cranium protects the brain and houses our eyes, ears, nose, and mouth
  • The mandible (lower jaw) is inferior to the maxilla and attaches posteriorly to the temporal bone
• Vertebral Column


  • Supports the weight of the head and trunk
  • Protects the spinal cord
  • Allows spinal nerves to exit the spinal cord
  • Provides a site for muscle attachment
  • Permits movement of the head and trunk
• Rib Cage and Sternum


  • Protects the heart and lungs within the thorax
  • Forms a semirigid chamber, which can increase and decrease in volume during respiration
  • True ribs articulate with the thoracic vertebrae and attach directly through their costal cartilages to the sternum
  • False ribs articulate with the thoracic vertebrae but do not attach directly to the sternum
• Appendicular Skeleton


  Consists of the bones of the upper limbs, the lower limbs, and the girdles
• Pectoral Girdle


  • Consists of scapula (shoulder blade) and a clavicle (collarbone)
• Bones of the Upper Limb

  • Arm (humerus)
  • Forearm (ulna and radius)
  • Wrist (8 carpal bones)
  • Hand (5 metacarpal bones and phalanges)
• Pelvic Girdle


  • The right and left hipbones (coxal bones) join each other anteriorly and the sacrum posteriorly to form a ring of bone
• Bones of the Lower Limb

  • Thigh (femur)
  • Kneecap (patella)
  • Leg (tibia and fibula)
  • Foot (7 tarsal bones, 5 metatarsal bones, and phalanges)
• Bone Shapes


  • Long bones
  • Flat bones
  • Short bones
  • Irregular bones
• Bone Cells


  • Osteogenic cells
  • Osteoblasts
  • Osteocytes
  • Osteoclasts
• Bone Formation


  1. Intramembranous ossification

  2. Endochondral ossification
• Intramembranous Ossification


  Bone forms from preexisting connective tissue membranes
• Osteocytes


  Bone cells that develop from osteoblasts as their structure and function changes
• Osteoclasts


  Cells that develop from monocytes and macrophages and are used in breaking down bone
• Ossification (osteogenesis)


  The process of bone formation by osteoblasts
• Endochondral ossification


  • Bone forms from a preexisting cartilage model
• Intramembranous ossification


  1. Ossification begins as mesenchymal cells form a template of the future bone

  2. Osteoblasts secrete osteoid and deposit calcium

  3. Clusters of osteoid unite around capillaries to form trabecular matrix

  4. Spongy bone forms and red bone marrow develops

  5. Bone is constantly remodeled by osteoclasts
• Endochondral ossification (in long bones)

  1. Mesenchymal cells differentiate into chondroblasts that form hyaline cartilage model

  2. Blood vessels bring osteoblasts to edges, forming bone collar

  3. Chondrocytes die in center, allowing blood vessel invasion and primary ossification center

  4. Cartilage continues to grow at epiphyses, increasing bone length

  5. Secondary ossification centers form after birth

  6. Epiphyseal plate is eventually replaced by epiphyseal line
• Lengthening of long bones

  Chondrocytes on epiphyseal side of plate divide, replacing cartilage with bone on diaphyseal side
• Thickening of long bones

  Appositional growth - osteoblasts add bone matrix on surface, osteoclasts break down bone on inner surface
• Bone remodeling


  Replacement of old bone tissue by new bone tissue, involving bone deposition by osteoblasts and bone resorption by osteoclasts
• Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous, and magnesium. Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for proper bone growth and maintenance.
• Bone remodeling allows bones to adapt to stresses by becoming thicker and stronger when subjected to stress.
• Bone repair (fracture)


  1. Blood vessels tear, forming hematoma

  2. Capillaries grow, phagocytic cells clear dead cells, fibroblasts and osteoblasts reform bone

  3. Fibrocartilaginous callus forms, then converts to bony callus of spongy bone

  4. Bony callus is remodeled by osteoclasts and osteoblasts to restore compact bone
• Osteoblasts


  A type of immature bone cell responsible for bone formation
• Osteocytes


  Mature bone cells that develop from osteoblasts and make up the majority of bone tissue