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• Components of Skeletal system

  • Bone
  • Cartilage
  • Tendon
  • Ligament
• Skeletal system Functions


  • Support
  • Protect
  • Movement
  • Storage
  • Blood cell production
• Extracellular Matrix


  Bone, cartilage, tendon, and ligaments of the skeletal system are all connective tissues. Their characteristics are largely determined by the composition of their extracellular matrix.
• Extracellular Matrix composition


  It always contains collagen, ground substance, and other organic molecules, as well as water and minerals.
• Collagen


  • A tough, ropelike protein
• Proteoglycans


  • Large molecules consisting of many polysaccharides attaching to and encircling core proteins. They form large aggregates and attract water.
• Extracellular Matrix of tendons and ligaments

  • Contains large amounts of collagen fibers, making these structures very tough, like ropes or cables
• Cartilage Extracellular Matrix


  • Contains collagen and proteoglycans. Collagen makes cartilage tough, whereas the water-filled proteoglycans make it smooth and resilient. As a result, cartilage is relatively rigid, but it springs back to its original shape after being bent or slightly compressed. It is an excellent shock absorber.
• Bone Extracellular Matrix


  • Contains collagen and minerals, including calcium and phosphate. The ropelike collagen fibers lend flexible strength to the bone. The mineral component gives bone compression (weight-bearing) strength. Most of the mineral in bone is in the form of calcium phosphate crystals called hydroxyapatite.
• Bone shape classifications


  • Long
  • Short
  • Flat
  • Irregular
• Long bones


  • Longer than they are wide, examples are upper and lower limb bones
• Short bones


  • Approximately as wide as they are long, examples are the bones of the wrist and ankle
• Flat bones


  • Have a relatively thin, flattened shape, examples are bones of the skull and sternum
• Bone Marrow


  • Red marrow (blood forming cells)
  • Yellow marrow (mostly fat)
• Spongy (Cancellous) Bone Tissue


  • Located at the epiphyses of long bones and center of other bones. It has trabeculae, which are interconnecting rods, and spaces that contain marrow. It has no osteons.
• Bone Cells


  • Osteoblasts (responsible for bone formation and repair/remodeling)
  • Osteocytes (maintain bone matrix and form from osteoblasts after bone matrix has surrounded it)
  • Osteoclasts (contribute to bone repair and remodeling by removing existing bone, called bone reabsorption)
• Intramembranous Ossification


  Osteoblasts begin to produce bone within connective tissue. This occurs primarily in the bones of the skull. Osteoblasts line up on the surface of connective tissue fibers and begin depositing bone matrix to form trabeculae. The process begins in areas called ossification centers and the trabeculae radiate out from the centers. Usually, two or more ossification centers exist in each flat skull bone and mature skull bones result from fusion of these centers as they enlarge. The trabeculae are constantly remodeled and they may enlarge or be replaced by compact bone.
• Endochondral Ossification


  Bone formation within a cartilage model. The cartilage model is replaced by bone. Initially formed is a primary ossification center, which is bone formation in the diaphysis of a long bone. A secondary ossification center is bone formation in the epiphysis.
• Steps in Endochondral Ossification

  Chondroblasts build a cartilage model, the chondroblasts become chondrocytes

  2. Cartilage model calcifies (hardens)

  3. Osteoblasts invade calcified cartilage and a primary ossification center forms diaphysis

  4. Secondary ossification centers form epiphysis

  5. Original cartilage model is almost completely ossified and remaining cartilage is articular cartilage
• As osteoblasts deposit a new bone matrix on the surface of bones between the periosteum and the existing bone matrix


  The bone increases in width, or diameter (appositional growth)
• Bone Growth in Length

  Growth in the length of a bone, which is the major source of increased height in an individual, occurs in the epiphyseal plate. This type of bone growth occurs through endochondral ossification. Chondrocytes increase in number on the epiphyseal side of the epiphyseal plate. Then the chondrocytes enlarge and die. Much of the cartilage matrix that forms around the enlarged cells is removed by osteoclasts, and the dying chondrocytes are replaced by osteoblasts. The osteoblasts start forming bone by depositing bone lamellae on the surface of the calcified cartilage. This process produces bone on the diaphyseal side of the epiphyseal plate.
• Bone Remodeling involves the removal of existing bone and the formation of new bone
• Appositional growth


  The process where the bone matrix between the periosteum and the existing bone matrix increases in width, or diameter
• Bone growth in length

  1. Chondrocytes increase in number on the epiphyseal side of the epiphyseal plate

  2. The chondrocytes enlarge and die

  3. The cartilage matrix becomes calcified

  4. Osteoclasts remove much of the cartilage that forms around the enlarged cells

  5. Osteoblasts start forming bone by depositing bone lamellae on the surface of the calcified cartilage
• Endochondral bone growth


  The process that produces bone on the diaphyseal side of the epiphyseal plate
• Bone remodeling


  1. Removal of existing bone by osteoclasts

  2. Deposition of new bone by osteoblasts

  3. Occurs in all bones

  4. Responsible for changes in bone shape, bone repair, adjustment of bone to stress, and calcium ion regulation
• Bone repair


  1. Broken bone causes bleeding and a blood clot forms

  2. Callus forms which is a fibrous network between 2 fragments

  3. Cartilage model forms first then osteoblasts enter the callus and form a cancellous bone that continues for 4-6 weeks after injury

  4. Cancellous bone is slowly remodeled to form compact and cancellous bone
• Bone calcium homeostasis


  Bone is a major storage site for calcium<|>Movement of calcium in and out of bone helps determine blood levels of calcium<|>Calcium moves into bone as osteoblasts build new bone<|>Calcium moves out of bone as osteoclasts break down bone<|>Calcium homeostasis is maintained by parathyroid hormone (PTH) and calcitonin
• Axial skeleton


  Composed of the skull, the vertebral column, and the thoracic cage
• Cranial bones


  • Frontal bone
  • Parietal bones
  • Occipital bone
  • Temporal bones
  • Sphenoid bone
  • Ethmoid bone
• Facial bones


  • Maxillae
  • Palatine bones
  • Zygomatic bones
  • Lacrimal bones
  • Nasal bones
  • Vomer
  • Inferior nasal conchae
  • Mandible
• Paranasal sinuses


  Several of the bones associated with the nasal cavity have large cavities within them, called the paranasal sinuses which open into the nasal cavity
• Hyoid bone


  An unpaired, U-shaped bone that is not part of the skull and has no direct bony attachment to the skull or any other bones. It provides an attachment for some tongue muscles, and it is an attachment point for important neck muscles that elevate the larynx.
• Regions of the vertebral column

  • Cervical
  • Thoracic
  • Lumbar
  • Sacral
  • Coccygeal
• Functions of the vertebral column

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