Skeletal System

• Components of Skeletal System

  • Bones
  • Cartilages
  • Tendons
  • Ligaments
• Cartilages


  • Connective tissue that is typically found between bones for cushioning so that bones do not have an impact with each other or do not rub on each other
• Tendons


  • Connective tissues that connect muscle to bones
• Ligaments


  • Connective tissues that connect bone to bone
• Skeletal System Functions


  • Support: Framework for body
  • Protect: Protects the organs it surrounds (Thoracic cage protects heart and lungs)
  • Movement: Tendons attach muscle to bone
  • Contractions
  • Storage: Calcium, Phosphorus, and Adipose tissue
  • Blood cell production: Red bone marrow produces blood cells and platelets
• Extracellular Matrix


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

  • Collagen (tough, ropelike protein)
  • Ground substance
  • Other organic molecules
  • Water
  • Minerals
• Proteoglycans


  Large molecules consisting of many polysaccharides attaching to and encircling core proteins, they attract and retain water
• Extracellular Matrix of Tendons and Ligaments

  • Contains large amounts of collagen fibers, which makes them tough and strong
• Extracellular Matrix of Cartilage

  • Contains collagen and proteoglycans, collagen makes cartilage tough, water-filled proteoglycans make it smooth and resilient
• Cartilage is relatively rigid, but it springs back to its original shape after being bent or slightly compressed, it is an excellent shock absorber
• Extracellular Matrix of Bone

  • Contains collagen and minerals (calcium and phosphate), the ropelike collagen fibers lend flexible strength, the mineral component gives bone compression (weight-bearing) strength
• Bone Shape Classifications


  • Long
  • Short
  • Flat
  • Irregular
• Long Bones


  • Longer than they are wide, examples are upper and lower limb bones (femur, humerus, tibia, fibula, ulna, radius)
• Short Bones


  • Approximately as wide as they are long (more square and round), examples are the bones of the wrist and ankle (carpals and tarsals), help transfer force between long bones
• Flat Bones


  • Have a relatively thin, flattened shape, examples are the skull, sternum, ribs, scapulae
• Irregular Bones


  • Have odd shapes that do not fit readily into the other three categories, examples are the vertebrae and facial bones
• Long Bone Structures


  • Diaphysis: Shaft, Compact bone tissue (on outside)
  • Epiphysis: Ends of spongy bone tissue
  • Articular cartilage: Covers epiphysis and reduces friction
  • Epiphyseal plate: Site of active growth between diaphysis and epiphysis in adults is called the epiphyseal line and in juveniles is called the epiphyseal plate
  • Medullary cavity: Center of diaphysis red (blood cells) or yellow marrow (fat)
  • Periosteum: Membrane/covering around bone's outer surface, Thicker, has 2 layers, contains vessels and nerves
  • Endosteum: Membrane that lines the inner surface of bone inside the medullary cavity, thinner than periosteum and within diaphysis of bone
• Red Marrow


  Location of blood forming cells (baby-7 y/o or Juvenile)
• Yellow Marrow


  Mostly fat (7 y/o-adults)
• In newborns most bones have blood making red bone marrow, red blood cells
• In adults most red bone marrow is in the flat bones and the long bones of the femur and humerus, everything else has been pretty much converted to yellow marrow
• Compact Bone Tissue


  • Located: Outer part of diaphysis (long bones) and thinner surfaces of epiphysis and other bones, Structural unit: Osteon includes lamella, lacunae, canaliculus, central canal, osteocytes
• Spongy Bone Tissue


  • Located at the epiphyses of long bones and center of other bones such as flat and irregular, Has trabeculae, which are interconnecting rods, and spaces that contain marrow, Has no osteons
• Bone Cells


  • Osteoblasts: Responsible for the formation of bone and the repair and remodeling of bone
  • Osteocytes: Cells that maintain bone matrix and form from osteoblast after bone matrix has surrounded it
  • Osteoclasts: Contribute to bone repair and remodeling by removing existing bone, called bone (reabsorption)
• Bone Remodeling


  A constant cycle occurring to keep bone strong and new, removing existing bone matrix and osteoblast will form the new bone matrix and osteocytes will maintain the new ones
• Types of Bone Formation

  • Intramembranous ossification: Formation that occurs within connective tissue membranes
  • Endochondral ossification: Formation that occurs inside hyaline cartilage
• Intramembranous Ossification


  1. Osteoblasts line up on the surface of connective tissue fibers and begin depositing bone matrix to form trabeculae

  2. The process begins in areas called ossification centers (within skull connective tissues) and the trabeculae radiate outward from the centers

  3. 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

  4. The trabeculae are constantly remodeled (broken down/remade) so that the fetus can enlarge and grow and they may enlarge or be replaced by compact bone or surroundings where parts of skull bones will be spongy
• Endochondral Ossification


  1. Hyaline Cartilage creates a mold

  2. Chondroblasts build a cartilage model (in the general shape of a bone)

  3. Cartilage model calcifies (hardens)

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

  5. Make and deposit bone matrix

  6. Secondary ossification centers form epiphysis

  7. Deposit bone matrix in epiphysis

  8. Original cartilage model is almost completely ossified and the remaining cartilage is articular cartilage (end of long bones to protect them at the joints)
• Bone Growth in Width (Appositional Growth)

  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
• Bone Growth in Length

  1. Chondrocytes (cartilage cells) increase in number on the epiphyseal side of the epiphyseal plate

  2. Then the chondrocytes enlarge

  3. The cartilage matrix becomes calcified

  4. The dying chondrocytes are replaced by osteoblasts

  5. The osteoblasts start forming bone by depositing bone lamellae on the surface of the calcified cartilage

  6. This process produces bone on the diaphyseal side of the epiphyseal plate
• Bone Remodeling Involves


  • Removal of existing bone by osteoclasts
  • Deposition of new bone by osteoblasts
  • Occurs in all bones
  • Responsible for changes in bone shape, bone repair, adjustment of bone to stress, and calcium ion regulation
• Bone and 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
• Paranasal Sinuses


  • Frontal Sinus (Frontal Bone)
  • Ethmoid Sinus (Ethmoid Bone)
  • Sphenoid Sinus (Sphenoid Bone)
  • Maxillary Sinus (Maxillary Bone)
• Structural Classification of Joints

  • Fibrous joint: United by fibrous connective tissue
  • Cartilaginous: United by means of cartilage connected between bones
  • Synovial: Joined by a fluid cavity, two bones come together with a fluid cavity
• Functional Classification of Joints

  • Synarthrosis: Non-movable joint
  • Amphiarthrosis: Slightly movable joint
  • Diarthrosis: Freely movable joint
• Types of Movement


  • Flexion: Bending
  • Extension: Straightening
  • Abduction: Movement away from midline
  • Adduction: Movement toward the midline
  • Pronation: Rotation of the forearm with palms down
  • Supination: Rotation of the forearm with palms up
  • Rotation: Movement of a structure about the long axis
• Effects of Aging on the Skeletal System and Joints

  • Decreased Collagen Production
  • Joints have less collagen
  • Loss of Bone Density
  • Degenerative Changes