Skeletal System

List and explain the five general functions of the skeletal system.

  • Provides structural support, frame work for soft tissue + organ attachment

  • Stores minerals: contribute to body fluid concentration - Calcium

    • Lipids: store energy as yellow bone marrow ← adipose tissue

  • Blood production: red bone marrow → red blood cells, white blood cells

  • Protection - surrounds soft tissue + organs

  • Leverage: like levers for muscles to produce movement



Two Main Components of Bone

  • Spongy Bone - in epiphyses of long bones and inside short/flat bones - trabeculae branches

    • Sandwich- compact:diploe:compact


  • Compact Bone - in diaphyses

    • Forms medullary cavity (protective layer)

    • Has osteons arranged in lamellae rings


Cellular Functions

Osteoblasts

  • Function: These cells synthesize and secrete the organic bone matrix called osteoid.

  • Origin: They differentiate from osteogenic progenitor cells in the periosteum.

  • Location: They are found on the growing surfaces of bone tissue.

  • Fate: They eventually become trapped in the matrix and transform into osteocytes.

2. Osteocytes

  • Function: These mature cells maintain the daily metabolism of bone tissue.

  • Origin: They develop directly from osteoblasts surrounded by mineralized bone matrix.

  • Location: They reside in small cavities called lacunae within the matrix.

  • Communication: They connect with each other through tiny channels known as canaliculi.

3. Osteoclasts

  • Function: These large cells break down and resorb bone matrix minerals.

  • Origin: They originate from the fusion of hematopoietic monocyte-macrophage progenitor cells.

  • Location: They sit in shallow depressions on the bone surface called resorption bays.

  • Structure: They are large, multinucleated cells featuring a specialized ruffled border membrane.



Types of Bones

  1. Long Bones - significantly longer than they are wide, featuring a shaft and two distinct ends.

• Examples: The femur, humerus, and radius serve as prime examples of this classification.


  1. Short Bones - roughly cube-shaped and provide stability and support with limited gliding movement.

• Examples: The carpal bones of the wrist and tarsal bones of the ankle.


  1. Flat Bones - thin, flattened, and typically curved, providing protection for internal organs.

• Examples: The sternum, scapula, and cranial bones illustrate this bone category.


  1. Irregular Bones - complex, specialized shapes that do not fit the other standard categories.

• Examples: The vertebrae of the spinal column and the sphenoid bone of the skull.


  1. Sutural Bones - small, flat, oddly shaped

• Examples: Between flat bone in the skull


  1. Sesamoid - small, round, flat

• Examples: Near knees, hands, feet joints - Patella + Pisiform




Compare Interstitial Growth vs Appositional Growth

Interstitial Growth

• Definition: Interstitial growth causes an increase in the length of long bones.

• Location: It occurs primarily at the epiphyseal growth plate within the bone.

• Mechanism: Cartilage cells divide and elongate, eventually becoming replaced by calcified bone tissue.

• Duration: This growth process stops when the epiphyseal plates close at adulthood.


Explain the role of vitamin D in a healthy skeleton

  • Calcium absorption

  • Regulate mineral balance


  • Parathormone: Increases blood calcium levels to parathyroid glands

    • Stimulates osteoclasts to break down bone matrix

  • Calcitonin: Lowers blood calcium levels to stimulate thyroid gland

    • Inhibits osteoclast activity and promotes calcium deposition into bones


Repair Cycle

1. Form Hematoma Clot

  • Bleeding: Blood vessels tear during the break and bleed into the fracture site.

  • Clot: A mass of clotted blood called a hematoma forms at the site.

  • Inflammation: Bone cells deprived of nutrients begin to die, causing localized swelling.

2. Form Soft Callus

  • Invasion: Capillaries grow into the hematoma and phagocytic cells clear debris.

  • Fibroblasts: Fibroblasts and osteoblasts migrate to the fracture from the periosteum.

  • Cartilage: They secrete collagen and a fibrocartilaginous callus bridges the broken ends.

3. Form Hard Callus

  • Osteogenesis: New trabeculae appear in the fibrocartilaginous callus over the next few weeks.

  • Conversion: The soft callus is gradually replaced by immature bone, forming a hard callus.

  • Unity: This bony callus firmly unites the two broken bone fragments together.

4. Remodel Bone

  • Duration: This final phase takes several months following the initial injury.

  • Resorption: Excess material on the diaphysis exterior and medullary cavity is removed.

  • Structure: The bone is reshaped to return to its original, strong structural condition.