SKELETAL

Overview of the Skeletal System

  • The skeletal system consists of:

    • Bones

    • Cartilages

    • Tendons

    • Ligaments

  • Functions of the skeletal system:

    1. Support the body

    2. Protection of organs

    3. Body movement

    4. Mineral storage

    5. Blood cell production

    6. Energy storage

Bone Cells

  • Osteoblasts:

    • Derived from osteogenic cells

    • Responsible for the formation of new bone

    • Produce matrix of bone

    • Process of new bone formation is called ossification

  • Osteocytes:

    • Maintain bone matrix

    • Form from osteoblasts after bone matrix surrounds them

    • Housed in lacunae; extensions in canaliculi

  • Osteoclasts:

    • Responsible for bone resorption

    • Important for mobilizing crucial calcium ions (Ca2+)

    • Contribute to bone repair and remodeling

Bone Histology

  • Bone matrix composition:

    • 35% organic (collagen, proteoglycans)

    • 65% inorganic (calcium phosphate crystals - hydroxyapatite)

  • Bone is organized into thin, concentric sheets called lamellae

  • Classification of bone according to matrix:

    • Spongy Bone: Less matrix, more space

    • Compact Bone: More matrix, less space

Compact (Cortical) Bone

  • Solid outer layer surrounding each bone

  • Functional unit is osteon:

    • Concentric rings of matrix surrounding central canal

    • Central canals contain blood vessels, nerves, connective tissue

  • Lacunae: Spaces for osteocytes between lamellae

  • Canaliculi: Connect lacunae, transport nutrients, remove wastes

Spongy (Cancellous or Trabecular) Bone

  • Composed of trabeculae (interconnecting rods or plates)

  • Spaces between trabeculae filled with bone marrow and blood vessels

  • Trabecular surfaces covered with osteoblasts and osteoclasts

Bone Classification

  • Five classifications of bone shapes:

    1. Long Bones: Longer than wide (e.g., humerus, femur)

    2. Short Bones: Approximately as wide as long (e.g., carpal bones)

    3. Flat Bones: Thin, flattened shape (e.g., sternum)

    4. Sesamoid Bones: Develop within tendons (e.g., patella)

    5. Irregular Bones: Shapes that do not fit other categories (e.g., vertebrae)

Structure of a Long Bone

  • Diaphysis:

    • Thick walls of compact bone surrounding a medullary cavity

  • Epiphyses:

    • Ends of long bone containing mostly spongy bone with outer compact layer

    • Covered with hyaline cartilage at joints (articular cartilage)

    • Epiphyseal plate allows bone growth in length, later becomes epiphyseal line

Bone Marrow

  • Cavities in spongy bone and medullary cavity filled with marrow:

    • Red Marrow: Location of blood-forming cells

    • Yellow Marrow: Mostly adipose tissue

  • In newborns: Long bones mostly filled with red marrow

  • In adults: Most red marrow found in flat bones

Periosteum and Endosteum

  • Periosteum:

    • Connective tissue layer wrapping outer surface of bone

    • Two layers: outer fibrous layer and inner cellular layer (osteoblasts and osteoclasts)

  • Endosteum:

    • Single cell layer lining internal surfaces of all cavities within bones

Bone Formation

  • Bone formation occurs during fetal development via two processes:

    • Intramembranous Ossification:

      • Occurs within embryonic connective tissue membranes

      • Primarily in flat bones of the skull

    • Endochondral Ossification:

      • Starts with a hyaline cartilage model

      • Occurs in most bones, including long bones

Intramembranous Ossification

  • Begins when osteoblasts produce bone within an embryonic connective tissue membrane

  • Initiated from ossification centers radiating trabeculae outward

  • Trabeculae can be remodeled or replaced by compact bone

Endochondral Ossification

  • Begins from a hyaline cartilage model

  • Primary Ossification Center: Develops in the diaphysis

  • Secondary Ossification Centers: Form in each epiphysis

  • Bone lengthens until osteoblasts outpace chondrocytes; epiphyseal plate is replaced by epiphyseal line

Bone Remodeling

  • Involves:

    • Resorption of bone by osteoclasts

    • Deposition of new bone by osteoblasts

  • Responsible for:

    • Changes in bone shape

    • Bone repair

    • Adjustment to stress

    • Regulation of calcium ions

  • Influences:

    • Exercise increases deposition of bone matrix

    • Adequate calcium and Vitamin D are critical

    • Sedentary lifestyle and aging lead to reduced bone mass

  • Osteoporosis: Significant loss of bone mass, raising fracture risk

  • Astronauts can experience rapid bone mass decline in microgravity (up to 10 years in six months).