Chapter 5: The Skeletal System

Overview of the Skeletal System

The skeletal system provides the structural framework for the human body, supporting movement, protecting organs, and serving as a site for blood cell formation and mineral storage.

• Parts of the Skeletal System:

  • Bones (skeleton)

  • Joints

  • Cartilages

  • Ligaments

• Key Question: Why is a bone an organ and not a tissue?

  • Answer: A bone is considered an organ because it is composed of several types of tissues (bone tissue, cartilage, connective tissue, blood vessels, nerves) working together for a specific function.

Divisions of the Skeleton

• Axial Skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).

• Appendicular Skeleton: Bones of the limbs and girdles (shoulder and pelvic girdles).

• The adult skeleton has approximately 206 bones.

Functions of Bones

Bones perform several vital functions in the body:

• Support: Provide structural support for the body and soft organs.

• Protection: Protect vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serve as levers for muscles to act upon, enabling movement.

• Mineral and Fat Storage: Store minerals (mainly calcium and phosphorus) and fats (in yellow bone marrow).

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

Bones of the Human Body

Types of Bone Tissue

• Compact Bone: Dense, smooth, and forms the outer layer of bones.

• Spongy Bone: Composed of small needle-like pieces (trabeculae) and open spaces; found mainly at the ends of long bones and inside flat bones.

Classification of Bones by Shape

Bones are classified into four main types based on their shapes:

Anatomy of a Long Bone

Gross Structure

• Diaphysis: Shaft of the bone; composed of compact bone.

• Periosteum: Fibrous connective tissue membrane covering the diaphysis; contains blood vessels and nerves.

• Epiphyses: Ends of the bone; mostly spongy bone covered by a thin layer of compact bone.

• Articular Cartilage: Hyaline cartilage covering the external surface of the epiphyses; reduces friction at joint surfaces.

• Epiphyseal Plate/Line: Plate of hyaline cartilage in growing bones (epiphyseal plate); replaced by bone in adults (epiphyseal line).

• Medullary Cavity: Cavity inside the shaft; contains yellow marrow (fat) in adults and red marrow (for blood cell formation) in infants.

Microscopic Anatomy of Bone

• Osteon (Haversian System): Structural and functional unit of compact bone; consists of a central canal surrounded by concentric lamellae (rings) of bone matrix.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Small cavities containing osteocytes (bone cells), found between lamellae.

• Canaliculi: Tiny canals connecting lacunae to each other and to the central canal; allow for nutrient and waste exchange.

• Central (Haversian) Canal: Runs longitudinally through the osteon; contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Run perpendicular to the central canals; connect blood and nerve supply of the periosteum to those in the central canals and medullary cavity.

• Nutrient Foramen: Small opening on the bone surface for blood vessels to enter the bone.

Bone Formation and Growth

Ossification (Bone Formation)

• Ossification: The process of bone formation from pre-existing connective tissue.

• Bone Components:

  • Organic part: Collagen fibers provide flexibility.

  • Inorganic part: Calcium salts deposited in the matrix make bone hard.

Types of Ossification

• Intramembranous Ossification: Forms flat bones of the skull and clavicle from fibrous membranes.

• Endochondral Ossification: Forms most bones of the body from hyaline cartilage models.

Bone Growth

• Longitudinal Growth: Occurs at the epiphyseal plates; new cartilage is continuously formed and then replaced by bone.

• Appositional Growth: Process by which bones increase in diameter (thickness).

Bone Remodeling

Bone remodeling is a continuous process of bone deposition and resorption throughout life, allowing bones to adapt to stress and repair microdamage.

• About 3-5% of bone mass is remodeled annually.

• Remodeling is influenced by blood calcium levels and mechanical stress on the skeleton.

• Osteoblasts: Cells that build new bone matrix.

• Osteoclasts: Cells that break down bone matrix, releasing calcium into the blood.

Factors Affecting Bone Growth and Remodeling

• Hormones:

  • Growth hormone: Stimulates growth of cartilage at epiphyseal plates.

  • Sex hormones (estrogen, testosterone): Stimulate bone growth and closure of epiphyseal plates at puberty.

  • Parathyroid hormone (PTH): Stimulates osteoclast activity, increasing blood calcium levels.

  • Calcitonin: Inhibits osteoclast activity, lowering blood calcium levels.

• Vitamins:

  • Vitamin A: Activates osteoblasts.

  • Vitamin C: Required for collagen synthesis.

  • Vitamin D: Promotes calcium absorption from the intestine.

Summary Table: Bone Classification by Shape

Additional info: The notes also briefly mention fetal skeleton development, emphasizing that ossification begins early in fetal life and continues into early adulthood. Exercise and mechanical stress are important for maintaining bone strength and density throughout life.