The Skeletal System: Structure, Function, and Physiology

TOPIC IX: SKELETAL SYSTEM

A) Skeletal System Overview

The skeletal system is a dynamic organ system that provides structural support, protects vital organs, enables movement, and plays a crucial role in physiological processes such as blood cell production and mineral storage.

• Structural Support: The skeleton forms the framework that supports the body and maintains its shape.

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

• Movement: Bones serve as levers for muscles, enabling movement at joints.

• Blood Cell Production: Red bone marrow produces all blood cells (red blood cells, white blood cells, and platelets) in a process called hematopoiesis.

• Mineral Storage: Bones store minerals, especially calcium (Ca2+), which is essential for nerve signaling, muscle contraction, and blood clotting.

B) Bone Ossification

Ossification is the process of bone formation. In the embryo, the skeleton is initially composed of fibrous membranes and hyaline cartilage. Bone formation begins around the 8th week of development and continues into early adulthood.

• Intramembranous Ossification:

  • Forms flat bones (e.g., some skull bones, mandible, clavicle).

  • Develops from fibrous connective tissue membranes.

  • Process:

    • Osteoblasts produce bone along connective tissue fibers.

    • Cells at the periphery form the periosteum; osteoblasts beneath lay down compact bone.

  • Fontanels: Unossified membranes in the skull present at birth, allowing for growth and flexibility during birth.

• Endochondral Ossification:

  • Forms most bones of the body.

  • Uses hyaline cartilage as a model for bone construction, which is gradually replaced by bone tissue.

  • Process:

    • Periosteum forms; osteoblasts beneath lay down compact bone.

    • Articular cartilage and epiphyseal plates remain as cartilage (not ossified) for growth.

C) Bone Growth (Postnatal)

After birth, bones continue to grow in length and diameter through distinct processes.

• Growth in Length (Endochondral Growth):

  • Occurs at the epiphyseal plates (growth plates).

  • Cartilage grows and is replaced by bone near the diaphysis (shaft).

  • Growth in length stops when the epiphyseal plate ossifies into an epiphyseal line (closure).

• Growth in Diameter (Appositional Growth):

  • Osteoblasts beneath the periosteum secrete bone matrix on the external surface.

  • Osteoclasts resorb bone on the endosteal (internal) surface, maintaining the proper cavity size.

Factors Affecting Bone Growth and Remodeling

• Mechanical Stress:

  • Muscle action and physical activity stimulate osteoblast activity, increasing bone formation.

  • Moderate exercise increases bone mass; lack of movement decreases osteoblast activity, leading to bone loss.

  • Weights or electric currents may speed healing by increasing osteoblast activity.

• Nutrition:

  • Calcium (Ca2+) and phosphate (PO43-) are required for bone matrix production.

  • Vitamin C is necessary for collagen synthesis.

  • Vitamin D increases intestinal absorption of calcium; deficiency can cause rickets (soft bones).

• Hormones:

  • Growth hormone (GH) and thyroid hormones stimulate bone growth.

  • Estrogen and testosterone increase osteoblast activity; estrogen causes ossification of the epiphyseal plate (closure).

  • Calcitonin inhibits osteoclast activity, increases Ca2+ movement from blood into bone, and promotes bone growth.

  • Parathyroid hormone (PTH) increases osteoclast activity and decreases osteoblast activity, releasing Ca2+ into the blood.

D) Osteoporosis

Osteoporosis is a disease characterized by reduced bone density and mass, resulting in fragile bones that are more susceptible to fractures.

• Risk Factors:

  1. Increasing age (decreased production of sex hormones)

  2. Post-menopause (decreased estrogen production)

  3. Inadequate diet (low vitamin D, Ca2+)

  4. Illness or excess parathyroid hormone

  5. Too little or excessive exercise (stress increases cortisol)

  6. Certain drugs (e.g., cortisone, alcohol), and smoking (decreases estrogen production)

E) Bone Repair

Bone repair is a multi-step process that restores bone structure and function after a fracture.

1. Formation of a blood clot (hematoma) at the fracture site.

2. Replacement of the clot by a callus (fibrous network and fibrocartilage islets).

3. Ossification of the callus via both intramembranous and endochondral ossification.

4. Process takes 4–6 weeks; immobilization (e.g., cast) is required to prevent re-fracture during healing.