Skeletal System: Ossification

• Functions of the skeletal system include support, protection of vital organs, muscle attachment for movement.
• Red bone marrow produces red blood cells (RBCs), white blood cells (WBCs), and platelets.
• The skeletal system stores calcium and performs fat storage in yellow bone marrow.

Ossification

• Bone ossification starts during the 8th week of development and ends at 18 years in females, 21 years in males.

Intramembranous Ossification

• Flat bones of the skull, some facial bones, the mandible and clavicle form through intramembranous ossification.
• This process develops from fibrous connective tissue membrane.
• Osteoblasts create spongy bone along fibers.
• The periosteum forms, and osteoblasts underneath create compact bone.
• Fontanels are unossified membranes that remain in the skull at birth.

Endochondral Ossification

• Most bones develop from hyaline cartilage.
• Cartilage is laid down as the rough shape of the bone.
• Cartilage ossifies into spongy bone.
• Periosteum forms, and osteoblasts beneath deposit compact bone.
• Articular cartilage and epiphyseal plates are cartilage that is not ossified.

Post-Natal Bone Growth

• Endochondral growth happens at the epiphyseal plates, facilitating growth in length.
• Cartilage grows, and the matrix near the diaphysis transforms into bone.
• Growth in length ceases as the epiphyseal plate ossifies into the epiphyseal line.
• Appositional growth increases bone diameter.
• Osteoblasts beneath the periosteum of long bones facilitates growth in diameter.
• Osteoclasts in the medullary endosteum resorb bone, ensuring cavity size matches bone growth.

Factors Affecting Bone Growth and Remodeling

• Bone growth and remodeling are mechanically influenced
• Moderate exercise increases osteoblast activity in stressed areas, favoring building over breakdown.
• Injury restricts movement, diminishing osteoblast activity without altering osteoclast activity, which decreases bone mass.
• Weights/electrical currents potentially accelerate osteoblast activity.
• Calcium and phosphorus are crucial for bone matrix production.
• Vitamin C is essential for collagen production.
• Vitamin D is required for small intestine calcium absorption; deficiency leads to soft bones, known as rickets.
• Growth hormone, thyroid hormone, insulin, estrogen, and testosterone stimulate bone growth.
• Estrogen and testosterone increase osteoblast activity.
• Estrogen hastens epiphyseal plate ossification, affecting both sexes but earlier in females due to higher estrogen.
• Calcitonin inhibits osteoclast activity, promoting calcium movement from blood to bone.
• Parathyroid hormone raises blood calcium by increasing bone resorption through osteoclast activity.

Osteoporosis

• Osteoporosis involves a decrease in bone mass.
• Risk factors include aging, inadequate vitamin D and calcium intake, excess parathyroid hormone, and insufficient/excessive exercise.
• Low body fat and high stress (cortisol) contribute.
• Menopause in females, marked by estrogen decline, reduces osteoblast and increases osteoclast activity.
• Smoking diminishes estrogen levels.
• Drugs like cortisone and alcohol lower osteoblast activity.

Bone Repair

• Fracture repair begins with a blood clot.
• The clot transforms into a callus consisting of a fibrous collagen network and fibrocartilage islets.
• Callus then ossifies through intramembranous and endochondral ossification.
• Spongy bone is then replaced by compact bone around the fracture site.
• Osteoclasts resorb dead bone, and the process takes 4-6 weeks (longer for larger bones).
• Casting is needed, where movement can refracture the forming matrix.