Overview
This lecture covers the mechanisms of bone growth, including longitudinal and appositional growth, their regulation by hormones, and associated disorders.
Longitudinal Bone Growth
- Occurs at the epiphyseal plate between the epiphysis and metaphysis of long bones.
- Five zones: reserve cartilage (inactive), proliferation (cell division), hypertrophy (cell growth), calcification (matrix hardening, chondrocyte death), ossification (bone matrix deposition).
- Only zones of proliferation, hypertrophy, calcification, and ossification contribute to growth.
- Growth continues until proliferation is caught by ossification, typically ceasing at ages 18-21.
- Growth plate closure varies by sex and individual; faster closure is promoted by increased hormone levels.
Appositional Bone Growth
- Increases bone width through osteoblasts depositing new bone beneath the periosteum.
- Osteoclasts resorb bone on the endosteal surface, creating the medullary cavity and reducing bone weight.
- New bone is added as circumferential lamellae and remodeled into osteons as needed.
- Width growth is influenced by hormones, nutrition, and physical activity (weight-bearing exercise).
Hormonal Regulation of Bone Growth
- Growth hormone from the anterior pituitary stimulates cell division, protein synthesis, and both types of bone growth.
- Testosterone promotes both width and length growth, increases bone density, and accelerates growth plate closure.
- Estrogen promotes growth plate closure sooner than testosterone and inhibits osteoclasts, protecting against bone resorption.
- Lack of estrogen after menopause increases the risk of osteoporosis due to greater bone breakdown.
Disorders Related to Growth Hormone
- Excess growth hormone before puberty causes gigantism due to continued growth plate activity.
- Excess after plate closure causes acromegaly (bone and tissue thickening, not height increase).
- Growth hormone deficiency before plate closure results in dwarfism due to insufficient bone growth.
Key Terms & Definitions
- Epiphyseal Plate — Cartilage growth zone between the epiphysis and metaphysis in long bones.
- Osteoblast — Bone-forming cell that deposits new bone matrix.
- Osteoclast — Bone-resorbing cell that breaks down bone tissue.
- Osteocyte — Mature bone cell maintaining bone tissue.
- Appositional Growth — Increase in bone width.
- Longitudinal Growth — Increase in bone length.
- Gigantism — Excessive growth due to high growth hormone before growth plate closure.
- Acromegaly — Bone thickening due to high growth hormone after plate closure.
- Dwarfism — Insufficient growth due to low growth hormone during development.
Action Items / Next Steps
- Review hormone functions from previous biology/anatomy chapters.
- Prepare for the next lecture on bone remodeling and repair.