Cartilage and Bone Growth Overview

Overview

This lecture covers the mechanisms of cartilage and bone growth, focusing on interstitial and appositional growth, bone formation processes (intramembranous and endochondral ossification), hormonal regulation, aging effects, and bone repair.

Cartilage Growth

• Cartilage grows via interstitial and appositional growth.
• Interstitial growth occurs within the cartilage when chondrocytes divide and produce more matrix.
• Appositional growth thickens cartilage by adding new matrix at the perichondrium (outer edge).
• Both growth types are necessary for normal cartilage development.
• Adult cartilage has limited repair ability, especially after the growth phase ends.

Bone Growth and Formation

• Bone forms by intramembranous and endochondral ossification.
• Intramembranous ossification creates flat bones (e.g., skull) from mesenchymal tissue.
• Endochondral ossification forms most bones by replacing a hyaline cartilage template with bone.
• Primary ossification starts in the diaphysis (shaft); secondary centers form in epiphyses (ends).
• Bone growth continues at the epiphyseal growth plates until early adulthood.

Epiphyseal Plate and Bone Elongation

• Growth plates lengthen bones by adding cartilage, which is then replaced by bone.
• The growth plate has distinct zones: resting cartilage, proliferation, hypertrophy, calcification, and ossification.
• Bone elongation ends when the growth plates are replaced by bone (epiphyseal lines).

Bone Remodeling and Mechanical Stress

• Interstitial and appositional growth continue in bone remodeling.
• Osteoblasts deposit new bone matrix; osteoclasts resorb bone.
• Mechanical stress and hormones regulate bone remodeling.
• Lack of stress (e.g., immobility or weightlessness) leads to bone loss.

Hormonal Regulation of Bone

• Calcitonin (from thyroid C cells) lowers blood calcium by inhibiting osteoclasts.
• Parathyroid hormone increases blood calcium by stimulating osteoclasts.
• Growth hormone and sex hormones promote bone growth, especially during childhood and puberty.

Aging and Bone Health

• Aging reduces osteoblast activity and collagen production, causing bone loss (osteopenia).
• Postmenopausal women have increased osteoclast activity, leading to osteoporosis.
• Osteoporotic bones are more porous and prone to fractures.

Bone Repair Process

• Bone healing starts with a blood clot, followed by formation of a cartilage (fibrocartilaginous) callus.
• Osteoblasts replace cartilage with woven bone, which remodels into mature bone.
• The healed site remains thicker due to increased bone deposition.

Key Terms & Definitions

• Interstitial Growth — Growth from within cartilage or bone by cell division.
• Appositional Growth — Growth by adding new layers at the surface (outer edge).
• Ossification/Osteogenesis — The process of bone formation.
• Intramembranous Ossification — Bone develops directly from mesenchymal tissue.
• Endochondral Ossification — Bone forms by replacing a cartilage template.
• Chondrocyte — Cartilage cell.
• Perichondrium — Membrane surrounding cartilage.
• Periosteum — Membrane surrounding bone.
• Osteoblast — Bone-forming cell.
• Osteoclast — Bone-resorbing cell.
• Epiphyseal Plate — Growth plate in long bones.
• Calcitonin — Hormone that lowers blood calcium.
• Parathyroid Hormone — Hormone that raises blood calcium.
• Osteopenia — Reduced bone mass.
• Osteoporosis — Severe bone loss, leading to fragile bones.

Action Items / Next Steps

• Review the steps of intramembranous and endochondral ossification.
• Memorize the roles of key hormones in bone growth and calcium balance.
• Study the structure and function of the epiphyseal plate.
• Prepare for test questions on cartilage/bone growth mechanisms and repair.