Bone Growth and Development

Bone Growth and Development Notes

Intramembranous Ossification Overview

  • Simpler of the two methods: Bone forms directly on or within loose fibrous connective tissue membranes.
  • Forms: Irregular and flat bones.
  • Four basic steps:
  1. Development of ossification center
  2. Calcification of membrane
  3. Formation of trabeculae
  4. Development of periosteum

Steps of Intramembranous Ossification

1. Development of Ossification Center
  • Mesenchymal cells cluster and differentiate into osteogenic cells, then into osteoblasts.
  • Osteoblasts secrete bony organic matrix and become surrounded by it.
2. Calcification
  • Matrix secretion stops, and the matrix hardens, converting osteoblasts into osteocytes.
  • Formation of lacunae and canaliculi occurs.
3. Formation of Trabeculae
  • Trabeculae fuse to form cancellous bone.
  • Blood vessels invade spaces, leading to red bone marrow formation by the end of the process.
4. Development of Periosteum
  • Mesenchyme at the periphery condenses to form the periosteum.
  • Cortical bone replaces surface layers of spongy bone.
  • Remodeling occurs, giving the bone its adult shape and size.

Endochondral Ossification Overview

  • More complex than intramembranous ossification: Involves a cartilage model (anlagen) forming within loose fibrous connective tissue membranes.
  • Forms: Long and short bones.
  • Five basic steps:
  1. Development of cartilage anlagen
  2. Growth of the cartilage model
  3. Development of primary ossification center
  4. Development of secondary ossification centers
  5. Formation of articular cartilage and epiphyseal plate

Steps of Endochondral Ossification

1. Development of Cartilage Anlagen
  • Initial formation of cartilage model.
2. Growth of the Cartilage Model
  • Chondrocytes undergo mitosis, leading to:
  • Interstitial growth: Increases length by adding cells.
  • Appositional growth: Increases width by matrix deposition.
  • Chondrocytes in mid-region hypertrophy, some bursting to increase pH and trigger calcification.
3. Development of Primary Ossification Center
  • Penetration of nutrient artery through perichondrium and cartilage model.
  • Formation of periosteal bone collar around diaphysis from osteoblasts in perichondrium.
  • Blood vessels induce growth of ossification center to replace cartilage with bone.
4. Development of Secondary Ossification Centers
  • Entered by branches of epiphyseal arteries.
  • Usually begins around the time of birth.
  • Resembles primary ossification, but without forming a medullary cavity.
5. Formation of Articular Cartilage and Epiphyseal Plate
  • Cartilage covering the epiphysis becomes articular cartilage.
  • Hyaline cartilage remains between diaphysis and epiphysis.
  • Epiphyseal plate (growth plate) allows for lengthening of the bone until growth stops when replaced by bone, forming the epiphyseal line.

Bone Growth

  • Begins in utero: Continuously occurs throughout life.
  • Length: Increases until adulthood.
  • Thickness/Density: Varies throughout life, influenced by various factors.
Interstitial Growth
  • Involves growth in length, through the epiphyseal plate with four zones:
  1. Zone of Resting Cartilage: Anchors the epiphyseal plate.
  2. Zone of Proliferating Cartilage: Chondrocytes arranged in stacks, undergoing mitosis.
  3. Zone of Hypertrophic Cartilage: Larger chondrocytes maturing in columns.
  4. Zone of Calcified Cartilage: Mostly dead chondrocytes where osteoclasts dissolve cartilage and osteoblasts replace with bony matrix.
  • Clavicle ossifies first and stops last, while most long bones complete ossification between ages 14-19.
Appositional Growth
  • Growth in width, involving four steps:
  1. Formation of Ridges: Osteoblasts form ridges around periosteal arteries.
  2. Ridges Fuse Together: Forms endosteum, where osteoblasts build layers to form osteons.
  3. Circumferential Lamellae Production: Osteoblasts thicken the bone.
  4. Repeat Process: More osteons continuously form.
  • Osteoclasts maintain medullary cavity size while growing the shaft outwards.

Factors Affecting Bone Growth

  • Dietary Needs: Essential minerals (Calcium, Phosphorus, etc.) and vitamins (C, K, A).
  • Hormonal Influence: Human growth hormone (hGH) promotes IGFs which stimulate growth. Sex steroids (estrogens and androgens) increase osteoblast activity but eventually lead to growth plate closure.

Laws of Bone Growth and Deposition

  • Wolff's Law: Bone structure adapts based on the forces applied, emphasizing the dynamic remodeling of bone. Bone renormalization does not always follow predicted patterns based purely on mechanical factors.