Bone Growth and Remodeling - Physical Bone Growth

Bone Growth and Remodeling

Physical Bone Growth

  • Focuses on bone growth after the skeleton is formed and bones are ossified.
  • Aims to achieve appropriate bone size based on age.
  • Occurs throughout development.

Types of bone growth:

  • Interstitial Growth:
    • Bone growth in length.
    • Occurs inside the tissue.
    • Takes place at the epiphyseal plate (growth plate).
    • The epiphyseal plate is located at the head of the long bone between the epiphysis and metaphysis.
    • Cartilage in the epiphyseal plate grows, allowing the bone to extend in length.
  • Appositional Growth:
    • Bone growth in width.
    • Involves depositing more bone on the outside of the cell, expanding the width of the long bone.

Hormonal Regulation of Bone Growth

  • Regulated by the endocrine system (e.g., testosterone and estrogen).
  • Different growth factors influence bone growth at varying levels.
  • Estrogen:
    • Quick stimulator of bone growth.
    • Effects tend to wear off quickly.
  • Testosterone:
    • Slower in stimulating bone growth.
    • Effects are more lasting and persistent.
  • Females tend to grow quickly and reach their maximum height earlier due to estrogen.
  • Males tend to grow continually over a longer period due to testosterone, resulting in a taller average height.

Bone Remodeling

  • Balance between bone growth/formation and bone resorption.
  • Bone formation: bone being laid down or deposited
  • Bone resorption: bone being broken down.
  • More bone formation leads to skeleton growth.
  • More bone resorption leads to skeleton shrinkage.
  • During growth, bone formation is greater than resorption.
  • In stable adulthood, formation and resorption balance to maintain the skeleton.

Interstitial Growth in Detail

  • Takes place at the epiphyseal plate between the epiphysis and metaphysis.
  • In adults, the epiphyseal plate ossifies and becomes the epiphyseal line, ceasing growth.
  • Cartilage in the epiphyseal plate expands, pushing the head of the bone lengthwise.
  • As cartilage expands, it calcifies and grows, and the diaphysis follows.
  • The combination of cartilage cell expansion and osteoblast mineralization extends the length of the long bone.
  • Articular cartilage at the ends of long bones maintains joint integrity, providing a frictionless surface.
  • Epiphyseal plate: cartilage plate (growth plate).
  • Epiphyseal line: adult fused bone, no longer growing in length.

Identifying Growing vs. Adult Hand X-rays

  • Short bones -- carpals.
  • Long bones -- metacarpals.
  • In a growing hand, long bones (e.g., second metacarpal) are composed of three individual bones with unfused ossification centers due to the epiphyseal plate.
  • In a fully formed adult hand, the bones are fused into one complete bone with the epiphyseal line.

Appositional Growth

  • Bone grows in width through cells growing on the edges of the long bone and laying down more bone.
  • Periosteum:
    • Outer lining of the bone.
    • Contains progenitor cells (pre-osteoblasts) that turn into active osteoblasts.
    • Osteoblasts lay down bone on the outside, increasing width.
  • Endosteum:
    • Contains osteoclasts that remodel the bone internally, expanding from the inside.
    • Osteoclasts remove old bone and replace it with fresh bone.
  • Osteoblasts that lay down too much bone become trapped and transform into osteocytes.
    • Act to regulate, maintain, and survey bone.
  • Process occurs in layers, forming osteons in the cortical long bone (diaphysis) during appositional growth.