Bone Growth During Childhood and Adolescence

Bone Tissue

• 60% of bone tissue is composed of inorganic components, including calcium phosphate (Ca3(PO4)2) and calcium hydroxide (Ca2(OH)2)
• Bone cells:
  • Osteoprogenitors: develop into mature bone cells, found in periosteum and endosteum, and are mitotic cells
  • Osteoblasts: form bone matrix, also known as builders
  • Osteocytes: reside in lacunae, are mature bone cells, and communicate with each other via canaliculi
  • Osteoclasts: break down and resorb bone matrix, are large multinucleated cells (~30), and resorb Ca2+ into the bloodstream

Bone Tissue Comparison

• Compact bone (cortical or dense bone tissue):
  • Dense and solid to the unaided eye, but porous when viewed under a microscope
  • External surface: flat bones have two layers with spongy bone in between (e.g. clavicle)
• Spongy bone (trabecular or cancellous bone tissue):
  • Trabeculae: open latticework of narrow plates
  • Internal surface
  • Abundant in epiphysis (forces coming in all directions)
  • Two unique characteristics: light and supports/protects red bone marrow (e.g. hip bones, ribs, sternum, vertebrae, proximal humerus and femur)

Compact Bone Organization

• The basic structural and functional unit of mature compact bone is the osteon
  • Cylindrical in shape
  • Parallel to the shaft
• Osteon components:
  • Canals: central (also known as osteonic or Haversian), perforating (also known as Volkman or interosteonic), and canaliculi
  • Calcium deposition: secretion of ECM stops, osteocytes are formed, and Ca2+ is deposited into ECM
  • Formation of trabeculae: form around blood vessels, and are surrounded by connective tissue (CT) which becomes red bone marrow
  • Development of periosteum: from surrounding mesenchyme, surface layers develop compact bone

Ossification

• Ossification: the formation and development of bone
  • AKA osteogenesis
  • Begins 6-8 weeks of embryonic development and continues into adulthood
  • Two general patterns: intramembranous and endochondral ossification
• Endochondral ossification:
  • Begins with a hyaline cartilage model
  • Produces the majority of bones in the body
  • Six stages of endochondral ossification:
    1. Development of cartilage model
    2. Growth of cartilage model
    3. Development of primary ossification center
    4. Development of medullary cavity
    5. Development of lamellae
    6. Formation of trabeculae

Blood and Nerve Supply

• Arteries and veins:
  • Periosteal: enter periosteum in diaphysis through interosteonic (perpendicular) canals
  • Nutrient: enters dominant away from growth end of bone
  • Metaphyseal: between diaphysis and epiphyseal plate
  • Epiphyseal: in epiphysis, supplies red bone marrow
• Nerves:
  • Accompany blood vessels
  • Periosteum: rich in nerve supply

Ossification Patterns

• Intramembranous ossification:
  • Develops from mesenchyme
  • Produces flat bones of the skull, some facial bones, the mandible, and central portions of the clavicle
  • Four stages of intramembranous ossification:
    1. Development of ossification center
    2. Short bones: length = width
    3. Flat bones: thin surfaces
    4. Irregular bones: vertebral column, sacrum, innominate, facial bones

Long Bone Anatomy

• Diaphysis: long cylindrical shaft
  • Singular
• Epiphyses: at ends of bone
  • Strengthens joints
  • Attachment site for ligaments and tendons
• Metaphysis: between diaphysis and epiphyses
  • Contains epiphyseal plate, which becomes a line or scar when bone stops growing
• Articular cartilage: thin layer of hyaline covering epiphysis
  • Reduces friction
• Periosteum: covers external surfaces of bones, except where there is articular cartilage
  • Dense irregular connective tissue
  • Anchor for blood vessels, nerves, and tendons
• Medullary cavity: in diaphysis
  • Usually contains yellow marrow
• Endosteum: covers most of the medullary cavity
  • Single layer of cells (bone-building osteoblasts)
  • Under torsional force as cells lay down new bone

Bone Surface Markings

• Depressions (fossa, groove) and openings (foramen, meatus, canal)
  • Form joints or allow passage of blood vessels and nerves
• Processes, projections of outward growth
  • Serve as attachment sites for tendons and ligaments

Histology of Bone Tissue

• Bone matrix:
  • 40% organic components
  • Cells: osteoblasts, osteocytes, and osteoclasts
  • Collagen fibers: give flexibility and resist bending
  • Chondroitin sulfate: provides framework for bone growth
• Bone growth:
  • Growth in length: interstitial growth of cartilage on epiphyseal side of plate + replacement of cartilage by bone on diaphyseal side
  • Growth in thickness: appositional growth = growth in width, occurs throughout lifespan

Bone Development

• Bone growth during infancy, childhood, and adolescence:
  • Growth in length: interstitial growth of cartilage on epiphyseal side of plate + replacement of cartilage by bone on diaphyseal side
  • Growth in thickness: appositional growth = growth in width, occurs throughout lifespan
• Bone growth and development:
  • Vit D (hormone): promotes Ca2+ and phosphorus absorption in intestine
  • Vit K and B12: synthesis of bone proteins
• Fractures:
  • Any break in the bone
  • Common types: open (compound), comminuted, greenstick, impacted, and Pott's
  • Three steps to repair bone fractures:
    1. Reactive phase: fracture hematoma, nearby cells die, and phagocytes and osteocytes clean up remains
    2. Restorative phase: formation of fibrocartilaginous and bony callus
    3. Bone remodeling phase: dead portions absorbed by osteoclasts, and compact bone replaces spongy bone in periphery