Bone Growth


  • Bone growth at the epiphyseal growth plate is endochondral (aka interstitial) is linear.
  • Bone growth that is periosteal (aka appositional) is via thickening (widening).


Hyaline cartilaginous model (template).

  • The diaphysis is the shaft.
  • The epiphyses are the articulating ends of the long bone.
  • The metaphyses separate the diaphysis and epiphysis.
  • The medullary cavity forms as the primary ossification center, degenerates, cavitates and remodels via interstitial growth. Endochondrium delineates it – which forms a layer of lamellar bone and osteoprogenitor cells.
  • The marrow cavity is filled with hematopoetic marrow (which comprises red and white blood cell precursors)
  • Vasculature invades the cavity to fill it with marrow.


Reserve zone

  • Filled with typical fetal hyaline cartilage cells, responsible for growth in length – these cells “lead the growth pack,” in essence.

Proliferative zone

  • Filled with chrondrocytes that proliferate but do NOT hypertrophy, regulated by Indian hedgehog – this prevents the growth plates from inactivating until puberty when the child reaches full growth the growth plates degenerate to epiphyseal lines.

Hypertrophic zone

  • Filled with hypertrophic chondrocytes (we can identify their nuclei and lipid droplets) – they undergo apoptotic enlargement and are the future site of ossification (bony matrix extension).
  • They mineralize the surrounding cartilage, attract vasculature via VEGF (vascular endothelial growth factor), and that vasculature then recruits chondroclasts to degrade the carilage and form osteoblasts, which secrete the osteoid (bony matrix).

Vascular Invasion.


From epiphysis to diaphysis, we label the…

  • Reserve zone (filled with fetal hyaline cartilage cells)
  • Proliferative zone (filled with mitotically active chondrocytes)
  • Hypertrophic zone (which undergo apoptotic enlargement)
  • Zone of vascular invasion (which provides the osteoprogenitor cells for bone proliferation).


From central to external…

  • The hematopoetic marrow of the marrow cavity.
  • The endosteum, which delineates the marrow cavity, itself.
    -The endosteum comprises the inner circumferential lamellae, which distinguishes this inner bony layer from the compact bone external to it.
    -The osteoprogenitor cells that line the endosteum and form the lamellae that derive the spongy bone that form the trabecular meshwork of the primary ossification center.
  • External to the endosteum, lies layer of compact bone, which, namely, comprises osteons that encompasses circular layers of lamellae around a Haversian (aka central) canal.
  • We show, for reference, an osteocyte trapped between lamellae.
  • A collagenous layer of periosteum surrounds the compact bone.
  • Periosteum divides into an outer, collagenous layer, and inner, osteogenic layer.
  • Osteoblasts from the periosteum create new osteons along the longitudinal grooves and ridges within what is referred to as the outer circumferential lamellae: it forms concentric osteons (with Haversian canals), and it also forms interstitial lamellae, which fill the spaces between osteons.
  • The inner circumferential lamellae thickens, as well.


From marrow to periosteum, label the…

  • Hematopoetic marrow
  • Inner circumferential lamellae
  • Osteon within the compact bone (indicate its Haversian canal).
  • Periosteum external to the compact bone.
  • An osteon in the process of being formed – In time, bone matrix will be released and the deep pink of the bony matrix would be observed as this region of growth takes on a biology of bone matrix.


  • principal inorganic substance (mineral) is hydroxyapatite, which is hydroxylated calcium/phosphate. Other constituents include magnesium, potassium, sodium, bicarbonate, and citrate.
    The principal organic substances are collagen type 1 fibers and ground substance.

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