Page 229 - Fundamentals of anatomy physiology

Spotlight Figure 6–11
Endochondral Ossification

Endochondral ossification begins with the formation of a hyaline
cartilage model. This model serves as the pattern for almost all bone
development. Limb bone development is a good example of this
process. By the time an embryo is six weeks old, the proximal bones
of the limbs, the humerus (upper limb) and femur (lower limb),
have formed, but they are composed entirely of cartilage.
These cartilage models continue to grow by expansion of
the cartilage matrix (interstitial growth) and the
production of more cartilage at the outer surface
(appositional growth).

Initiation of Ossification in the Developing Bone (Steps 1–4)

1 As the cartilage 2 Blood vessels grow 3 Blood vessels penetrate 4 Remodeling occurs as

enlarges, chondrocytes around the edges of the cartilage and invade growth continues, creating
near the center of the the cartilage, and the the central region. a medullary cavity. The
shaft increase greatly cells of the perichon- Fibroblasts migrating osseous tissue of the shaft
in size. The matrix is drium convert to with the blood vessels becomes thicker, and the
reduced to a series of osteoblasts. The shaft differentiate into cartilage near each
small struts that soon of the cartilage then osteoblasts and begin epiphysis is replaced by
begin to calcify. The becomes ensheathed producing spongy bone shafts of bone. Further
enlarged chondrocytes in a superficial layer of at a primary ossification growth involves increases
then die and disinte- bone. center. Bone formation in length and diameter.
grate, leaving cavities then spreads along the
within the cartilage. shaft toward both ends
of the former cartilage
model.

Enlarging Perichondrium
chondrocytes within

calcifying matrix

Epiphysis

Bone collar

Blood Diaphysis Medullary Medullary
vessel cavity cavity

Hyaline Periosteum Primary Metaphysis
cartilage formed from ossification
perichondrium center
Disintegrating
chondrocytes Superficial
of the cartilage bone

model Spongy
bone

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