Page 230 - Fundamentals of anatomy physiology

Watch the > ™ on Endochondral Ossification.

Increasing the Length of a Developing Long Bone (Steps 5–7)

During the initial stages of ossification, osteoblasts move away from the primary
ossification center toward the epiphyses. But they do not complete the ossification of
the model immediately, because the cartilages of the epiphyses continue to grow. The
region where the cartilage is being replaced by bone lies at the metaphysis, the junction
between the diaphysis and an epiphysis. On the diaphyseal (shaft) side of the metaphysis,
osteoblasts continually invade the cartilage and replace it with bone, while on the
epiphyseal side, new cartilage is produced at the same rate. The situation is like a pair of
joggers, one in front of the other. As long as they are running at the same speed, they can
run for miles without colliding. In this case, the osteoblasts and the epiphysis are both
“running away” from the primary ossification center. As a result, the osteoblasts never
catch up with the epiphysis, and the bone continues to grow longer and longer.

5 Capillaries and osteoblasts 6 7 At puberty, the rate of epiphyseal
The epiphyses eventually become filled
migrate into the epiphyses, with spongy bone. The metaphysis, a cartilage production slows and the
creating secondary relatively narrow cartilaginous region called rate of osteoblast activity accelerates.
ossification centers. the epiphyseal cartilage, or epiphyseal As a result, the epiphyseal cartilage
plate, now separates the epiphysis from the gets narrower and narrower, until it
Hyaline cartilage diaphysis. On the shaft side of the ultimately disappears. This event is
metaphysis, osteoblasts continuously called epiphyseal closure. The former
Epiphysis invade the cartilage and replace it with location of the epiphyseal cartilage
bone. New cartilage is produced at the becomes a distinct epiphyseal line
same rate on the epiphyseal side. that remains after epiphyseal growth
has ended.
Articular cartilage
Articular cartilage Epiphyseal line
Spongy
bone

Metaphysis Epiphyseal Spongy
Periosteum cartilage bone
Compact
bone Diaphysis Medullary
cavity
Secondary Within the epiphyseal cartilage, the
ossification chondrocytes are organized into zones. A thin cap of the original cartilage
model remains exposed to the joint
center Chondrocytes at the cavity as the articular cartilage. This
epiphyseal side of the cartilage prevents damaging the
cartilage continue to joint from bone-to-bone contact.
divide and enlarge.
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Chondrocytes degenerate
at the diaphyseal side.

Osteoblasts migrate upward
from the diaphysis and
cartilage is gradually
replaced by bone.