184
UNIT 2
Covering, Support, and Movement of the Body
6
5
The epiphyses ossify.
At birth, most of our long bones have
a bony diaphysis surrounding remnants of spongy bone, a
widening medullary cavity, and two cartilaginous epiphy-
ses. Shortly before or aFer birth,
secondary ossification
centers
appear in one or both epiphyses, and the epiphyses
gain bony tissue. (Typically, the large long bones form sec-
ondary centers in both epiphyses, whereas the small long
bones form only one secondary ossification center.) ±e
cartilage in the center of the epiphysis calcifies and dete-
riorates, opening up cavities that allow a periosteal bud to
enter. ±en bone trabeculae appear, just as they did earlier
in the primary ossification center.
In short bones, only the primary ossification center is
formed. Most irregular bones develop from several distinct
ossification centers.
Secondary ossification reproduces almost exactly the
events of primary ossification, except that the spongy bone
in the interior is retained and no medullary cavity forms
in the epiphyses. When secondary ossification is complete,
hyaline cartilage remains only at two places:
On the epiphyseal surfaces, as the
articular cartilages
At the junction of the diaphysis and epiphysis, where it
forms the
epiphyseal plates
Hyaline
cartilage
Area of
deteriorating
cartilage matrix
Epiphyseal
blood vessel
Spongy
bone
formation
Epiphyseal
plate
cartilage
Secondary
ossification
center
Blood
vessel of
periosteal
bud
Medullary
cavity
Articular
cartilage
Childhood to adolescence
Birth
Week 9
Month 3
Spongy
bone
Bone
collar
Primary
ossification
center
1
2
3
4
5
Bone collar forms
around the diaphysis
of the hyaline
cartilage model.
Cartilage in the
center of the diaphysis
calcifies and then
develops cavities.
The periosteal
bud invades the
internal cavities and
spongy bone forms.
The diaphysis
elongates and a
medullary cavity forms.
Secondary ossification
centers appear in the
epiphyses.
The epiphyses ossify.
When completed, hyaline
cartilage remains only in
the epiphyseal plates and
articular cartilages.
Figure 6.8
Endochondral ossification in a long bone.
3
The periosteal bud invades the internal cavities and
spongy bone forms.
In month 3, the forming cavities are
invaded by a collection of elements called the
periosteal
bud
, which contains a nutrient artery and vein, nerve fi-
bers, red marrow elements, osteogenic cells, and osteo-
clasts. ±e entering osteoclasts partially erode the calcified
cartilage matrix, and the osteogenic cells become osteo-
blasts and secrete osteoid around the remaining calcified
fragments of hyaline cartilage, forming bone-covered car-
tilage trabeculae. In this way, the earliest version of spongy
bone forms in a developing long bone.
4
The diaphysis elongates and a medullary cavity forms.
As
the primary ossification center enlarges, osteoclasts break
down the newly formed spongy bone and open up a med-
ullary cavity in the center of the diaphysis. ±roughout
the fetal period (week 9 until birth), the rapidly growing
epiphyses consist only of cartilage, and the hyaline cartilage
models continue to elongate by division of viable cartilage
cells at the epiphyses. Ossification “chases” cartilage for-
mation along the length of the shaF as cartilage calcifies,
erodes, and then is replaced by bony spicules on the epiphy-
seal surfaces facing the medullary cavity.
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