198
UNIT TWO
seal plate form four layers, each of which may be several
cells thick, as shown in
f gure 7.9
. The F rst layer, or
zone of
resting cartilage,
is closest to the end of the epiphysis. It is
composed of resting cells that do not actively participate in
growth. This layer anchors the epiphyseal plate to the bony
tissue of the epiphysis.
The second layer of the epiphyseal plate, or
zone of
proliferating cartilage,
includes rows of many young cells
undergoing mitosis. As new cells appear and as extracellular
matrix forms around them, the cartilaginous plate thickens.
The rows of older cells, left behind when new cells
appear, form the third layer, or
zone of hypertrophic carti-
lage,
enlarging and thickening the epiphyseal plate still more.
Consequently, the entire bone lengthens. At the same time,
invading osteoblasts, which secrete calcium salts, accumu-
late in the extracellular matrix adjacent to the oldest carti-
laginous cells, and as the extracellular matrix calciF es, the
cells begin to die.
The fourth layer of the epiphyseal plate, or
zone of calci-
F
ed cartilage,
is thin. It is composed of dead cells and calci-
F
ed extracellular matrix.
In time, large, multinucleated cells called
osteoclasts
(os
te-o-klasts) break down the calciF ed matrix. These large
cells originate from the fusion of single-nucleated white blood
cells called monocytes (see chapter 14, p. 532). Osteoclasts
secrete an acid that dissolves the inorganic component of
the calciF ed matrix, and their lysosomal enzymes digest the
organic components. Osteoclasts also phagocytize compo-
nents of the bony matrix. After osteoclasts remove the extra-
cellular matrix, bone-building osteoblasts invade the region
and deposit bone tissue in place of the calciF ed cartilage.
In bone cancers, abnormally active osteoclasts destroy bone tissue.
Interestingly, advanced cancer of the prostate gland can have the
opposite effect. If such cancer cells reach the bone marrow, they
stimulate osteoblast activity. This promotes formation of new bone
on the surfaces of the bony trabeculae.
The process of forming an endochondral bone by the
replacement of hyaline cartilage is called
endochondral ossi-
F
cation.
Its major steps are listed in table 7.1 and illustrated
in
f gure 7.8
.
In a long bone, bony tissue begins to replace hyaline car-
tilage in the center of the diaphysis. This region is called the
primary ossiF
cation center,
and bone develops from it toward
the ends of the cartilaginous structure. Meanwhile, osteo-
blasts from the periosteum deposit a thin layer of compact
bone around the primary ossiF
cation center. The epiphyses
of the developing bone remain cartilaginous and continue to
grow. Later,
secondary ossiF
cation centers
appear in the epi-
physes, and spongy bone forms in all directions from them.
As spongy bone is deposited in the diaphysis and in the epi-
physis, a band of cartilage called the
epiphyseal plate
(ep
ı˘-
F z
e-al pla
¯t) remains between the two ossiF
cation centers
(see F
gs. 7.2, 7.3
b,
and 7.8).
Growth at the Epiphyseal Plate
In a long bone, the diaphysis is separated from the epiphysis
by an epiphyseal plate. The cartilaginous cells of the epiphy-
Cell process
in canaliculus
Osteocyte
Lacuna
FIGURE 7.7
Scanning electron micrograph (falsely colored) of an
osteocyte isolated in a lacuna (4,700×).
TABLE
7.1
|
Major Steps in Bone Development
Intramembranous Ossif
cation
Endochondral Ossif
cation
1.
Sheets of relatively undiF
erentiated connective tissue appear at sites
of future bones.
1.
Masses of hyaline cartilage form models of future bones.
2.
Partially diF
erentiated connective tissue cells collect around blood
vessels in these layers.
2.
Cartilage tissue breaks down. Periosteum develops.
3.
Connective tissue cells further diF
erentiate into osteoblasts, which
deposit spongy bone.
3.
Blood vessels and diF
erentiating osteoblasts from the periosteum
invade the disintegrating tissue.
4.
Osteoblasts become osteocytes when bony matrix completely
surrounds them.
4.
Osteoblasts form spongy bone in the space occupied by cartilage.
5.
Connective tissue on the surface of each developing structure forms a
periosteum.
5.
Osteoblasts beneath the periosteum deposit a thin layer of compact
bone.
6.
Osteoblasts on the inside of the periosteum deposit compact bone
over the spongy bone.
6.
Osteoblasts become osteocytes when bony matrix completely
surrounds them.
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