94
UNIT 2
Basic Tissues
Figure 5-12A.
Endochondral ossif
cation, f
nger.
H&E,
3
20; inset
3
68
Endochondral ossif cation
is a process of bone formation
in which hyaline cartilage serves as a cartilage model (pre-
cursor). Cartilage proliferation occurs, then calciF
cation,
and gradually the cartilage is replaced by bone (see ±igs.
5-12B and 5-13A). This is an example of a long bone (F n-
ger), showing the
epiphyseal plate
(cartilage plate) with
the
primary ossif cation center
(primary marrow cavity).
There is a thick layer of dense connective tissue covering
the peripheral region of the cartilage, called the
perichon-
drium
. The connective tissue layer that covers the outer
surface of the bone is called
periosteum
. The primary ossi-
F cation center contains blood vessels, newly formed bone
tissue, osteoblasts, osteoclasts, calciF
ed cartilage matrix,
and dead chondrocytes. (PC, primary ossiF
cation center.)
Bone
Bone
matrix
matrix
Periosteum
Periosteum
Bone
matrix
Epiphyseal
plate
Periosteum
Perichondrium
Perichondrium
Cartilage
Cartilage
matrix
matrix
Bone
Bone
matrix
matrix
Perichondrium
Cartilage
matrix
Bone
matrix
Articular
cartilage
PC
PC
PC
A
1. Reserve zone
2. Proliferation zone
3. Hypertrophy zone
4. Calcification zone
5. Ossification zone
Reserve zone
Proliferation zone
Hypertrophy zone
Calcification zone
Ossification zone
B
Figure 5-12B.
Epiphyseal plate, f
nger.
H&E,
3
71; small images
3
96
The
epiphyseal plate
is a region of hyaline
cartilage at the ends (epiphyses) of the shafts of
long bones. Its chondrocytes are undergoing the
process of proliferation, hypertrophy, and cal-
ciF
cation, during the process of endochondral
ossiF
cation. The epiphyseal plate can be divided
into F
ve functionally distinct zones beginning
at the epiphyseal end: (1) In the r
eserve zone,
cartilage chondrocytes are inactive and individ-
ual cells are not arranged in isogenous groups.
These cells are small and randomly scattered
in the matrix. (2) In the
proli±eration zone
,
chondrocytes undergo frequent mitosis and are
arranged in groups of columns (indicative of
interstitial growth of cartilage) in this region.
Chondrocytes are fl
at, and their size is increased
leading to increased length of the cartilage. (3)
In the
hypertrophy zone
, chondrocytes become
mature, and their size increases markedly (big
and fat cells). Isogenous groups are clearly evi-
denced and cells actively deposit matrix (type
X and XI collagen). (4) In the
calcif
cation
zone
, cartilage matrix becomes calciF ed, and
chondrocytes die because nutrients and oxygen
cannot diffuse through the calciF
ed cartilage
matrix. The matrix in this region is F
lled with
hydroxyapatite (a complex phosphate of cal-
cium). (5) In the
ossif
cation zone
, blood vessels
invade and create primary marrow; osteopro-
genitor cells arrive in this region and differen-
tiate into osteoblasts to start depositing bone
matrix (osteoid or new bone) on the surface
of the calciF
ed cartilage. Osteoclasts are also
present and function as phagocytes to remove
unwanted calciF
ed cartilage matrix and dead
chondrocytes.
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