80
UNIT 2
Basic Tissues
Bone Development and Growth
Figure 5-11A
Intramembranous Ossif
cation, Fetal Head
Figure 5-11B
Osteoblasts
Figure 5-12A
Endochondral Ossif cation, Finger
Figure 5-12B
Epiphyseal Plate, Finger
Figure 5-13A
A Representation o± the Development o± the Long Bone
Figure 5-13B
Clinical Correlation: Osteosarcoma
Figure 5-14A
Bone Remodeling, Nasal
Figure 5-14B
Osteoclast
Table 5-2
Bone
Synopsis 5-4
Pathological and Clinical Terms ±or Cartilage and Bone
Cartilage
Introduction and Key Concepts
for Cartilage
Cartilage
and
bone
are two types o± supporting connective
tissues. Cartilage is an
avascular
specialized ±orm o± connective
tissue whose support ±unction is a result o± a f rm
extracellular
matrix
that has variable fl exibility depending on its location.
This type o± tissue is able to bear mechanical stress without
permanent de±ormation. Cartilage has ±eatures that are di±±er-
ent ±rom other types o± connective tissues but, like bone, has
the characteristic o± isolated cells embedded in extensive matrix.
Most cartilage is covered by a layer o± dense irregular connective
tissue called
perichondrium
, which contains a rich blood supply
and is innervated by nerve f
bers conveying pain. The excep-
tions are f brocartilage and articular cartilage o± the joint, which
do not have perichondrium. Perichondrium is important ±or the
growth (
appositional growth
) and maintenance o± cartilage; it
has two layers. The
outer ± brous layer
o± the perichondrium
contains connective tissue f
bers, f
broblasts, and blood vessels.
These perichondrial vessels represent an essential blood supply
±or cartilage. Because cartilage itsel± is avascular, these vessels
are the route through which nutrients access the matrix by di±-
±usion. The
inner cellular layer
o± the perichondrium consists o±
chondrogenic cells
, which are able to di±±erentiate into
chondro-
blasts
(Fig. 5-2). The ±unctions o± cartilage include the support
o± so±t tissues, the ±acilitation o± smooth movement o± bones at
joints, and the mediation o± growth o± the length o± bones dur-
ing bone development.
Cartilage Cells
The main types o± cells in cartilage are
chondrogenic cells
,
chon-
droblasts
, and
chondrocytes
. (1)
Chondrogenic cells
are located
in the
perichondrium
and di±±erentiate into chondroblasts to par-
ticipate in
appositional growth
o± cartilage (Fig. 5-7). These cells
are di±f
cult to identi±y under the light microscope with H&E
stain. (2)
Chondroblasts
are young chondrocytes, which derive
±rom
chondrogenic cells
, and are able to actively manu±acture
the matrix o± cartilage. The chondroblasts have ribosome-rich
basophilic cytoplasm. They synthesize and deposit cartilage
matrix around themselves. As the matrix accumulates and
separates the chondroblasts ±rom one another, the cells become
entrapped in small individual compartments called
lacunae
and
are then re±erred to as “chondrocytes.” (3)
Chondrocytes
are
mature chondroblasts that are embedded in the lacunae o± the
matrix. Chondrocytes retain the ability to divide and o±ten pres-
ent as an
isogenous group
, two or more chondrocytes arranged
in a group that was derived ±rom a single progenitor cell
(Fig. 5-2). The isogenous group represents the active division
o± cells, which contribute to
interstitial growth
(see below,
Cartilage Growth
). In most cartilage, chondrocytes are arranged
in an isogenous group. However, in some locations such as in
f brocartilage, chondrocytes are more likely to be arranged in
groups o± small columns or rows instead o± isogenous groups.
This is also a sign o± interstitial growth.
Cartilage Matrix
The matrix o± cartilage is nonmineralized and consists o± f
bers
and ground substance.
Collagen ±
bers
are mainly
type II
in
the matrix, although some cartilage may also contain
type I
or
elastic ±
bers
. The major components o±
ground substance
include glycosaminoglycans (GAGs), proteoglycans, and glyco-
proteins. The matrix o± cartilage surrounding each chondrocyte,
or immediately adjacent to chondrocytes o± isogenous groups, is
called
territorial matrix
. This newly produced matrix has abun-
dant proteoglycans and less collagen and stains more intensely
in routine H&E preparations. Another type o± matrix, which
surrounds the regions o± territorial matrix and f
lls the rest o± the
space, is called
interterritorial matrix
. This type o± matrix stains
more lightly than does the territorial matrix (Figs. 5-2 and 5-4).
Types of Cartilage
Cartilage can be classif
ed into three types based on the charac-
teristics o± the matrix. All three types o± cartilage contain type II
collagen; in addition, some types contain type I collagen or elas-
tic f
bers in the extracellular matrix. Types o± cartilage include
hyaline cartilage
,
elastic cartilage
, and
± brocartilage
.
HYALINE CARTILAGE
is characterized by the presence o± a
glassy, homogeneous matrix that contains
type II collagen
, which
is evenly dispersed within the ground substance. Most hyaline
cartilage is covered by
perichondrium
, except at the articular
sur±aces o± joints. Hyaline cartilage is the most common type
o± cartilage; is ±ound in the articular ends o± long bones, nose,
larynx, trachea, bronchi, and the distal ends o± ribs; and is the
template ±or endochondral bone ±ormation (Figs. 5-2 to 5-4 and
5-12). Hyaline cartilage covers the smooth sur±ace o± joints,
previous page 95 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 97 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off