CHAPTER 5
Cartilage and Bone
89
Bone
Introduction and Key
Concepts for Bone
Bone
is a special type of supporting connective tissue, which
has a hard, mineralized, extracellular matrix containing
osteo-
cytes
embedded in the matrix. It is different from cartilage in
that bone is calciF ed and, hence, is harder and stronger than
cartilage. In addition, it has many blood vessels penetrating the
tissue. Bone protects internal organs, provides support for soft
tissues, serves as a calcium reserve for the body, provides an
environment for blood cell production, detoxiF es certain chem-
icals in the body, and aids in the movement of the body. In gen-
eral, the external surface of the bone is covered by
periosteum
,
a layer of connective tissue containing small blood
vessels,
osteogenic cells, and nerve F bers conveying pain information.
The inner surface of the bone is covered by
endosteum
, a thin
connective tissue layer composed of a single layer of osteo-
progenitor cells and osteoblasts that lines all internal cavities
within bone; this lining represents the boundary between the
bone matrix and the marrow cavities. Bone cells include
osteo-
genic cells
,
osteoblasts
,
osteocytes
, and
osteoclasts
. These cells
contribute to bone growth, remodeling, and repair.
Bone Matrix
Bone
is primarily characterized by a hard matrix, which
contains
calcium, phosphate, other organic and inorganic materials, and
type I collagen F bers. Compared to cartilage, bone contains
only about 25% water in the matrix, whereas cartilage matrix
contains about 75% water. This combination makes bone
hard, F rm, and very strong.
Bone
matrix
has
organic
and
inor-
ganic
components
. (1)
Organic
(
noncalcif
ed
)
matrix
is mainly
type I collagen
with nonmineralized ground substance (chon-
droitin sulfate and keratin sulfate). It is found in the freshly
produced bone matrix,
osteoid
(also called
prebone
), which is
produced by
osteoblasts
. This matrix stains light pink in H&E
preparations (±ig. 5-11A). (2)
Inorganic
(
calcif
ed
)
matrix
,
mainly in the form of
hydroxyapatite
, contains crystalline min-
eral salts, mostly of calcium and phosphorus. After osteoid is
produced, this fresh matrix undergoes a mineralization process
to become the calciF ed matrix (±ig. 5-11B).
Bone Cells
The main types of cells in bone are
osteoprogenitor cells
,
osteo-
blasts
,
osteocytes
, and
osteoclasts
: (1)
Osteoprogenitor cells
are located in the
periosteum
on the surface of the growing
bone and can differentiate into osteoblasts. (2)
Osteoblasts
produce the bone matrix. They are cuboidal or low columnar
in shape and have a well-developed
Golgi complex
and
RER
,
which correlates with their protein-secreting function (±ig.
5-11). The overall process of mineralization relies on the eleva-
tion of calcium and phosphate within the matrix and the func-
tion of
hydroxyapatite crystals
. This is brought about by com-
plex functions of the osteoblast. (3)
Osteocytes
are small, have
cytoplasmic processes, and are unable to divide. These cells
originate from osteoblasts and are embedded in the bone
matrix. Osteoblasts deposit the matrix around themselves and
end up inside the matrix, where they are called “osteocytes.”
Each osteocyte has many long, thin processes that extend into
small narrow spaces called
canaliculi
. The nucleus and sur-
rounding cytoplasm of each osteocyte occupy a space in the
bone matrix called a
lacuna
. Thin processes of the osteocyte
course through thin channels (canaliculi) that radiate from
each lacuna and connect neighboring lacunae (±ig. 5-9B,C).
(4)
Osteoclasts
are large, multinucleated cells, which derive
from
monocytes
, absorb the bone matrix, and play an essential
role in bone remodeling (±ig. 5-14A,B).
Types of Bone
There are several ways to classify bone tissues. Microscopically,
bone can be classiF
ed as
primary
bone
(
immature
, or “
woven
bone
) and
secondary
bone
(
mature
, or
lamellar bone
). Bones
can also be classiF
ed by their shapes as follows:
long bones
,
short bones
,
fl at bones
, and
irregular bones
(Table 5-2).
Mature
bone
can be classiF
ed as
compact bone
and
cancellous bone
based on gross appearance and density of the bone.
Compact
bone
, also called
cortical bone
, has a much higher density and
a well-organized osteon system. It does not have trabeculae
and usually forms the external aspect (outside portion) of the
bone (±igs. 5-8 to 5-10B).
Cancellous bone
, also called
spongy
bone
, has a much lower density and contains
bony
trabecu-
lae
or
spicules
with intervening bone marrow (±ig. 5-8A,C). It
can be found between the inner and the outer tables of the
skull, at the ends of long bones, and in the inner core of other
bones.
Bone Development
Bone development
can be classiF
ed as
intramembranous ossiF
-
cation
and
endochondral ossiF cation
, according to the mecha-
nism of its initial formation. (1)
Intramembranous
ossif
cation
is the process by which a condensed mesenchyme tissue is
transformed into bone. A cartilage precursor is not involved;
instead, mesenchymal cells serve as
osteoprogenitor cells
, which
then differentiate into
osteoblasts
. Osteoblasts begin to deposit
the bone matrix (±ig. 5-11A,B). (2)
Endochondral ossif cation
is the process by which
hyaline cartilage
serves as a
cartilage
model
precursor. This hyaline cartilage proliferates, calci-
F es, and is gradually replaced by bone. Osteoprogenitor cells
migrate along with blood vessels into the region of the calciF
ed
cartilage. These cells become osteoblasts, which then begin to
deposit the bone matrix on the surface of the calciF
ed cartilage
matrix plate. Endochondral ossiF
cation involves several events
(see ±igs. 5-12 and 5-13A for a summary of these processes). The
development of long bone is a good example of endochondral
formation. In this particular case, the hyaline cartilage under-
goes proliferation and calciF
cation in the
epiphyseal plates
. This
epiphyseal cartilage can be divided into F
ve recognizable zones:
reserve zone
,
proliferation zone
,
hypertrophy zone
,
calciF
cation
zone
, and
ossiF
cation zone
(see ±ig. 5-12B).
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