CHAPTER 5
Cartilage and Bone
95
Figure 5-13A.
A representation of the development of a long bone.
Most
long bones
are formed by endochondral ossiF cation, a process of bone formation involving hyaline cartilage serving as a cartilage
model, cartilage proliferation and calciF cation, and gradual replacement by bone. Long bone formation includes the following steps: (1)
Cartilage model
: A small piece of hyaline cartilage is formed by mesenchymal tissue, and the outer part of this tissue condenses to form
a
perichondrium
. (2)
Developing cartilage model
: Cartilage assumes the shape for the future bone. (3)
Formation of bone collar
: As the
cartilage proliferates, the perichondrium in the middle shaft region transforms into
periosteum
. Osteoprogenitor cells in the periosteum
differentiate into osteoblasts, which start to form the
bone collar
(periosteal bone) by intramembranous ossiF cation. (4)
Formation of
primary ossi± cation centers
: The cartilage plate (epiphyseal plate) continues to proliferate and then calcify (see ±ig. 5-12A,B). The bone
collar (contains bone matrix, osteoblasts, and osteoclasts) triggers blood vessels to invade and create a primary marrow cavity. (5)
For-
mation of bony trabeculae
: Osteoprogenitor cells in the periosteum migrate with blood vessels into the region of the calciF ed cartilage.
These cells become osteoblasts and begin to deposit
osteoid
(prebone) on the surface of the calciF ed cartilage matrix. At the same time,
osteoclasts remove dead chondrocytes and extra calciF ed cartilage matrix, thereby producing
bony trabeculae
. (6)
Formation of second-
ary ossi± cation centers
: A similar bone ossiF cation takes place at the distal ends of long bones (
epiphyses
) called
secondary ossi± cation
centers
. (7)
Continuation of primary and secondary ossi± cation
: Repetition of the endochondral ossiF cation process results in more
bone being produced and more cartilage being absorbed in both primary and secondary ossiF cation centers. ±inally, the cartilage in the
epiphyseal plates disappears, and the primary ossiF cation center meets the secondary ossiF cation center at about age 20 in humans.
D. Cui
D. Cui
Blood vessel
Bony
trabeculae
Secondary
ossification
center
Perichondrium
Perichondrium
Perichondrium
Periosteum
Periosteum
Periosteum
Bone
collar
Diaphysis
Epiphysis
Primary
ossification
center
1. Cartilage model
2. Developing
cartilage model
3. Formation
of bone collar
4. Formation of
primary ossification
center
7. Continuous primary and
secondary ossification
6. Formation of the
secondary ossification center
5. Formation of
bony trabeculae
A
CLINICAL CORRELATION
Figure 5-13B.
Osteosarcoma.
Osteosarcoma
, also known as
osteogenic sarcoma
, is the most
common primary malignant neoplasm of bone and occurs most
commonly in the second decade of life. Conventional osteosarcoma
tends to affect the long bones, including the distal femur, proxi-
mal tibia, and proximal humerus, and is most often a disease of
the
metaphysis
(±ig. 5-8). Clinically, patients may experience pain,
decreased range of motion, edema, and localized warmth. Histolog-
ically, the
tumor cells
tend to be
pleomorphic
with a variety of sizes
and shapes. Central to the diagnosis of osteosarcoma is the presence
of
osteoid
(
prebone
) produced by the malignant cells (tumor cells).
Osteoid is a dense, pink, amorphous material. Conventional osteo-
sarcoma is an aggressive tumor and preferentially metastasizes to
the lungs. Treatment involves surgery and chemotherapy.
B
Osteoid
Tumor cells
previous page 110 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 112 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off