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Figure 5-4. Osteoclast (solid arrow) resorbs
bone. Osteoblasts (open arrows) form bone.
Source: Dellmann and Eurell, 1998. Reproduced
a with permission of John Wiley & Sons, Inc.
b
tissue. Calcification of tissue other than
osteoid is usually associated with some
pathologic process.
Regardless of the location, the sequence
of actual bone formation consists of
osteoblasts laying down osteoid tissue that
is subsequently calcified under the influ-
ence of the enzyme alkaline phosphatase.
A local area of bone formation is called a
center of ossification (Fig. 5‐5). The envi-
ronment in which bone forms determines
whether the type of ossification is either
endochondral or intramembranous.

Endochondral (Intracartilaginous)
Ossification
Figure 5-3. Remodeling that occurs as a long
bone increases in size. Both resorption and depo- During fetal development, most of the
sition of bone take place. a, size and shape of skeleton first develops as a cartilage pattern
young bone; b, size and shape of mature bone. or model, and then the cartilage of this
model is gradually replaced by bone. This
derived from macrophages, take an active process is called endochondral ossification.
part in bone destruction by releasing The center of ossification that develops
organic acids and enzymes. in the midshaft region of a long bone is
the primary ossification center (Fig. 5‐5).
Secondary ossification centers then
Ossification develop near the ends of long bones. These
ossification centers grow and expand, but
Ossification is the formation of true bone a region of cartilage, the physis, still
by deposition of calcium salts in a matrix separates the centers during growth and
of osteoid tissue. Calcification refers to development. Chondrocytes within this
the deposition of calcium salts in any region continue to proliferate and produce