Page 266 - Atlas of Histology with Functional Correlations

membrane proteins.

The inorganic component of bone matrix consists of the minerals calcium

and phosphate in the form of hydroxyapatite crystals. The association of coarse
collagen fibers with hydroxyapatite crystals provides the bone with its hardness,
durability, and strength. In addition, as the need arises, actions of parathyroid
hormone from the parathyroid gland and calcitonin from the thyroid gland on the

bone adjust and maintain a proper mineral content in the blood.

PROCESS OF BONE FORMATION

(OSSIFICATION)

Bone development begins in the embryo, the fetus, and continues after birth into
adolescence. This development occurs by two processes: endochondral
ossification and intramembranous ossification. Although the resulting bones
are produced by two different methods, they exhibit the same histologic structure
or morphology (see Fig. 7.1).

Endochondral Ossification

Most long bones, vertebrae, ribs, and the pelvis develop by endochondral
ossification through replacement of the temporary hyaline cartilage models. This

method allows the hyaline cartilage model to initially grow in length and width.
Mesenchymal cells proliferate and differentiate into chondroblasts that produce
the cartilage model for the bone. The cartilage model, surrounded by

perichondrium, grows by both interstitial and appositional means, producing
the short and long bones of the body. As the growth progresses, the
chondroblasts divide, hypertrophy (enlarge), and mature, and the hyaline
cartilage model begins to calcify. As calcification progresses, diffusion of
nutrients and gases through the calcified cartilage matrix decreases.

Consequently, chondrocytes degenerate and die, leaving a fragmented and
porous calcified matrix as scaffolding for the deposition of bony material.

As the bony material is deposited around the calcifying cartilage, the inner
perichondrial cells become osteogenic and form a thin bony collar around the

midpoint of the shaft. The connective tissue around the new bone now becomes
the periosteum, and the mesenchymal cells in the inner layer of periosteum

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