Biomedical Engineering Reference
In-Depth Information
Appendix A: Bone Types Based on
Pattern of
Development and Region*
In Section 1.1, bone types based on a macroscopic approach, microscopic
observation, and geometric shape are described. Those typologies are used
as the major reference throughout the topic. For completeness, bone types
based on pattern of development and region are presented here.
A.1 Bone Types Based on Pattern of Development
On the basis of the pattern of development of a bone, bones can be clas-
sified into three categories. In fact, bone formation (osteogenesis) begins
during prenatal development and persists through adulthood. Bones of
infants and children are softer than those of adults because they have
not yet been ossified (the process of synthesizing cartilage into bone).
Osteogenesis occurs in two ways: by intramembranous ossification and
by endochondral ossification. Both types form bone by replacing existing
cartilage, but they differ in the way they go about it. Two types of cells
that are of great importance in the process are osteoblasts and osteoclasts.
Osteoblasts, used mainly in intramembranous ossification, are the special-
ized cells in bone tissue that deposit calcium into the protein matrix of
bone (collagen). Osteoclasts, used in endochondral ossification, dissolve
calcium previously stored away in bone and carry it to tissues whenever
needed.
A.1.1 Membranous Bones
Bones formed during intramembranous ossification are called membranous
bones or, occasionally, dermal bone. These bones ossify from mesenchymal
condensation in intrauterine life.
It can be observed under a microscope that membranous bones first appear
as flat, membrane-like layers of early connective tissue. These layers are pro-
vided with a constant flow of nutrient blood supply by networks of blood
vessels formed between the layers. Early connective tissue cells first arrange
themselves among the layers and then differentiate into bone-forming cells
called osteoblasts. The osteoblasts then remove calcium from the blood and
*
http://en.wikipedia.org/wiki/Bone (accessed March 2011).
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