DEVELOPMENT OF AN INDIVIDUAL BONE
a. General. The human skeleton is "preformed" in the early fetus, but the early
form is not of bony material. There are two types of bones according to their preformed
basis: membranous bones and cartilage bones. These are in the location and have the
general shape of the adult bones they will later become.
(1) Membranous bones. The outer skull bones are an example of
membranous bones. Osteoblasts invade a membrane to form a center of ossification
(formation of bone). Bone-forming activity spreads out from this center until a full bone
plate is formed.
(2) Cartilage bones. In the fetus, many bones, for example, long bones, exist
first as models formed of cartilage.
b. Sesamoid Bones. Sesamoid bones are small masses of bone that develop
in tendons at points where great forces are applied to the tendons. The most obvious
and largest sesamoid bone is the patella, or kneecap.
c. Ossification Centers. An ossification center is a growing mass of actual
bone within the preformed material, as noted above.
(1) Initial bone formation involves destruction of the preforming material and
replacement with bony tissue.
(2) In the development of long bones, there are two types of ossification
(a) Diaphyseal--in the shaft region.
(b) Epiphyseal--in the end(s).
(3) As a long bone grows in length, the preforming material grows faster than
the ossification center can tear it down. Ultimately, with time, the preforming material is
overcome and growth ceases.
d. Growth in Bone Width. A bone grows wider through the activity of the
osteogenic layer of the periosteum. Remember, the periosteum covers most of the
outer surface of the bone.
4-6. TYPES OF BONES
Bones of the skeleton can be grouped into the following major types: long, short,
flat, and irregular. Each type has a somewhat different construction pattern.