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
centers:
(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.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
