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.
(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.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.