4.2 Bone As An Individual Organ

BASIC STRUCTURE OF AN INDIVIDUAL BONE

See figure 4-1 for the basic structure of an individual bone.

Figure 4-1. A mature long bone (femur).

(1) Cortex. The cortex is the outer layer of the individual bone. It is made up of compact (dense) bony tissue.

(2) Medulla. The medulla is the central portion of the individual bone. It generally consists of cancellous (spongy) bone tissue. In some bones, particularly long bones, the medulla may include a space without any bony tissue. This space is called the medullary or marrow cavity.

b. Marrow. Marrow serves as a filler of the inside of bones. There are two types of bone marrow--yellow bone marrow and red bone marrow. Yellow bone marrow is mostly yellow fat tissue. Red bone marrow is the only site in adults for the formation of red blood cells (hematopoiesis).

Parts of an Individual Long Bone

Shaft (diaphysis). The shaft is the central portion of a long bone. Here, the cortex is thickened as required by applied physical stresses.

Ends (epiphyses). The ends of long bones are made up mainly of cancellous (spongy) bone tissue. An articular cartilage covers each area where a bone contacts another bone(s). This articular cartilage is made up of hyaline cartilage tissue and provides a smooth surface for motions.

d. Periosteum. The periosteum is a covering of the bone surface area not covered by articular cartilage. It has two layers--the innermost layer and the fibrous layer.

(1) The innermost layer, which lies against the outer surface of the bone, consists of bone-forming cells (osteoblasts). It is the osteogenic (bone-forming) layer.

(2) The outermost layer is a FCT (fibrous connective tissue) layer.

(3) The periosteum is well supplied with blood vessels and sensory-type nervous tissue.

e. Blood Supply of an Individual Bone. A system of blood vessels enters and spreads out through the periosteum. Additional blood vessels, called "nutrient vessels," penetrate the cortex of the bone and spread out through the marrow. The passageways for penetration of these vessels are called the nutrient canals.

DEVELOPMENT OF AN INDIVIDUAL BONE

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.

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.

a. Long Bones. The basic structure of a long bone is illustrated in figure 4-1. Example: femur.

b. Short Bones. The short bones, such as those of the wrist and feet, have a thin layer of compact bone surrounding an inner mass of spongy bone.

Example: carpal bones.

c. Flat Bones. The flat bones are constructed with two plates of compact bone, which enclose between them a layer of spongy bone. The spongy bone is richly supplied with blood vessels and red marrow.

Example: the cranial frontal bone.

d. Irregular Bones. The irregular bones are those that do not fit into the three categories above.

Example: a vertebra.