MODULE SUPPLEMENT: MUSCULO-SKELETAL SYSTEM
Bone Components
Cortical or compact bone forms the shafts of long bones and the cortices or outer shell of other bones.
Approximately 75-80% of the body's bone is cortical or compact bone which is especially important for support. Compact bone has a concentric structure which includes the Haversian system or osteon. The concentric circles are made up of calcified matrix material (the lamellae) that encircles the canal (The Haversian canal) within which travel blood vessels, lymphatics, and nerve fibers. The lamella are actually tiny circular plates of bone between the Haversian systems. Spaces within these lamellae are called lacunae and contain the osteocytes or bone cells that evolve from osteoblasts.
For a schematic of compact bone see:
Compact Bone
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and for a histological picture of compact bone see:
Compact Bone Histology
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Trabecular or canellous bone (spongy bone) is found especially in the ends of the long bones (the epiphyses) and predominates in the vertebral bodies of the spine and the other odd shaped bones (short and flat bones).
Trabecular bone makes up the remaining 20-25% of the bone. It is within the spaces of this porous bone that the bone marrow is found. Trabecular bone provides support with light weight.
For a good picture of how fragile and open this bone is, please see:
High-Resolution Trabecular Bone Images
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Bones tend to have a number of surfaces. This can be important to bone remodeling because it can occur on each of these surfaces.
There are three major types of bone "cells"
a. Osteoblasts
b. Osteoclasts
c. Osteocytes
Osteoblasts deposit bone. Osteoblasts also secrete large amounts of alkaline phosphatase when they are active and this is thus used as a measure of bone activity. Some osteoblasts become encased within the bone matrix and the "Osteocytes" or the bone cells.
Osteoclasts absorb bone and are under the influence of parathyroid hormone and a number of local factors that may be secreted by the osteoblasts.
Bone is constantly being remodeled, as noted in the schematic below. Remodeling is extremely important in that it allows the bone to renew itself, grow to deal with specific stressors, and repair microfractures or other trauma.
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