9.3: Bones
- Page ID
- 84050
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Though the bones are of many sizes and shapes, they are all built of the same types of materials. Each component is in the same position relative to the others, and each contributes to one or more of the five functions of the skeletal system.
Bone Tissue Components
The bony material (bone matrix) in bones contains three types of cells. Osteoblasts are cells that produce bone matrix by first secreting fibers made of collagen and then coating the fibers with mineral materials. In this way, the osteoblasts build bone, repair damaged or broken bone, and place extra amounts of certain minerals found in the blood into bones for storage (Figure 9.5). Osteoblasts also activate osteoclasts (see below) by secreting signaling materials (e.g., IL-6) (see Chapter 15).
As the osteoblasts work, many of them become surrounded and entrapped by the mineralized material they produce. These cells become quiescent and are called osteocytes. They will remain in a retired state unless a severe condition such as a fracture develops in the bone.
Osteoclasts dissolve some minerals in the matrix whenever the concentration of these minerals in the blood drops too low, restoring blood mineral concentrations to normal levels. This action is regulated by many factors (e.g., IL-6, hormones). Osteoclasts also remove unwanted bone material when bones have to be remodeled or repaired. Osteoblasts often fill the vacated areas with new bone matrix at a later time.
The activities of the three types of bone cells are carefully controlled by a variety of hormones and other substances so that minerals are simultaneously being added to and removed from the bones (see Chapter 14). Since the speed of these two processes is not always equal, the bones may gain minerals at one time and lose them at another. These control mechanisms usually assure preservation of homeostasis in the body.
Since minerals cannot be easily deposited into bone matrix unless fibers are present, the fibers in bone matrix play a role in mineral storage. The fibers make up about one-third of the bone matrix in young adults; the remainder consists of mineral salts. The fibers hold the minerals together and keep them from cracking. Therefore, the reinforcing fibers contribute to three other functions of the skeletal system: support, protection from trauma, and movement.
About 90 percent of the minerals in the matrix consist of calcium and phosphorus. The presence of minerals provides the means by which the skeletal system stores minerals. The minerals also make the bone hard and rigid. These two properties allow the bone to provide good support and protection from trauma. The hard and rigid bones also provide secure anchoring points and levers to aid the muscles in performing varied movements.
Bone Tissue Types
Cortical Bone
Bone cells produce two types of bone tissue. One type forms the outer layer of the bone (cortical bone). This bone tissue is made up of many long thick tubes of bone matrix called osteons (Figure 9.4 and Figure 9.5). The osteons are welded tightly together with bone matrix; therefore, this type of tissue is also called compact bone. Old osteons are always being gradually dissolved and replaced with new osteons.
Trabecular Bone
The second type of bone tissue is inside the bone. It consists of small pieces of bone matrix called trabeculae; therefore, this type of bone tissue is called trabecular bone. The trabeculae are of varied shapes, including needles, chips, and flakes, which are fused together by bone matrix. However, unlike the osteons in cortical bone, the irregular arrangement of the trabeculae leaves many open spaces. Since this arrangement provides a structure resembling that of a sponge, this is also called spongy bone. However, since trabecular bone is quite hard, rigid, and rough rather than soft and spongy, it might be better named coral bone.
Other Tissues
Bones contain more than just bone tissue. For example, the hollow shell formed by cortical bone and the arrangement of trabeculae in spongy bone leave much space inside a bone, which is filled with bone marrow (Figure 9.4). The outer surface of compact bone is covered by a tough layer of fibrous material called the periosteum, which serves as a place of attachment for ligaments and tendons. Additionally, where the surface of a bone meets another bone to form a joint, the bone may have a coating of cartilage, which may help join the bones or help them move. Finally, bones have blood vessels and nerves to serve the parts already mentioned. Therefore, bones are complex, dynamic, living parts of the body that change continuously. Some alterations are reversible physiological changes, such as removing and adding minerals, while others are biological age changes.