are able to pass through perforating Volkmann’s canals (named after Alfred Volkmann: 1800–1877) to other osteocytes and maintain their viability. It should be noted here that several blood vessels enter the bone within the periosteum and ultimately make their way throughout the Haversian systems.
Figure 4-5 Interior of bone.
Cartilage is both similar to and different from bone. Both structures consist more of an intracellular substance than that of actual cells. They both contain a countless number of collagenous fibers that strengthen their intracellular matrices. However, the type of collagenous fibers that form the matrices embedded in each structure is different.
In cartilage, Type II collagen is used to form the firm gel-like substance found in its matrix, while Type I collagen is used to form the harder, cement-like and calcified matrix found in that of bone. This is the reason for the flexibility found in cartilage, whereas bone is much more rigid. Cartilage cells, called chondrocytes, are located in lacunae – just as with the osteocytes. Conversely, no blood vessels are found in cartilage. Lacunae are suspended in the gel-like firm matrix. Therefore, nutrients must disperse throughout the matrix of cartilage to reach the chondrocytes.
■Bone Development, Growth, and Aging
The human skeleton begins to form early during embryogenesis (beginning of an embryo), approximately six weeks after fertilization. However, at this stage the skeleton is basically a model of cartilage. During the process of bone development, called osteogenesis8 (osteo=bone; genesis=origin), the bones undergo a significant increase in size and shape. This development is continuously monitored and carefully regulated.
The growth process of bone development, such as the cartilage in an embryo with osseous tissue (bone tissue) is called ossification. It should also be noted here that the term ossification is reserved specifically to the formation of bone. While the term calcification (the accumulation of calcium salts that leads to the hardening of structures) also occurs during ossification, it can also take place in other tissues. Two types of ossification9 take place in the human body:
•Intramembranous ossification
•Endochondral ossification
Intramembranous ossification, also called “dermal ossification”, is the term applied to the process of forming bone directly from mesenchymal or fibrous connective tissue. Intramembranous ossification normally only occurs in the deep layers of the dermis, resulting in development and growth of dermal bones. Examples of dermal bones include the flat bones of the skull, the mandible (jaw bone), and the clavicle (collar bone).
In response to abnormal stresses, such as a fracture, the human body relies on intramembranous ossification to form bone in other dermal areas, tendons, joints, and even skeletal muscle.
The second type of ossification, endochondral ossification, is the process of forming bone that has been modeled from cartilage; most bones in the human body are formed this way. As an example, the development of a long bone in one of the limbs.
During embryogenesis, which occurs approximately six weeks after conception, the proximal portion of a limb is present – albeit completely composed of cartilage. Chondrocytes, or cartilage cells, perform an essential role during this stage of the process, providing a platform for longitudinal growth by means of an arrangement of proliferation, extracellular matrix (ECM) secretion, and hypertrophy.
Chondrocytes grow, but eventually disintegrate and die, leaving behind a cavity invaded by blood vessels, providing a medium for the differentiation of fibroblasts into osteoblasts, or bone-forming cells. It is at this time, during the primary center of ossification, where actual bone development proceeds and ultimately results in a diaphysis (shaft) of a long bone.
A second site of bone development, called the secondary ossification center, takes place in the epiphysis or end of a long bone. Located between the diaphysis and epiphysis is a cartilaginous structure called the epiphyseal plate.10 As long as the epiphyseal plate remains between the diaphysis and epiphysis of a long bone, growth will continue to occur.
Eventually, when the entire epiphyseal cartilage is converted into bone, growth stops and all that remains of the epiphyseal plate is an epiphyseal line demarcating the location where the two ossification centers have merged together. It should be noted that the timing of the epiphyseal closure differs from bone to bone, and individual to individual.
The human skeleton is a dynamic, living tissue – continuously and internally monitored and regulated for nutrition, growth, and repair. Osteolysis, or bone resorption, is an erosion process constantly occurring via osteoclast (bone-resorbing cells) secretion of acids and proteolytic enzymes.
Essentially, osteoclasts eliminate the bone matrix, while osteoblasts enhance it. The management and balance of both of these types of cells is extremely important in bone regulation and in the control of calcium and phosphate concentrations in the human body. As the human body ages, so does the skeleton; the bones become thinner and weaker over time. This is normal aging. The actual physiological phenomenon that occurs is deficient ossification process, known as osteopenia.
Usually, this process begins to occur between the ages of 30 and 40, when osteoclastic activity disproportionately increases in comparison to osteoblastic processes. Once this reduction in ossification, women tend to lose approximately eight percent of their skeletal mass every ten years, while men only lose approximately three percent of their skeletal mass in the same amount of time – mainly through hormonal decreases. Not all components of the skeleton are affected equally – the epiphysis, vertebrae, and mandible lose a higher percentage of their portions – resulting in easily broken limbs, a decrease in height, and the loss of teeth, respectively.
■Divisions of the Skeleton
The human skeleton contains a total of 206 individual bones and several accompanying cartilages and ligaments. The skeletal system is divided into two main divisions:
•the axial skeleton
•the appendicular skeleton
The axial skeleton, which encompasses the main mechanical core of the human body, contains exactly 80 bones, while the appendicular skeleton, which includes the bones of the limbs and their respective attachments to the pectoral and pelvic girdles, contains 126 bones. We begin our discussion with a closer look at the axial skeleton.
■Axial Skeleton
The axial skeleton offers a support system that functions to ensure the protection of the vital organs located in the ventral and dorsal body cavities.
Figure 4-6 Axial skeleton structure.
It also serves as a platform for the attachment of skeletal muscles responsible for:
•movement of the head, neck, and trunk
•aiding respiratory processes
•stabilization and security of inserting components of the appendicular skeleton
While movement is limited in the axial skeleton, the bones and joints are solidly fortified with ligaments, and therefore
