Figure 4-3 Bone structure.
■Bone Histology
Histology defines structure and organization, so in regard to the skeletal system, bone histology means a branch of anatomy that focuses on the minute structures of human (or animal) tissues that is discernable through a microscope.
Several types of bone cells4 are found in the human body:
•Osteoblasts – bone-forming cells
•Osteocytes – responsible for the maintenance of mineral and organic elements in bone
•Osteoclasts – involved in breakdown of bone tissue and involved with resorption processes
Figure 4-4 Bone cells.
The skeletal system contains two main types of connective tissue: bone and cartilage. Further, each bone is comprised of two types of osseous tissue: compact bone, and spongy bone.
Compact bone is a dense and solid type of bone. Spongy bone, also called cancellous bone,5 is a relatively weaker and porous type of bone (hence, resembling a sponge). It is composed of “needle-like threads” of bone, called trabeculae encompassed by gaps of space filled with red marrow.
Compact bone, because it does not contain a network of gaps or open spaces, is a dense and rigid structure, organized into a matrix of several functional units of bone called osteons, or Haversian systems.6
All osteons are packed tightly together and oriented the same way, creating immense strength and providing a great basis for support. The Haversian systems are circular structures composed of a hardened medium arranged in multiple layers, resembling the rings of an onion.
Each concentric ring or layer in the osteon is called a lamella. The lamellae (plural) surround and encircle the central canal, also known as the Haversian canal, named after Dr. Clopton Havers (1657–1702),7 which contains a blood vessel.
In contrast to compact bone, spongy bone often lacks complete osteons or Haversian systems due to extremely thin trabeculae. However, spongy bone is more metabolically active than its denser counterpart, due to its much larger surface area.
Bones, because of their hard, rigid matrix, are often thought to be lifeless structures. However, wedged in between the hard layers of the lamellae in tiny pockets called lacunae are living bone cells; the aforementioned osteocytes.
Lacunae are connected to one another and to the central canal via microscopic channels or canals, called canaliculi. Because of this interconnection, nutrients from the blood vessel in the central canal 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,