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Biology of Bone

In the skull, most of the bones forming the base of the skull begin as precursor cartilage models that enlarge and become ossified. In the adult skull the remnants of these early structures can be found in the sphenooccipital region. The bones of the cranial vault are thin plate-like bones that condense and ossify within mesenchymal membranes.

CARTILAGE:

Cartilage is a specialized type of connective tissue that exists in several different formats and serves several functions. As a structural material it is phylogenetically old and has been either a permanent or temporary element of skeletal systems for even the earliest of animals. It is a nearly avascular tissue, with only small vessels on it surface and a network of canals that carry veins and an artery. It receives metabolites by diffusion from surrounding fluids and tissues and thus its thickness is limited to a few millimeters. It has a very low metabolic rate and resists forces of tension, compression and shearing. It is flexible, compressible and possesses rebound properties. Nearly all bones of the skeleton begin as cartilage models that subsequently become mineralized (ossified) becoming exceedingly strong.

The five types of cartilage are differentiated from one another based on their location and cellular architecture.

  1. Hyaline: These cartilages appear opalescent and glassy and are somewhat flexible and strong. Hyaline cartilages can be found in the nasal septum, larynx, and tracheobronchial tree.

  2. Articular hyaline: Articular cartilages are found covering articular surfaces and are smooth, forming a nearly frictionless surface that allows free, painless movements between bones. If disease or injury results in the loss of cartilage at articular surfaces normal movements become exceedingly painful.

  3. Costal: These occur at the sternal ends of the first 10 ribs forming gliding type synovial joints between the rib cartilages and the sternum permitting respiratory movements of the thorax.

  4. White fibrocartilage: Large amounts of type I collagen in its matrix gives this cartilage great strength and resilience. It is found between adjacent vertebrae on the intervertebral discs, articular discs, glenoid and acetabular labrum and grooves in bones that transmit tendons.

  5. Yellow elastic: Contains yellow elastic fibers and it is flexible and resilient. It can be found in the larynx, the epiglottis and the external ear.

The cells of cartilage are chondrocytes. These generate a matrix of collagen fibers and a hydrophilic gel-like ground substance. Small spaces within this matrix are called lacunae and each contains a chondrocytes. When cartilage is actively growing, it does so by two mechanisms: interstitial and appositional. Interstitial growth occurs with meiotic activity of chondrocytes throughout the cartilage mass. Interstitial growth is expansive. Appositional growth is a lamellar style of growth in which the new material is deposited on the surface of the cartilage.

BONE:

All bone is vitally active tissue that like cartilage is formed by specialized cells into a cell / matrix arrangement. The cells are known as osteocytes. While bone can be remarkably strong it also possesses vigorous regenerative ability. Initially osteoblasts generate a matrix that then becomes mineralized and entombs the osteoblasts. At that point, the osteoblasts become osteocytes and function to support the bone around them. Bone has an additional cell type, osteoclasts that erode the mineralized matrix of bone. The balance of osteoblast and osteoclast activity contributes to control of the amount of mineralized bone in a process known as remodeling.

Bone is exquisitely sensitive to external forces. When chronic pressure is directly applied to bone it responds by to eroding to minimize the pressure. This can be illustrated by the presence of the networks of shallow grooves seen on the internal surface of the lateral skull formed by meningeal vessels trapped between the dura and the inner table of the skull. Conversely, when a void or defect is created, osteoblasts respond by depositing new bone to fill the defect.

Bone also responds to forces of tension (pulling) by deposition of additional material. This is particularly evident at the bone-muscle interface. Tendons are connective tissue that functions to attach muscle to bone. The greatest tension and thus a need for thicker bone occurs at the periphery of the attachment. Conversely the tension at the central region of the connection is smaller and the requirement for bone thickness is also less. A good example of this can be seen in the wings of the ilium where so many large postural muscles attach producing a thick, heavy margin and thinner flat central pan. The lengthy insertion of the adductor muscles and the deep fascia of the thigh on the femur produces a prominent ridge of thickened bone called linea aspera. On the lateral surface of the skull, the thickened superior and inferior temporal lines of the parietal and frontal bones indicate the origin of the temporalis.

Grossly bones are classified into two basic shape classes: long bones or flat. At the macroscopic level, bone is deposited in two styles: lamellar (compact) and cancellous (spongy or trabecular) bone. Compact bone forms the strong outer cortical layer of most bones, while cancellous bone occupies central cavities of bone. This arrangement, a strong shell of compact bone enclosing a space filled by trabecular bone, is a fundamental arrangement resulting in reduction of overall mass of the bones and conferring a tremendous strength and resistance to external forces. The spaces of trabecular bone are filled by marrow. In some bone of the skull, these internal spaces are enlarged and lined by respiratory epithelium (paranasal air sinuses) and are contiguous with the nasal cavity.

Trabecular bone is structurally critical to the overall strength of bones particularly for those that withstand constant impact or pressure like the long bones of the lower extremity. Impact forces transmitted up the shaft of the bone are redistributed along the cortex and the cancellous trabeculae. This arrangement spreads the stress of weight bearing or impact along the length of the bone and helps relieve the magnitude of damaging forces.

Bone development takes place in two basic formats. Endochondral bone formation occurs when a cartilage precursor is invaded by osteoblasts and converted to bone. Intramembranous bone formation occurs when mesenchyme differentiates into bone forming regions within membranes that occur at the site of bone formation. Some of the bones forming the base of the cranium arise from a mix of the two formats.