Skeletal tissue

Skeletal tissues are modified connective tissues, whereby the cells and fibres have a particular organization which becomes condensed so that the tissue is rigid.

Cartilage

Cartilage is supplementary to bone, being formed wherever strength, rigidity and some elasticity are required. In fetal development, cartilage is often a temporary tissue being later replaced by bone. However, in many places cartilage persists throughout life. Although a rigid tissue, cartilage is not as hard or strong as bone. It is also relatively non-vascular being nourished by tissue fluids. A vascular invasion of cartilage often results in the death of the cells during the process of ossification of the cartilage and its eventual replacement by bone. Except for the articular cartilage of synovial joints, cartilage possesses a fibrous covering layer, the perichondrium.

There are three main types of cartilage: hyaline cartilage, white fibrocartilage and yellow fibrocartilage.

Hyaline cartilage

This forms the temporary skeleton of the fetus from which many bones develop. Its remnants can be seen as the articular cartilages of synovial joints, the epiphyseal growth plates between parts of an ossifying bone during growth, and the costal cartilages of the ribs. At joint surfaces it provides a certain degree of elasticity to offset and absorb shocks, as well as providing a relatively smooth surface permitting free movement to occur. With increasing age, hyaline cartilage tends to become calcified and sometimes ossified.

White fibrocartilage

White fibrocartilage contains bundles of white fibrous tissue which give it great tensile strength combined with some elasticity so that it is able to resist considerable pressure. It is found at many sites within the musculoskeletal system: 1) within the intervertebral discs between adjacent vertebrae; 2) in the menisci of the knee joint; 3) in the labrum deepening the glenoid fossa of the shoulder joint and the acetabulum of the hip joint; 4) in the articular discs of the wrist, sternoclavicular, acromioclavicular and temporomandibular joints, and 5) as the articular covering of bones which ossify in membrane, e.g. the clavicle and mandible.

White fibrocartilage may calcify and ossify.

Yellow fibrocartilage

Yellow fibrocartilage contains bundles of elastic fibres with little of no white fibrous tissue. It does not calcify or ossify, and is not found within the musculoskeletal system.

Bone

Bone is extremely hard with a certain amount of resilience. It is essentially an organic matrix of fibrous connective tissue impregnated with mineral salts. The connective tissue gives the bone its toughness and elasticity, while the mineral salts provide hardness and rigidity, the two being skillfully blended together. It must be remembered that the mineral component provides a ready store of calcium, which is continuously exchanged with that in body fluids, with the rate of exchange and overall balance of these mineral ions being influenced by several factors including hormones.

Each bone is enclosed in a dense of fibrous tissue, the periosteum, with its form and structure adapted to the function of support and the resistance of mechanical stresses. Being a living tissue, bone is continually being remodeled to meet these demands; this is particulary so during growth. The structure of any bone cannot be satisfactorily considered in isolation, for it is dependent upon its relationship to adjacent bones and the type of articulation between them, as well as the attachment of muscles, tendons and ligaments to it.

The internal architecture of bone reveals systems of trabeculae running in many directions(picture below), arranged to resist compressive, tensile and shearing stresses. Surrounding these trabecular systems, which tend to be found at the ends of long bones, is a thin layer of condensed or compact bone(picture below). The network of the trabeculae, because of its appearance, is known as cancellous or spongy bone. In the region of the shaft of a long bone is an outer, relatively thick ring of compact bone surrounding a cavity, which in life contains bone marrow.

Red and white blood cells are formed in red bone marrow, which after birth is the only source of red blood cells and the main source of white blood cells. In infants, the cavities of all of the bones contain red marrow. However, this gradually becomes replaced by yellow fat marrow, so that at puberty red marrow is only found in the cavities associated with cancellous bone. With increasing age many of these regions containing red marrow are replaced by yellow marrow. Nevertheless, red marrow tends to persist throughout life in the vertebrae, the ribs and sternum, and the proximal ends of the femur and humerus.

For descriptive purposes bones can be classified according to their shape:

1. Long bones are found within the limbs, and consist of a shaft(diaphysis) and two expanded ends(epiphyses).
2. Short bones are the bones of the wrist and part of the foot, the carpal and tarsal bones respectively.
3. Flat bones are thin and tend to be curved in spite of their classification; they include the bones of the skull vault and the ribs. Structurally, they consist of two layers of compact bone enclosing cancellous bone.
4. Irregular bones are those which fit none of the above categories, and include the vertebrae and many of the bones of the skull and face.

Bone development

Bone develops either directly in mesoderm by the deposition of mineral salts, or in a previously formed cartilage model. When the process of calcification and then ossification takes place without an intervening cartilage model, the process is known as intramembranous ossification, with the bone being referred to as membrane bone. However, if there is an intervening cartilage model, the process is known as endochondral ossification, with the bone being referred to as cartilage bone. This latter process is by far the most common.

Intramembranous ossification

The site of bone formation is initially indicated by a condensation of cells and collagen fibres accompanied by the laying down of organic bone matrix, which becomes impregnated with mineral salts. The formation of new bone continues in a similar manner to bone developed in cartilage. Intramembranous ossification occurs in certain bones of the skull, the mandible and the clavicle.

Endochondral ossification

Again the first step in the process is the accumulation of mesodermal cells in the region where the bone is to develop. A cartilage model of the future bone develops from these mesodermal cells(figure a). In long bones the cartilage model grows principally at its ends, so that the oldes part of the model is near the middle. As time progresses, the cartilage matrix in this older region is impregnated with lime salts so that it becomes calcified. Consequently, the cartilage cells, being cut off from their nutrient supply, die. The greater part of the calcified cartilage is subsequently removed and bone is formed around its few remaining spicules(figure a). Ultimatively, the continual process of excavation of calcified cartilage and laying down of bone leads to the complete removal of the calcified cartilage(figure a).

The cartilage at the ends of the bone continues to grow due to the multiplication of its cells. However the deeper layers gradually become calcified and replaced by bone. Therefore the increase in length of a long bone is due to active cartilage at its ends, while an increase in width is by deposition of new bone on that already existing.

When first laid down, bone is cancellous in appearance, having no particular pattern of organization. It is reffered to as woven bone. In the repair of fractures, the newly formed bone also has this woven appearance. However, in response to stresses applied to the bone by muscles, tendons, ligaments and the forces transmitted across joints, the woven bone gradually assumes a specific pattern in response to these stresses.

Growth and remodeling of bone

During growth there is an obvious change in the shape of the bone. However, it should be remembered that even in the adult, bone is being continuously remodelled, principally under the direct control of hormones to stabilize blood calcium levels, but also in response to long-term changes in the force patterns applied to the bone.

Both growth and remodeling depend on the balanced activity of two cell types, one of which removes bone tissue(osteoclasts) and the other which lays down new bone(osteoblasts). In a growing bone, for example, new bone is laid down around the circumference of the shaft in order to increase its diameter. At the same time the deepest layers of bone are being removed, thereby maintaining a reasonable thickness of cortical bone and enlarging the marrow cavity(figure b). Should the combined process of deposition and absorption fail to match, then either a very thick or a very thin shaft results.

Ossification centres

The regions where bone begins to be laid down are known as ossification centres. It is from these centres that the process of ossification spreads. The earliest, and usually the principal, centre of ossification in a bone is referred to as a primary centre. Primary centres of ossification appear at different times in different bones, but are relatively constant between individuals, and also appear in an orderly sequence. The majority of such centres appear between the seventh and twelfth week of intrauterine life. They are virtually all present before birth. In long bones, the primary ossification centre appears in the shaft of the bone(figure c).

Secondary ossification centres appear much later than primary ones, usually after birth, being formed in parts of the cartilage model into which ossification from the primary centre has not spread(figure c). All of the long bones in the body, and many others, have secondary ossification centres. The bone formed from these centres is almost entirely cancellous.

That part of a long bone which ossifies from the primary centre is called the diaphysis, while that from the secondary centre is called an epiphysis. The plate of cartilage between these two regions is where the diaphysis continues to grow in length. Consequently, it is referred to as the epiphyseal growth plate(figure c). When this growth plate disappears the diaphysis and epiphysis become fused and growth of the bone ceases.