Bone

Bone tissue occurs in the different bones of the skeleton. Bone is a hard and rigid tissue. Like cartilage, bone consists of living cells with large amounts of ground substance or matrix. It is impregnated with organic salts such as calcium carbonate (7%) and calcium phosphate (85%). Small amounts of sodium and magnesium is also present. In addition to this, the matrix contains numerous collagenous fibres and a large amount of water. Collagen fibres together with the bone cells constitute the organic (living) matter in bone tissue. There are different groups of bone in the skeleton, inter alia long bones such as the humerus and femur.

Structure of a Bone

A long bone such as the femur, consists of a centre piece, the shaft (diaphysis) and a thickened head (epiphysis) at each end. The heads articulate with other bones in the joints and are covered with a thin layer of hyaline cartilage. The remainder of the bone is covered with a tough, strong membrane, the periosteum which is richly supplied with blood vessels. There is a small artery which penetrates the shaft near the centre to supply the bone tissue with blood. Beneath the periosteum is a layer of compact bone which is thicker in the shaft than in the two heads. The shaft encloses a hollow, the marrow cavity, which is lined with a thin soft membrane known as the endosteum. The marrow cavity contains a soft tissue richly supplied with fat cells and blood corpuscles, the yellow marrow. The epiphysis of a long bone consist of spongy (or cancellous) bone covered with a thin layer of compact bone. This is made up of bony bars (or trabeculae) arranged in such a way that they are able to resist any force which a applied upon the bone. Between the bars are many tiny cavities filled with a red marrow which contains numerous red blood corpuscles in different stages of development.

Microscopic Structure of Compact Bone

Under the microscope dense, compact bone shows a definite and a characteristic pattern of arrangement. The ground substance of bone is arranged in concentrated layers (lamellae) round the small canals which run parallel to the long axis (shaft) of the bone. These canals, called Haversian canals, are interconnected with one another via Volkmann's canals and contain a blood vessel, a nerve and a lymph vessel. Each Haversian canal is surrounded by concentric layers of bone matrix (called lamallae) and concentric rings of bone forming cells (osteoblasts). Bone cells remain alive and once they have completely surrounded by the hard bone matrix, they are called osteocytes. The osteocytes are embedded in fluid-filled cavities within the concentric lamellae. These cavities are known as lacunae and occur at regular intervals in these concentric layers of bone tissue. The lacunae are connected to one another and to the Haversian canals by a system of interconnecting canals known as canaliculi. Each Haversian canal, its concentric lamellae, lacunae with osteocytes and canaliculi forms a long cylinder and is called a Haversian system. Separate Haversian systems are joined to each other by means of interstitial lamellae.

Growth of Bone Tissue

In a child a long bone has a layer of cartilage between the head (epiphysis) and the shaft (diaphysis). The cartilage grows actively which causes an increase in the length of the bone. The layer does not thicken since the edges (on both sides) are constantly replaced by bone (become ossified). The bone grows in the length until the child reaches its adult size. The cartilage then also ossifies and disappears. At the same time the bone increases in thickness as a result of the formation of bone tissue immediately beneath the periosteum. The innermost layer, nearest to the marrow cavity, are constantly absorbed, which enlarges the size of the marrow cavity.

Functions of Bone Tissue

Structure of long bone with enlargement of a section of compact bone.

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