This is a solid rigid connective tissue. The greater part of the skeletal system in most of the vertebrates is made of bone. Like other connective tissues the bone is also basically made up of matrix and bone cells. The bones may be solid when they are small or they may be having a cavity as in the case of long bones such as humerus and femur.
The substances of the bone are distinguishable into three regions surrounding the central cavity. These three regions are periosteum, matrix and endosteum. The central cavity is usually referred to as the marrow cavity. It consists of the bone marrow. The bone marrow stores the reserve food material and also is the place from where new blood cells are produced. External to the marrow cavity is the endosteum. It consists of white fibrous tissue and contains the bone forming cells which produce new bone material.
Matrix: The matrix of the bone occupies a very large portion. It is very hard and made up of a protein called ossien. Collagen fibres are also found in the matrix but difficult to observe. The matrix is loaded with salts like calcium phosphate, calcium carbonate, sodium carbonate and magnesium sulphate. Of these the major quantity is of calcium phosphate.
The organic and inorganic constituents of the matrix are in the ratio of 30:70. Within the matrix are found a number of layers called lamellae. These are arranged in concentric rings around narrow longitudinal cavities called Haversian canals.
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The Lamellae that surround the haversian canals are called haversian lamellae. Some lamellae however are arranged concentrically around the marrow cavity. These are called circumferential lamellae. Within the lamellae are found small fluid spaces called bone lacunae. These give off fine radiating canals called canaliculi. Each lacuna has a bone cell or an osteocyte.
The canaliculi help in communication between different bone cells. Small protoplasmic strands extend from the bone cells and form a network throughout the matrix. The Haversian canal together with lamellae, lacunae and canaliculi forms a Haversian system. The spaces between the Haversian systems are filled with small interstitial lamellae.
There are some transverse canals which connect the Haversian canals. These are called Volkmann’s canals. Each Volkmann’s canal contains the supply of blood vessels, nerves, a lymph vessel and some bone cells. The blood vessels of the Haversian canal are connected to the blood vessels of periosteum and bone marrow. Nourishment and oxygen to the oestocytes is provided through these blood vessels.
External to the endosteum (lining the inner margin of the matrix) is found a layer of cells called the inner layer of osteoblasts. Similarly the outer margin of the matrix also has a layer of cells called the outer layer of osteoblasts.
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The periosteum is a tough sheath made up of white fibrous tissue enclosing the bone externally. It is to this sheath that tendons of the muscles are attached. The periosteum has blood vessels which send branches into the bone to provide nourishment.
There are some differences between the bone of a frog and that of a mammal. In the case of the frog bone the lamellae are concentrically arranged around the bone marrow cavity where as in the case of mammals the lamellae surround the Haversian cannals.