Types Of Bones

TYPES OF BONES

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Ossification (bone formation). The skeleton is formed entirely of cartilage in a early embryo. Bone formation occurs later. The process of bone formation is called ossification (os = bone; facio = to make) or osteogenesis. Bones are of the following types according to their source of formation. Cartilaginous or Replacing Bones. These bones develop form the pre-existing cartilage and practically replace the cartilage. They are also called endochondrial bones. Examples: humerus, femur. Investing or Dermal or Membrane Bones. These bones develop in the dermis of the skin as thin plates and sink to get attached over the original cartilaginous endoskeleton. In fact these bones become invested upon original cartilages hence their name. Examples: frontal, nasals, vomers and parietals of the skull. Sesamoid Bones. These bones are formed in the tendons at the joints. Examples: patella (kneecap). Visceral Bones. These are formed in the soft organs (= viscera). Examples: os cordis in the heart of some ruminants (e.g., deer), os penis in the penis of most bats, insectivores, rodents (e.g., rats), carnivores (e.g., dog, walrus), whales, some primates (not man), os clitoris in the clitoris of many carnivores, and os palpebrae in the eyelids of crocodiles. A small bone also develops in the crest of a bird and snout of a hog. • Osteoclasts. These cells are derived from osteoblasts and osteocytes, rich in acid phosphatase and contain slightly basophilic cytoplasm and are lysosome-rich, multinucleate cells which destroy bone matrix. They are also called bone destroying cells. Thus the osteoclasts take part in the bone resorption. When required, calcium and phosphate are released from the bone into the blood under the influence of the hormones, parathormone from the parathyroid glands and calcitonin from the thyroid gland.

DISORDERS OF BONES AND BONE TISSUE Arthritis is of three types – rheumatoid arthritis, osteoarthritis and gouty arthritis. (a) Rheumatoid Arthritis. It is caused by inflammation of synovial membrane caused by bacterial infection, allergy or hormonal imbalance. Diagnosis of the disease is carried out by finding rheumatoid factor which resembles immunoglobulin IgM. Inflammation of synovial membrane leads to its thickening and excessive secretion of synovial fluid. It puts a pressure on the joint and the joint becomes painful. Later the synovial membrane starts secreting and exudates which forms abnormal granules over the membrane. The exudate is called pannus. The granules cause erosion of cartilage surface. The fibrous tissue attached to bones becomes ossified. Movements become extremely painful, ultimately resulting in immobilization of the joint. Heat treatment and physiotherapy are helpful in the early stages. A completely damaged joint is replaced surgically. Staphylococcus bacteria which causes this also effects heart valves (Rheumatic heart disease) (b) Osteoarthritis. It is a form of arthritis in which one or many joints undergo degenerative changes like loss of articular cartilage and proliferation of bone and cartilage in the joint. It is very common in old age, where degeneration of hyaline cap by the regular use and deposition of calcium makes the surface hard. This causes one bone to move over the other bone and lead to pain in joints. Osteoarthiritis is common in the joints of hands, knees and spine.

(c) Gouty Arthrititis or Gout. It is a disease associated with an inborn error of uric acid metabolism that increases its production or interferes with uric acid excretion. Thus, uric acid concentration increases in blood. Excess uric acid is converted into monosodium urate crystals that precipitate from the blood and become deposited in joints and other tissues. Men are more often affected than women. Fracture. It is a break in a compact bone due to mechanical injury. It is very rare in children because their bones have more organic matter and are quite flexible. Fracture is common in old age because of less organic matter and more inorganic matter, whereby their bones becomes hard and brittle. Fracture is of following types : (i) Simple. Bone is completely broken into two parts. (ii) Compound. Bone completely breaks into two parts and the fractured pieces pierce into the skin. (iii) Comminuted. When a bone breaks down into more than two pieces, it is also called multiple or comminuted fracture.

(iv) Evulsive. A chip breaks out but remains attached to ligament. (v) Green Stick Fracture. When a bone undergoes simple crack but the bone remains intact. It is vary common in children, having high recovering power.

(B) On the basis of structure or shape. (i) Long Bones. These bones are long in size, e.g., humerus, femur, radius, tibia, fibula. (ii) Short Bones. These bones are short in size e.g., carpals, metacarpals and tarsals. (iii) Flat Bones. Bones are dorso-ventrally flattened , e.g., scapula, sternum, cranium, (iv) Irregular Bones. Irregular shape of bones, e.g., vertebrae, carpals and tarsals.