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Bones
Bones provide the framework and internal core structure for the attachment of muscles. Bone is a living rigid tissue which forms the support structures for the rest of the body. The process of bone formation is called ossification. The matrix of bone contains a dense arrangement of collagen fibres together with mineral salts of calcium, magnesium and phosphates. The calcium salts give bone its hardness and rigidity while collagen fibres give bones its flexibility and strength.
Functions of bone
- To serve as a firm support framework for the whole body.
- To protect such delicate structures as the brain and spinal cord.
- To serve as levers, working with attached muscles to produce movement.
- To serve as a storehouse for calcium salts, which may be reabsorbed into the blood if there is not enough calcium in the diet.
- To produce blood cells in the red marrow.
Microscopic structure of a long bone
Bones are made up of numerous hollow tunnels called Haversian canals. Haversian canals occur within the matrix of bone tissue and run parallel to the length of the bone. Each Haversian canal consists of a nerve to carry impulses, blood vessels to transport gases, food and wastes and a lymph duct to drain tissue fluid. Under the microscope they appear as black circles against a lighter background.
Each Haversian canal is surrounded by concentric rings of compact bone called lamellae. Each of these layers contains a ring of fluid-filled cavities called lacunae. Each of these lacuna will contain a number of bone cells called osteocytes. The lacunae are linked to each other and to the Haversian canal by a system of very tiny interconnecting canals called canaliculi. Strands of cytoplasm extend through these canals which supply the osteocytes with oxygen and nutrients and remove waste products. The Haversian canals, lacunae, osteocytes and canaliculi together form a unit called an Haversian System and a number of these systems make up compact bones.
Apart from osteocytes which are embedded in the lacunae of bone there are two other types of bone cells.
Osteoblasts: Bone forming cells. These cells allow the bone to change and remodel its shape as the organism grows and responds to stresses. If a bone is broken or if strengthening is needed, bone cells lay down new tissue and repair damaged tissue
Osteoclasts: Special bone cells for destroying and reabsorbing bone tissue.
Structure of long bone
Although there are many different types of bones in the skeleton, we will discuss the different parts of a specific type of bone: the long bone. The femur, tibia and fibula in the leg, and the humerus, radius and ulna in the arm are all examples of long bones.
Epiphysis: The head of each end of a long bone consists largely of spongy bone and is covered with hyaline cartilage.
Spongy bone: Found in the epiphysis of long bones and contains red marrow.
Red bone marrow: Found in the spaces between the trabeculae in spongy bone. This is where the red blood cells are made at the rate of 2 -3 million per second. White blood cell types are also produced here.
Trabeculae: The struts in the network of irregular bony plates in the epiphysis of bones which transfer stresses from the epiphysis to the diaphysis which has a much thicker layer of compact bone and resists stress better.
Diaphysis: Cylindrical shaft of a long bone composed of hard compact bone on the outside.
Periosteum: The membrane of dense fibrous connective tissue which surrounds the outside surface of the shaft of a long bone. It has blood vessels which enables it to nourish the bone and repair injuries. It also provides a surface for the attachment of muscles by means of tendons and ligaments.
Endosteum: The delicate connective tissue layer lining the inside surface of compact bone.
Marrow cavity: This is filled with yellow marrow which consists largely of fat.
