Bone in general is a form of connective tissue that has a cellular component as well as a matrix. The only difference between bone and other normal connective tissue is by the fact that its matrix is mineralized with extremely hard solid tissue. The bone is formed from calcium salts, especially calcium phosphate and the mineral is called hydroxyapatite. The matrix of bones contain in addition to calcium salts also several types of the protein collagen which can give the bone a form of elasticity.
The bone of humans have several functions. One of them is a protective function which can protect vulnerable organs of the body from injury. For example, the heart and lungs in humans which are extremely important and vulnerable organs are protected against trauma by the rib cage.
Also the brain tissue which has a vital role in the body functions is protected against trauma or injury by the skull or cranial bone. A surgery to the brain usually requires removal of part of the skull. In addition the spinal cord which is also of importance as equal as that of the brain especially in maintaining reflexes and voluntary movement for the body is protected from injury by the bones of the vertebral column.
In addition to its role in giving protection the bone has also a supportive role in which for example we could not be able to stand if we did not have the skeletal system of bones. Therefore bones have a supportive function in which it permits movement of the body through the different types of joints in the body such as the synovial joint of the knee and the joints of the vertebral column.
The bones in the body provide attachment points for the skeletal muscles which in turn can permit movement of the body parts such as the hands and legs. In addition, bone has an important physiological role that pertains to calcium homeostasis in the body. Bones of the body form a hudge reservoir for calcium minerals which its deposition and resorption are regulated by two important hormones in the body.
These two hormones are called parathyroid hormone and calcitonin. Parathyroid hormone is secreted in physiological doses from the parathyroid gland in response to low level of calcium in the blood such as occurs in chronic renal failure. This hormone functions by stimulating one type of bone cells and which is called osteoclasts to release calcium ions to the blood from the bone tissue.
Osteoclasts are one type of four bone cells that have physiological role in bone metabolism. Osteoclasts are also phagocytic cells that function also by stimulating bone tissue to release calcium to the blood circulation. Osteoclasts respond to hypocalcemic state through the parathyroid hormone.
The other hormone which has effect on the bone calcium is called calcitonin. This hormone is usually secreted by the parafollicular cells of the thyroid gland. It is usually secreted in response to high level of calcium ions in the blood. Calcitonin is an antagonist to parathyroid hormone functioning exactly in the opposite direction.
Calcitonin functions by stimulating the other cell type of bones and which is called osteoblasts. These cells usually function by stimulating the deposition of calcium salts on bone tissue. It also functions by forming the bone matrix. Once the matrix is formed these cells are referred to as osteocytes by a differentiation process. Osteoblasts are obtained in a diferentiation process from mother cells that are called osteogenic cells.
The other function of bones is blood cells formation. bone contains the important red bone marrow which is the site of formation of red blood cells precursors in a process that is called hemopoiesis. The last function of the bone is to store lipids in the form of triglycerides. They are contained in the yellow bone marrow and are a potential source of energy.
