These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and are involved in the transmission of nerve impulses. Bones also serve as a site for fat storage and blood cell production.
The unique connective tissue that fills the interior of most bones is referred to as bone marrow. There are two types of bone marrow: yellow bone marrow and red bone marrow. Yellow bone marrow contains adipose tissue, and the triglycerides stored in the adipocytes of this tissue can be released to serve as a source of energy for other tissues of the body. Red blood cells, white blood cells, and platelets are all produced in the red bone marrow. As we age, the distribution of red and yellow bone marrow changes as seen in the figure Figure 6.
An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery Figure 6. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.
Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures scoliosis can be severe enough to slip under the shoulder blade scapula forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally kyphosis causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists.
As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.
Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery. Long, short and irregular bones develop from an initial model of hyaline cartilage cartilage models. Once the cartilage model has been formed, the osteoblasts gradually replace the cartilage with bone matrix through endochondral ossification Robson and Syndercombe Court, Mineralisation starts at the centre of the cartilage structure, which is known as the primary ossification centre.
Secondary ossification centres also form at the epiphyses epiphyseal growth plates Danning, The epiphyseal growth plate is composed of hyaline cartilage and has four regions Fig 3 :.
Resting or quiescent zone — situated closest to the epiphysis, this is composed of small scattered chondrocytes with a low proliferation rate and anchors the growth plate to the epiphysis;. Growth or proliferation zone — this area has larger chondrocytes, arranged like stacks of coins, which divide and are responsible for the longitudinal growth of the bone;.
Hypertrophic zone — this consists of large maturing chondrocytes, which migrate towards the metaphysis. There is no new growth at this layer;.
Calcification zone — this final zone of the growth plate is only a few cells thick. Bones are not fully developed at birth, and continue to form until skeletal maturity is reached. In rare cases, a genetic mutation can disrupt cartilage development, and therefore the development of bone.
This can result in reduced growth and short stature and is known as achondroplasia. The human growth hormone somatotropin is the main stimulus for growth at the epiphyseal growth plates. During puberty, levels of sex hormones oestrogen and testosterone increase, which stops cell division within the growth plate.
As the chondrocytes in the proliferation zone stop dividing, the growth plate thins and eventually calcifies, and longitudinal bone growth stops Ralston and McInnes, Males are on average taller than females because male puberty tends to occur later, so male bones have more time to grow Waugh and Grant, Over-secretion of human growth hormone during childhood can produce gigantism, whereby the person is taller and heavier than usually expected, while over-secretion in adults results in a condition called acromegaly.
If there is a fracture in the epiphyseal growth plate while bones are still growing, this can subsequently inhibit bone growth, resulting in reduced bone formation and the bone being shorter. It may also cause misalignment of the joint surfaces and cause a predisposition to developing secondary arthritis later in life. A discrepancy in leg length can lead to pelvic obliquity, with subsequent scoliosis caused by trying to compensate for the difference. Once bone has formed and matured, it undergoes constant remodelling by osteoclasts and osteoblasts, whereby old bone tissue is replaced by new bone tissue Fig 4.
Bone remodelling has several functions, including mobilisation of calcium and other minerals from the skeletal tissue to maintain serum homoeostasis, replacing old tissue and repairing damaged bone, as well as helping the body adapt to different forces, loads and stress applied to the skeleton. Calcium plays a significant role in the body and is required for muscle contraction, nerve conduction, cell division and blood coagulation.
Serum calcium levels are tightly regulated by two hormones, which work antagonistically to maintain homoeostasis. Calcitonin facilitates the deposition of calcium to bone, lowering the serum levels, whereas the parathyroid hormone stimulates the release of calcium from bone, raising the serum calcium levels.
Osteoclasts are large multinucleated cells typically found at sites where there is active bone growth, repair or remodelling, such as around the periosteum, within the endosteum and in the removal of calluses formed during fracture healing Waugh and Grant, The osteoclast cell membrane has numerous folds that face the surface of the bone and osteoclasts break down bone tissue by secreting lysosomal enzymes and acids into the space between the ruffled membrane Robson and Syndercombe Court, These enzymes dissolve the minerals and some of the bone matrix.
The minerals are released from the bone matrix into the extracellular space and the rest of the matrix is phagocytosed and metabolised in the cytoplasm of the osteoclasts Bartl and Bartl, Once the area of bone has been resorbed, the osteoclasts move on, while the osteoblasts move in to rebuild the bone matrix. Osteoblasts synthesise collagen fibres and other organic components that make up the bone matrix.
They also secrete alkaline phosphatase, which initiates calcification through the deposit of calcium and other minerals around the matrix Robson and Syndercombe Court, As the osteoblasts deposit new bone tissue around themselves, they become trapped in pockets of bone called lacunae. Once this happens, the cells differentiate into osteocytes, which are mature bone cells that no longer secrete bone matrix.
The remodelling process is achieved through the balanced activity of osteoclasts and osteoblasts. If bone is built without the appropriate balance of osteocytes, it results in abnormally thick bone or bony spurs.
Conversely, too much tissue loss or calcium depletion can lead to fragile bone that is more susceptible to fracture. Typical features on X-ray include focal patches of lysis or sclerosis, cortical thickening, disorganised trabeculae and trabecular thickening.
As the body ages, bone may lose some of its strength and elasticity, making it more susceptible to fracture. This is due to the loss of mineral in the matrix and a reduction in the flexibility of the collagen. Adequate intake of vitamins and minerals is essential for optimum bone formation and ongoing bone health. Two of the most important are calcium and vitamin D, but many others are needed to keep bones strong and healthy Box 2.
Key nutritional requirements for bone health include minerals such as calcium and phosphorus, as well as smaller qualities of fluoride, manganese, and iron Robson and Syndercombe Court, Calcium, phosphorus and vitamin D are essential for effective bone mineralisation.
Vitamin D promotes calcium absorption in the intestines, and deficiency in calcium or vitamin D can predispose an individual to ineffective mineralisation and increased risk of developing conditions such as osteoporosis and osteomalacia.
Other key vitamins for healthy bones include vitamin A for osteoblast function and vitamin C for collagen synthesis Waugh and Grant, Physical exercise, in particular weight-bearing exercise, is important in maintaining or increasing bone mineral density and the overall quality and strength of the bone. This is because osteoblasts are stimulated by load-bearing exercise and so bones subjected to mechanical stresses undergo a higher rate of bone remodelling.
Reduced skeletal loading is associated with an increased risk of developing osteoporosis Robson and Syndercombe Court, When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously. Hematopoiesis , the formation of blood cells, mostly takes place in the red marrow of the bones. In infants, red marrow is found in the bone cavities. With age, it is largely replaced by yellow marrow for fat storage.
In adults, red marrow is limited to the spongy bone in the skull , ribs, sternum , clavicles, vertebrae and pelvis. Red marrow functions in the formation of red blood cells, white blood cells and blood platelets.
0コメント