The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart and lungs , and the pelvis helps protect the bladder, part of the intestines, and in women, the reproductive organs. Bones are made up of a framework of a protein called collagen , with a mineral called calcium phosphate that makes the framework hard and strong.
Bones store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium , directly affect how much calcium is stored in the bones. In this soft bone is where most of the body's blood cells are made.
The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound.
White blood cells help the body fight infection. Bones are fastened to other bones by long, fibrous straps called ligaments pronounced: LIG-uh-mentz. Cartilage pronounced: KAR-tul-ij , a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other. The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone.
These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.
Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:. Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system. Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood.
When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy. The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem.
The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum pronounced: ser-uh-BEL-um. When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract.
The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa. The cerebellum coordinates the muscle movements ordered by the motor cortex. For example, your ribs protect your lungs and heart, the bones of your vertebral column spine protect your spinal cord, and the bones of your cranium skull protect your brain see Figure 6.
On a metabolic level, bone tissue performs several critical functions. For one, the bone tissue acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. 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. 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, Bone is a dynamic structure, which is continually remodelled in response to stresses placed on the body.
Changes to this remodelling process, or inadequate intake of nutrients, can result in changes to bone structure that may predispose the body to increased risk of fracture. Part 2 of this series will review the structure and function of the skeletal system.
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