Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering
Bones are an important part of the human body, and their development and growth is a carefully regulated process that depends on the interactions of various cells, hormones, and vitamins. In this lesson, we will learn more about how bones grow and the disorders than can affect bone growth.
Overview of Bone Growth
Bones are a very important part of our bodies. They allow us to move, protect our internal organs, produce blood cells, and support all of the other tissues of the body. Bone begins to form before we are born, and bones continue to grow throughout childhood and adolescence. Several hormones and vitamins, including growth hormone and vitamin D, are critical for bone growth. In this lesson, you will learn more about how bones form and grow and what factors affect that process.
Early Bone Formation Before Birth
Bone formation begins very early in development, at about the third week of fetal life. Cartilage is produced in the shape of the future bone, and then osteoblasts, the cells that produce new bone, invade the cartilage and start to produce a substance called osteoid. Osteoid is made primarily of collagen and contains sites where calcium phosphate crystals will form. Calcium phosphate is the hard mineral that gives bone its strength. This process begins in the middle of the newly forming bone, at a site called the primary ossification center. As the bone develops, more bone is formed from the center outward towards the ends of the bone. Then, blood vessels grow into the bone, and it is reshaped by cells called osteoclasts so that there is a central cavity filled with bone marrow. At birth, the ends of bones are still mostly cartilage. The area at the end of all long bones, called the growth plate or secondary ossification center, allows more bone to be produced, causing the bone lengthen throughout childhood.
Bone Growth in Childhood
As children grow, their bones need to get longer. This happens at the ends of the bones at the growth plate. As the bones grow, cells in the growth plate first produce more cartilage to extend the bone and then osteoblasts come in and convert the cartilage to bone in the same way that they do when bones are first formed before birth. In this way, bones continue to grow longer and longer. Bones can continue to grow until about the age 16 to 20, when the growth plate turns completely to bone and no new cartilage can be added at the ends of the bones. When this happens, we say that the growth plate has closed. Damage to the growth plate by trauma or certain illnesses can cause the growth plate to close prematurely. This can cause one leg or arm to grow longer than the other.
Hormones Regulating Bone Growth
Regulation of bone growth in children is complex and mediated by the action of several hormones. The most important is growth hormone. Produced in the pituitary gland, growth hormone stimulates the production of new cartilage in the growth plates and causes the bones to grow longer. Damage to the pituitary gland and several genetic diseases, such as Turner syndrome and Prader-Willi Syndrome, can cause growth hormone deficiencies and lead to failure of the bones to grow to normal lengths. Growth hormone deficiency in childhood is called pituitary dwarfism, and if not treated, can result in children failing to grow to normal heights. A disorder called acromegaly is characterized by an overproduction of growth hormone in adults. This causes certain bones to continue to grow past puberty and leads to gigantism. Patients with acromegaly typically have enlarged hands and feet and protruding jawbones, as these are the bones that often continue to grow the most.
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The sex hormones, estrogen in girls and testosterone in boys, also play a large role in stimulating bone growth. Increases in these hormones at puberty are responsible for the growth spurt that typically occurs during adolescence, and these hormones are also responsible for closing the growth plates at the beginning of adulthood.
Nutrition, Exercise, and Bone Growth
Many vitamins and minerals are necessary for normal bone growth. Of these, the most important is calcium. The hard tissue that forms bones is constructed primarily of calcium phosphate. Without adequate dietary intake of calcium, bones will be weak and more easily fractured. In addition, vitamin D is necessary for calcium to be properly absorbed in the intestines. Vitamin D is found naturally in eggs and is added to milk and other dairy products. Vitamin D is also produced by skin through exposure to ultraviolet light. Deficiencies in calcium and vitamin D lead to a disorder called rickets in children and osteomalacia in adults. In children with rickets, the bones become so weak that the leg bones start to curve because they can no longer support the weight of the body.
Vitamins A and C are also necessary for normal bone development and growth. Vitamin A is needed by osteoblasts and osteoclasts to produce and remodel new bone, and vitamin C is required for collagen synthesis, which occurs as bones grow. Without adequate vitamin C, osteoblasts cannot produce enough collagen, causing bones to become abnormally fragile and thin.
Physical stress on bones also stimulates them to grow and to become stronger. With lack of exercise, the bone tissue shrinks and becomes more fragile and weak. This is why the bones of athletes are usually stronger than those of sedentary people. It is also why astronauts experience a 1% loss of their bone mass every month they spend in space.
Bone forms from a cartilage model that slowly is converted to hard tissue by osteoblast cells. This begins in the center of bones during fetal life and continues at the growth plates located at the ends of bones throughout childhood. Growth hormone is the primary hormone required for proper bone growth, and calcium, and vitamins A, C, and D, are necessary dietary nutrients. Growth hormone deficiency causes pituitary dwarfism, and vitamin D deficiency causes rickets and osteomalacia.
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