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Bone Function, Development, and Growth

Factors Affecting Bone Growth and Remodeling

Hormones, physical activity, and nutrition influence how bones grow and are formed.

A hormone is a chemical compound that signals growth and development and is secreted from endocrine glands. Bone growth, up to the point of late puberty, is directly influenced by several of these chemicals. Growth hormone, for example, is the primary stimulus that causes bones to increase in length and width. It is produced by the anterior pituitary gland, which is found in the brain, and starts being produced during infancy. Other hormones include the sex hormones testosterone and estrogen, as well as calcitonin and parathyroid hormone, which are responsible for monitoring and regulating blood calcium levels.

Correct levels of calcium are essential to maintaining the body. Having the correct amount of calcium in the blood is essential for producing the membrane potentials involved with transmission of nervous signals across neurons. Without this, muscles would be unable to contract because there would be no rapid influx of ions across the cell membranes. Additionally, the remodeling of bone also requires strict regulation of calcium in the blood. Bones serve as storage areas for calcium, to be released when needed. When blood calcium levels drop too low, parathyroid hormone (PTH) is released from the parathyroid glands. This hormone activates reabsorption, the process by which osteoclasts break down bone tissue, releasing calcium into the blood. If blood levels of calcium are too high, the thyroid gland releases calcitonin, a hormone that activates osteoblasts to remove calcium from the blood and deposit it into newly formed bone tissue. The normal range of calcium in the blood is 9 to 11 mg per 100 mL. Both calcitonin and parathyroid hormone work to maintain these levels.

Bone-Blood Calcium Homeostasis

Normal calcium blood levels are maintained through strict regulation by the thyroid and parathyroid glands.

Puberty and Bone Growth

During puberty, growth hormone levels increase, thereby increasing the rate of growth.

Puberty is a time when there are great changes in hormone levels within the human body. The sex hormones, testosterone and estrogen, are released into the bloodstream and target various organs to begin the changes necessary to reach sexual maturity. While testosterone is the male hormone and estrogen is the female hormone, both sexes produce certain amounts of both. In fact, it is the production of estrogen that plays a large role in the growth of bones. Estrogen is so important for bone growth that some testosterone in males converts into estrogen in order to accomplish this.

The typical growth spurt exhibited by teenagers is the result of an increase in the volume of estrogen. The levels of this hormone continue to increase over time, leading to the ultimate closure of the epiphyseal plate, which is the location where bone growth occurs. This ends the ability of bones to become any longer. Estrogen and testosterone are also responsible for the masculinization or feminization of bone structure. For example, the widening of the female hip bones during puberty is due to a thickening and lengthening of the pelvic bones. Testosterone in males is responsible for broadening the shoulders and widening the jaw bone during puberty.

If growth hormone, testosterone, or estrogen fall outside of the normal range, skeletal growth can be affected. For example, there is a condition called gigantism, which results from an excess of growth hormone being produced in childhood, before the epiphyseal plates close. A child with gigantism grows into an adult with a much larger skeleton than normal. If excess growth hormone is produced in adulthood, after the bones have stopped growing, the result is a condition called acromegaly. A person with acromegaly has exaggerated body parts, such as very large hands and feet, as well as prominent facial features. On the other hand, a deficiency of growth hormone could result in the child developing dwarfism, being shorter in height and having body parts that are reduced in size.

Bones and Physical Activity

Bones respond to mechanical stress by increasing in size and thickness.

Running, weight lifting, walking, and even the force of gravity can put additional stress on the bones. As a child learns to crawl and ultimately stand upright, the force of gravity pulls down on the skeleton, permanently altering its shape. For example, humans are born with a spinal cord that is a relatively straight line. Once the baby starts lifting its head and moves onto all fours, the bones of the spine start to curve upward. Additionally, when the child starts to walk, gravity pulls down on the spine, giving it its characteristic S shape.

Bones accommodate for the stresses they normally encounter. The bones of the skeleton are perfectly capable of supporting a person's weight. Wolff's law (named after German anatomist and surgeon Julius Wolff) states that bones will grow or remodel themselves in response to additional stresses, such as gravity or additional load. As more load is added to the bone (perhaps due to weight gain), the bones tend to bend toward one side. This bending leads to more tension and the possibility of a fracture on the other side of the bone.

To protect themselves from breaking under these conditions, bones are thickest in the tissue of the diaphysis. The diaphysis is the central part of a long bone between the two epiphyses at each end. The shaft walls of the diaphysis are made of hard compact bone and are thickest toward the middle of the shaft, where the amount of stress is at its greatest. To encourage this thickening, bones under stress release electrical signals into the canaliculi. This stimulus is detected by osteocytes in the area of the stress. Additional signals are produced by the osteocytes that stimulate the production of additional bone.

Wolff's law can also be used to explain several other phenomena that occur within the skeletal system:

  • People who are left-handed have thicker bones in their left arms than people who are right-handed.
  • Bones with a C curvature or an S curvature are thickest at the place where they may break.
  • Weight lifters have very thick bones at the points of attachment of their muscles because of the increased use of the muscles.
  • People who are bedridden and newborn babies have very thin bones.

Bone Reformation Due to Stress

When exposed to compressive forces, bones gradually remodel themselves to better withstand these forces over time.

Bones and Nutrition

Deficiencies in certain vitamins and minerals can affect bone growth and health.

Like other parts of the body, bones are very susceptible to damage and developmental abnormalities due to lack of proper nutrition. Bones need a supply of vitamins and minerals in order to grow and repair themselves properly. Many times, vitamin and mineral deficiencies result in abnormal deposition or reabsorption of bone. This leads to several serious bone diseases that can affect a person's health.

Rickets is a bone disease that is caused by a deficiency of vitamin D and calcium in the diet. It is more common in children than adults because the lack of vitamin D impacts the growth and hardening of the bones as the child grows. Common symptoms of rickets include bowed legs (the legs have an outer curvature of the bones producing a large space between the knees) and deformed skull, rib, and pelvic bones. The epiphyseal plate fails to harden, which causes the bones to keep growing and get farther apart. To combat rickets, it is suggested children (or adults) increase their intake of calcium through dairy products or supplements. Additionally, spending time in direct sunlight, or taking vitamin D supplements, will also increase the amount of vitamin D produced. This additional vitamin D will increase the rate at which calcium is absorbed into the body.

Effect of Rickets on Bone Development

Deficiency of vitamin D and calcium causes abnormal bone growth and development. This results in more brittle bones that have a higher tendency to fracture.
Another bone disease caused by abnormal bone deposition and reabsorption is osteoporosis, which occurs when bone reabsorption occurs at a faster rate than bone deposition. As a result, the breakdown of bones forms large cavities within the compact tissue, making the bones very brittle and quite likely to break. The composition of the bone remains the same, but the mass and density of the bone decreases. The most common population experiencing osteoporosis is postmenopausal women over 50 years of age, although men can also experience the symptoms.

There are several factors that contribute to a person's chances of getting osteoporosis. These include:

  • low calcium intake in the diet
  • genetics
  • decreased sex hormones
  • smoking
  • diabetes mellitus
  • lack of exercise

It is recommended that people who are susceptible to osteoporosis get a bone density test to determine the extent to which their bones have degraded. Also, removing factors such as smoking from one's lifestyle can help. Doctors recommend that people with osteoporosis exercise regularly and consume a diet that contains sufficient calcium. Osteoporosis is not totally preventable, but it can be delayed and treated.