The Basics Of Bone In Health And Disease - NCBI Bookshelf - NCBI

Key Messages

• The bony skeleton is a remarkable organ that serves both a structural function, providing mobility, support, and protection for the body, and a reservoir function, as the storehouse for essential minerals.
• During childhood and adolescence bones are sculpted by a process called modeling, which allows for the formation of new bone at one site and the removal of old bone from another site within the same bone. This process allows individual bones to grow in size and to shift in space.
• Much of the cellular activity in a bone consists of removal and replacement at the same site, a process called remodeling. The remodeling process occurs throughout life and becomes dominant by the time that bone reaches its peak mass (typically by the early 20s). Remodeling continues throughout life so that most of the adult skeleton is replaced about every 10 years.
• Both genes and the environment contribute to bone health. Some elements of bone health are determined largely by genes, and errors in signaling by these genes can result in birth defects. External factors, such as diet and physical activity, are critically important to bone health throughout life, and these factors can be modified.
• The growth of the skeleton, its response to mechanical forces, and its role as a mineral storehouse are all dependent on the proper functioning of a number of systemic or circulating hormones that respond to changes in blood calcium and phosphorus. If calcium or phosphorus are in short supply, the regulating hormones take them out of the bone to serve vital functions in other systems of the body. Too many withdrawals can weaken the bone.
• Many things can interfere with the development of a strong and healthy skeleton. Genetic abnormalities can produce weak, thin bones, or bones that are too dense. Nutritional deficiencies can result in the formation of weak, poorly mineralized bone. Many hormonal disorders can also affect the skeleton. Lack of exercise, immobilization, and smoking can also have negative effects on bone mass and strength.
• Osteoporosis, the most common bone disease, typically does not manifest until late in life, when bone loss begins due to bone breakdown and decreased levels of bone formation. Loss of bone mass leads to the development of structural abnormalities that make the skeleton more fragile.

The purpose of this chapter is to provide an overview of bone biology that will help the reader to understand:

• why humans have bones;
• how bones work;
• how bones change during life;
• what keeps bones healthy;
• what causes bone disease, including the most common form, osteoporosis; and
• the future of bone biology and what it means for preventing and treating bone disease.

While dealing with a subject that is highly technical in nature, this chapter attempts to explain bone biology in terms that a lay person can generally understand. It is intended to provide the reader with the background needed to understand the basis for some of the preventive, diagnostic, and treatment approaches related to bone disease that are discussed in detail later in this report. Those interested in a more detailed review of bone biology and bone disease can consult any of a number of recent texts (Bilezikian et al. 2001, Marcus et al. 2001, Favus 2003).