Bone remodeling

Bone remodeling refers to the ongoing process by which bone changes and replenishes itself. Children's bones grow and change until they reach physical maturity. Once maturity is reached, bone continues to be broken down and replaced by the remodeling process. Various factors affect remodeling, including age, physical activity, diet, and overall health. Having an efficient and effective bone remodeling process can play an important role in the health and function of a person or animal.

Background

Bone appears to be a hard, solid substance, but it is living tissue made up of a combination of cells in a mineral framework. When examined under a microscope, bone appears to have many holes in it, much like a honeycomb. Healthy bone has smaller holes and a more solid framework, while bone in which the remodeling process is not fully functional has larger holes and is more susceptible to breakage and other problems.

Bones perform several important functions. In addition to providing support for all the other tissues and organs of the body, they provide protection for the internal organs. Bones also house the bone marrow that makes new blood cells. Finally, bones are storage sites for minerals such as calcium; 99 percent of all the calcium in the human body is found in either bones or teeth.

Bones begin changing before birth, as the initial bones of an infant continually grow and change in size, shape, and even composition. This process is known as bone modeling. The ends of infant bones are made up of a substance called cartilage, a flexible connective tissue, while the center is bone. During childhood through early adulthood, this process occurs rapidly. In some cases, multiple smaller bones are replaced by a single larger one, and cartilage is gradually replaced by bone. By the mid- to late twenties, the human skeleton has reached its full size and maximum mass. From this point on, bone is remodeled, or changed, renewed, or altered, as bone is dissolved and replaced.

A number of factors can affect the remodeling process. Genetics play a role; the size and general condition of a person's bones are in part determined by heredity. However, the quality and type of diet, a person's weight, the amount and type of physical activity in which a person engages, drugs or other substances that are ingested, the aging process, and the health of other body systems can all affect how well a person's bones remodel and determine one's overall skeletal health. The immune system also influences bone remodeling through signaling between immune cells and bone cells, a relationship studied in osteoimmunology. When the remodeling process functions at less than peak efficiency, bone-health issues can develop. One of the most common issues is osteoporosis, in which the mineral components of the bone are thin, and there are large holes in the framework, making bone breakage much more likely.

Overview

There are multiple substances in bone, all of which are necessary for bone health and function. The outside layer is known as the periosteum; this thin membrane helps the bones connect to other tissues and contains cells that support bone health and growth. This periosteum covers the hard, solid framework usually thought of as bone, which contains minerals such as phosphorus and calcium. This framework is laced with a network of blood vessels, nerves, and special cells that help the body break down and make new bone. Also known as bone metabolism, this process continues until death.

Bone remodeling is a two-step process in which bone is first dissolved and then replaced. A remodeling cycle commonly takes several months, with reported durations of about 120 to 200 days depending on the type of bone. It occurs in five phases. First, during the activation phase, precursor cells known as pre-osteoclasts move to the area where bone is to be remodeled. The cells are activated and become mature osteoclasts. The term osteoclasts comes from the Greek word osteon, which means "bone," and klastes, which means "breaker." Osteoclasts are the cells that break down the bone matrix. In biology, a matrix is a tissue that contains the special cells and materials that make up a body part. The phase in which the osteoclasts break down the bone is known as resorption. The osteoclasts form a niche in the bone known as a resorption pit. As they do this, they release calcium into the bloodstream. Once they have completed this process, the osteoclasts undergo apoptosis (programmed cell death) or leave the resorption site.

The next phase is known as reversal. This is the point in remodeling at which resorption ceases. Special cells called mesenchymal stem cells gather in the resorption pits. They reproduce and grow into a new type of bone cell known as a pre-osteoblast. Next comes the formation phase, when the pre-osteoblasts become osteoblasts. These new bone cells begin making fresh bone material by releasing a substance called osteoid. Osteoid forms a new living matrix for the bone, which begins to attract calcium and phosphorus and mineralize to develop new solid bone. In the final stage, quiescence, the osteoblasts stop functioning and become lining cells on the surface of the newly formed bone. They remain dormant until it is time for the section of bone to remodel again. When osteoblasts become embedded in the new bone they have created, they are known as osteocytes. They help maintain the new bone.

Several different factors can trigger remodeling. One factor is the need for calcium in the blood. The body uses calcium for a number of functions besides forming bones. These include ensuring the healthy function of nerves and muscles and helping blood to clot properly. If one of these systems needs calcium, it is drawn from the bones or teeth. The parathyroid gland releases a hormone that triggers the osteoclasts to dissolve bone and release the needed calcium. This is one of the reasons it is important for a diet to have enough calcium; otherwise, the body will withdraw it from the bone, even if this makes the bones weak and prone to breakage, because the other functions are considered more essential. Other important nutrients for healthy bones include vitamin D and phosphorus.

Another trigger for remodeling is damage to the bone. If a bone is weakened, cracked, or broken, the remodeling process will begin to replace the missing or damaged bone area. Without the remodeling process, broken bones would not heal, and bones weakened by illness or the lack of calcium could not be restored.

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