Osteoclast
Osteoclasts are specialized bone cells responsible for the resorption and breakdown of bone tissue, functioning alongside osteoblasts, which are responsible for bone formation. These multi-nucleated cells play a crucial role in bone remodeling, a process essential for maintaining bone strength, facilitating growth in children, and aiding recovery from fractures. Osteoclasts originate from stem cells in the bone marrow and are attracted to specific bone areas that require resorption through signals from osteoblasts and parathyroid hormone.
During childhood, osteoclasts are vital for remodeling softer cartilage into stronger bone, allowing for rapid growth and skeletal development. In the context of aging, however, the balance between osteoclast activity and osteoblast function can become disrupted, particularly in post-menopausal women, leading to conditions like osteoporosis. This condition is characterized by weakened bones due to excessive resorption by osteoclasts without adequate replenishment by osteoblasts. Overall, the health and functionality of osteoclasts are significant for skeletal integrity throughout a person's life.
Osteoclast
Osteoclasts are one of two main categories of cells in the bones: osteoclasts and osteoblasts. Osteoclasts remove bone and work in conjunction with osteoblasts, which build bone. The bone absorption and removal performed by osteoclasts is important because it allows the body to keep existing bones strong and build new bone when necessary. Osteoclasts play a key role in how infants and children grow and in how bones heal after a fracture. They also contribute to the health of a person's skeleton during aging.
Background
The word osteoclast comes from the Greek words osteon, which means bone, and klastos, which means broken. Osteoclasts were identified and named in 1873 by Swiss anatomist Rudolph Albert Von Kolliker (1817–1905). Osteoclasts are multi-nucleated cells, or cells with more than one nucleus. The nucleus contains the genetic material that defines the cell's function and purpose. Osteoclasts generally have about five to twenty nuclei but can have as many as two hundred.
Osteoclasts are formed from stem cells in the bone marrow known as osteoclast precursors (OCPs), sometimes called preosteoclasts, and then carried in the blood through the circulatory system. Eventually, they settle in small ridges or indentations in the bone known as Howship's lacunae. Osteoclasts are directed to these places by the release of parathyroid hormone (PTH), which prompts the bone-building osteoblasts to release RANKL (Receptor Activator of Nuclear Factor-Kappa B Ligand). RANKL attracts osteoclasts to bone areas that need to be broken down, so new bone can be formed.
Once they attach firmly to the bone, osteoclasts create a protective compartment around the area. The part of this compartment directly attached to the bone becomes the ruffled membrane, which releases acid-containing proteins that begin dissolving the bone. As the bone is broken down, the minerals contained by the bone are dissolved and enzymes are generated that dissolve the collagen and other components of the bone. Osteoclasts are pre-programmed to self-destruct when their work is complete, so when the bone in the protective compartment has been completely removed, the osteoclast dies.
In most circumstances, once the osteoclast has completed its work, the osteoblasts rebuild the area with fresh bone; this is known as bone remodeling. However, in some cases, this process may be out of balance due to illnesses, such as cancer, and diet deficiencies. Some medications also alter the bone remodeling process. Age can also interfere with the effectiveness of the bone remodeling process. Children and younger people can generally regenerate bone at a faster rate than older people. Osteoporosis is the most common result of age-related bone remodeling problems.
Role in Childhood
Developing fetuses and newborn babies have skeletons with about three hundred parts, as opposed to the 206 that a child will have as an adult. These parts are soft and made mostly of cartilage, a firm but very flexible form of connective tissue found in the body. This cartilage gradually turns into bone through a process known as ossification. Children also undergo rapid growth in their earliest years, and the newly formed bone is frequently remodeled to grow longer, thicker, and stronger, so the skeleton can support the developing child. Osteoclasts play a key role in this, breaking down bone so that it can be rebuilt into larger sizes. The bone formation process related to growth will continue until the person reaches about the age of twenty-five, when the bones have reached maximum size.
Role in Fracture Healing
Osteoclasts work efficiently with osteoblasts to help fix fractured bones during childhood. Osteoblasts and osteoclasts continuously work at a fast pace during childhood, so the systems needed to create new bone are already very active at the time of a fracture. This means that it takes less time for the body to begin healing a fracture in a child than it does in an adult, where the system is called upon less often and needs to reactivate to remodel the broken bone.
Osteoclasts are part of the final segment of the healing process for a fractured bone. The broken parts are held together temporarily by collagen and other soft tissues before being replaced by new bone generated by osteoblasts. This process leaves a thickened, larger bump around the break. Osteoclasts will slowly dissolve this extra bone to restore the bone as close to its original shape as possible.
Role in Aging
Sometimes the balance between how quickly osteoclasts remove old bone and how quickly it is replaced by osteoblasts is out of balance. This is a relatively common occurrence in older people, particularly women who have gone through menopause, which is the cessation of the monthly egg-releasing cycle of the reproductive system. The change in the reproductive process alters the amount of the hormone estrogen in the body, and estrogen is part of the process that regulates the number of osteoblasts and osteoclasts the body produces. When more osteoclasts are produced than osteoblasts, bone is removed faster than it can be replaced. This results in bones that have holes and tunnels instead of being solid, decreasing bone strength and increasing the likelihood of a break. This condition is known as osteoporosis.
In addition, because osteoclasts function to maintain bone health throughout life, they tend to be attracted to aging bone. When the body is able to produce enough osteoblasts to rebuild the bone absorbed and removed by the osteoclasts, this tendency of the osteoclasts does not present a problem. However, this balance can be altered by diet as well as other health problems and the use of some types of medications. Because older people tend to have more health problems and require more kinds of medication, they have an increased likelihood of their osteoclasts dissolving bone faster than osteoblasts can replace it. This puts them at a greater risk of developing osteoporosis.
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