What is bone resorption?

Throughout life, our bones undergo constant remodeling, in which new bone tissue is formed and mature bone tissue is destroyed. Bone resorption is part of this remodeling process in which bone is destroyed by the activity of specialized cells called osteoclasts releasing the minerals that form it from the bone matrix into the bloodstream. Throughout this article, we will learn what it consists of and what role it plays in health and disease.

bone remodeling

The creation and destruction of bone is maintained in a healthy balance by shaping itself as necessary for the normal functioning of the organism, during the growth phases or for the repair of injuries. This balance is known as bone remodeling (it can also sometimes be seen as bone homeostasis) and involves several coordinated cellular and molecular processes. The most prominent cells in bone metabolism are osteoblasts, responsible for bone creation, and osteoclasts, responsible for bone destruction.

But these cells do not act alone, but must cooperate with other cells present in bone tissue, for example, immune cells, and respond to a complex signaling system involving hormones (parathyroid hormone (PTH), vitamin D, growth, steroids, calcitonin). ) together with cytokines and bone marrow growth factors (M-CSF, RANKL, VEGF, IL-6). In this way, the body can maintain the proper rate of bone creation and destruction.

Osteoclast-mediated bone destruction, therefore, must be understood as a physiological process that occurs under normal conditions and in a state of health, and it is this physiological destruction that is known as bone resorption.

The two main functions of bone resorption are the repair of damaged bone tissue (osteoclasts break down damaged bone so it can be replaced by healthy tissue) and the regulation of the metabolism of various minerals, especially minerals. calcium metabolism; bone resorption makes it possible to maintain adequate levels of calcium in the blood, essential for numerous functions, for example, for muscle contraction.

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osteoclastic activity.

Osteoclasts are multinucleated cells with basophilic cytoplasm in which a large number of mitochondria and lysosomes can be observed. They are attached to bone tissue and have the ability to reabsorb the surrounding bone matrix (resorption).

Resorption is produced by the secretion of collagenases and other enzymes, such as Cathepsin K, which break down the bone matrix structure. The products of this degradation, including calcium, magnesium, and phosphate ions, are endocytosed by the osteoclast, causing bone demineralization. The minerals then pass into the bloodstream and can be recycled to make new bones, used in other physiological processes, or eliminated along with other metabolic wastes.

regulation

Bone resorption can be stimulated or inhibited by signals from various parts of the body. One of the most important signals is the demand for calcium; If the body needs calcium, bone resorption is stimulated so that calcium is released from the bone, which is why getting adequate amounts of calcium in the diet is so important. It is also stimulated in response to inflammatory processes and injury to bone tissue with the aim of eliminating damaged tissue and replacing it with healthy bone.

One of the most prominent hormones in the regulation of bone resorption is parathyroid hormone (PTH, paratoid hormone or parathyroid hormone), which is secreted by the paratoid gland. This gland has calcium-sensitive membrane receptors that continuously monitor the concentration of calcium in the extracellular fluid. Low levels of calcium stimulate the release of PTH which exerts these two main actions:

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in bone: increases the activity and number of osteoclasts to release calcium into the bloodstream. in the kidney: stimulates the hydroxylation of vitamin D to form vitamin D3 (calcitrol or 1-alpha-1,25-dihydroxycholecalciferol), the active form of vitamin D, which increases intestinal calcium absorption and calcium reabsorption in the kidney (is exchanged by phosphorus that passes into the urine).

On the other hand, high blood calcium levels lead to a decrease in PTH secretion in the paratoid gland, which results in decreased osteoclast activity and ultimately a lower rate of bone resorption.

Hormonal inhibition of osteoclast activity depends primarily on calcitonin, a hormone secreted by the thyroid. Calcitriol intake, due to the greater availability of intestinal and renal calcium it produces, also reduces bone resorption.

medical implications

Bone resorption occurs continuously in the human body, keeping in balance with the formation of new bone. As age advances, the rate of resorption tends to exceed the rate of formation, leading to diseases such as osteoporosis, in which bone decalcifies, loses density and becomes more fragile.

The rate of bone resorption can also be higher than the rate of bone formation in diseases of very different origins, such as hypoparathyroidism, of endocrine origin, or hypovitaminosis D (rickets), of food origin. In addition, chronic inflammatory bone diseases such as rheumatoid arthritis and psoriatic arthritis cause a higher rate of bone resorption, which plays a key role in disease progression.

The lack of stimuli for the body to maintain bone tissue can also result in an unbalanced rate of bone resorption. For example, a sedentary lifestyle, lack of physical exercise or weightlessness that astronauts are subjected to cause loss of bone density.

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Bone density loss can be detected in several ways. A blood test that reveals the presence of high levels of certain minerals can be a sign of abnormally increased or imbalanced bone resorption. These signs can be confirmed with bone densitometry, an X-ray test specifically designed to measure bone mineral density.

Once a loss of bone density and mass has been diagnosed, a further investigation can be carried out to discover the underlying cause that is producing it and thus be able to decide on the best treatment. In most cases, in addition to possible specific treatment depending on the cause, calcium and calcitriol supplements are used. In other cases, treatment focuses on reducing the risks associated with the patient’s bone fragility.

Gallery

Bone Resorption Scheme Osteoclasts seen under the light microscope As age advances, bone resorption outweighs bone formation.

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