In the course of life, bone tissue undergoes changes, like any renewable tissue in the human body. These changes are associated with the destruction of cellular structures and the formation of new ones. Such bone renewal is called physiological regeneration of bone tissue.
When there is a shift towards the predominance of the process of destruction of cellular structures over the process of creating new ones, we talk about bone resorption – resorption of bone tissue. In this case, we have a pathogenetic process.
Bone tissues are formed by structures of mineral substance – hydroxyapatite and connective tissue. Connective tissue consists of collagen fibers and loose connective tissue.
Regulation of bone tissue remodeling processes is a complex mechanism that is controlled by various systemic and local factors. These factors are important in order to understand which of them we can influence in order to start the processes of sanogenesis in the bones in order to avoid age-related loss of bone tissue.
Systemic factors include:
Local regulators are: growth factors, prostaglandins, cytokines, granulocyte-macrophage colony stimulating factor – GM-CSF, lymphotoxin-alpha.
Some of these hormones and factors, first of all, stimulate bone resorption, others have a predominantly inhibitory effect, however, both act on the principle of feedback. They can also be conditionally divided into pathogenesis factors – resorption stimulators, and sanogenesis factors – resorption inhibitors.
Stimulate bone resorption:
Parathyroid hormone (PTH). Activate osteoclasts.
Glucocorticoids (GC). In excess, they reduce calcium absorption in the intestines and calcium reabsorption in the kidneys, which leads to hypocalcemia. GC inhibit the function of osteoblasts; reduce the level of sex hormones. The level of GC increases significantly with a stress reaction.
Thyroid hormones. With their excess in the body, there is a sharp activation of the process of bone resorption. When thyroid function is impaired, there is a loss of bone mass and bone mineral density, which leads to osteoporosis.
Vitamin D – belongs to the prohormones of the steroid group, participates in bone mineralization and supports calcium homeostasis. Together with PTH, Vitamin D stimulates bone resorption by increasing the number of osteoclasts.
Local factors that enhance bone resorption.
The stress factor should be considered separately. It also refers to hormonal factors. After all, under the influence of stressful factors, there is an increased production of the hormone cortisol. Cortisol, in turn, reduces the activity of osteoblasts – cells responsible for the synthesis of bone tissue. Bone density decreases, resulting in a shift towards pathogenesis, the rate of bone destruction is higher than the rate of renewal. This leads to the development of osteoporosis. During stress, calcium withdrawal from the bones increases. Also, during the stress reaction, the level of norepinephrine increases, which also inhibits the activity of osteoblasts.
Inhibit (slow down) bone resorption:
Calcitonin, a hypocalcemic hormone, inhibits bone resorption due to its inhibitory effect on osteoclasts. Suppresses the breakdown of collagen. Calcitonin is a functional antagonist of PTH.
Sex hormones (estrogens, androgens, progestins) – participate in the formation of the skeleton, control the peak of bone mass and the rate of its further decline, maintain mineral homeostasis. In this series, estrogens play a leading role, regulating the metabolism of bone tissue, both in women and men.
Estrogens have a direct effect on all bone cells due to the fact that estrogen receptors are found on all types of bone cells. They reduce bone resorption by inhibiting the activity of osteoclasts, as well as their differentiation in the early stages from progenitor cells. The indirect effect of estrogens is carried out through the suppression of local resorbing factors. Estrogens have a protective effect on bone tissue from the resorptive effect of PTH.
Estrogen deficiency in women of reproductive age and during menopause is a leading factor in the development of osteopenia and osteoporosis, the development of which is based on an imbalance in the process of bone remodeling with a predominance of the rate of bone resorption.
Several important conclusions can be drawn from the above:
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