physiology

Bone renewal

Bone turnover: importance and biological basis

Despite the characteristic hardness and resistance, the bone is not a static tissue, but it changes continuously and continuously repairs itself. This process is called "bone remodeling".

REMEMBER:

  • It is called turnover or bone remodeling that cyclic process in which the oldest bone is removed to be replaced with another younger tissue.
  • We talk about osteogenesis to indicate the formation of bone tissue; of reabsorption to indicate its disintegration.
  • Every year about 10% of our total bone mass is renewed.

Under a fine endocrine control, the remodeling processes follow one another by modifying the structure of the bone tissue based on the demands

Responsible for bone renewal are two types of cells, called osteoclasts and osteoblasts, respectively. The former, polynuclear and rich in microvilli, secrete proteolytic acids and enzymes which, by destroying the bone matrix, release the minerals contained in it.

The erosive action of the osteoclast manifests itself with the formation of the Howship gap. Once a first gap is formed, the osteoclast is detached from the matrix, moving by ameboid motion on a portion of bone contiguous to that just reabsorbed. Here, it adheres again and forms yet another gap.

Thanks to this process, about 500 mg of calcium are removed daily from the bone (0.05% of the total calcium). Moreover, at the time of need, various populations of osteoclasts are able to reabsorb even large portions of bone in a relatively short time.

Following the process of bone erosion, osteoblasts intervene, cells with diametrically opposite functions. In fact, they guarantee the formation and deposition of organic matrix in the cavities generated by the catabolic action of the osteoclasts.

As soon as this matrix reaches a sufficient thickness, it is readily mineralized, thanks to the interposition of calcium. This mineralization process goes on for months, during which the density of the new bone progressively increases.

Thus, osteogenesis occurs in two phases:

  • matrix formation (osteoid);
  • matrix mineralization.

Why is bone turnover important?

  • For the repair of stress-induced microfractures caused by normal physical effort
  • To reinforce bone tissue in response to appropriate stimuli
  • To regulate plasma levels of calcium and phosphorus

What regulates the activity of these cells, favoring osteoblastic or osteoclastic action?

The process is quite complex and understanding it means having a solid base from which to learn about and cure diseases in which there is a loss of balance between osteoblastic and osteoclastic action, such as in osteoporosis and bone metastases.

The drugs of the future will act by regulating the transcription of some genes to promote osteoblast activity and apoptosis (cell death) of osteoclasts.

The main adjustment factors include:

  • a- the level of calcium in the blood
  • b- the mechanical load due to the force of gravity and to the muscular mechanical stresses

The skeleton responds to exercise, to muscle strain and to the force of gravity by strengthening itself; on the other hand it weakens.

Hormonal influence and other factors

Although the length of the bones remains constant in adulthood, the bone tissue continues to host an active cell population, which keeps it in a state of dynamic equilibrium. Various hormones influence bone formation, growth and remodeling, stimulating either osteoblasts or osteoclasts.

Calciotropic hormones: specifically regulate calcium homeostasis
parathyroiddecreases bone strength (stimulates osteoclastic resorption)
CALCITONINincreases bone strength (inhibits osteoclastic resorption)
VITAMIN D:

after having been activated in the liver and kidneys it increases the absorption of calcium and phosphorus in the intestine and decreases its excretion in the urine

Systemically active hormones : affect bone metabolism
Androgen:they increase it
OESTROGENS:

they increase it (that's why women are more prone to osteoporosis after menopause)

THYROID HORMONES

they increase it, in synergy with GH, but if present in excess they decrease it

GH:

promotes skeletal growth in childhood and adolescence; an excess in juvenile age determines gigantism (a defect dwarfism), while in adult age it causes acromegaly (bone enlargement evident above all in the limbs and face).

IGF-1 and IGF-2

growth factors that, together with insulin and in synergy with GH, increase bone density and statural growth

PROLACTIN:

increases the synthesis of active vitamin D, promoting intestinal calcium absorption and thus increasing the amount of mineral available for milk production

GLUCOCORTICOIDSThey destroy the bone matrix by inducing osteopenia

In addition to signals of endocrine origin, bones are also sensitive to mechanical stimuli. The fabric that composes them reacts positively to the stimuli induced by the loading activities (works and sports that induce compressive stresses on the bone, such as football, dancing, running, much less cycling and swimming).

On the contrary, a prolonged immobilization (for example following a fracture) is accompanied by a rarefaction of the bone tissue. This explains why certain sports, including dance, prevent the appearance of osteoporosis in the elderly.

There are also local stimuli entrusted to particular messengers, such as the transforming growth factor (transforming growth factor) -ß (TGF-ß) and the insulin-like growth factors (IGF), produced by osteoblasts and stimulating their activity.

Note, in the image, the thickest arrow under the genetic factors, to emphasize the greater weight of this element on the others. the role of genetics in the variability of bone mineral mass (BMD) among individuals is quantifiable at around 60-70% (the prevalence of osteoporosis is greater among white and Asian individuals than among those of the black race).