Presented results were obtained in the years 1990-2020, during studies which were carried out with the use of various methods, including mineralogical. Studies on tissue biomineralization (calcification) show connection between developing osteoporosis and tissue biomineralization. This article presents the mechanisms of both osteoporosis and mineralization of selected human tissues. The relations between these processes were presented

Osteporosis is known as the process of bone destruction – reduction of their strength parameters, which in turn leads to their fragility, brittleness and lowering of osteogenic properties, i.e. reduction of the ability of forming bone fusion after fractures. In studies of these phenomena with the use of densitometry and other research techniques, no attention is paid to what happens with the elements removed from bone in the course of developing osteoporosis. Where do they go?

With the aging of organisms, calcification of various types of tissues and organs develops (1-29). The phenomenon is lethal for the organisms and in many cases may lead to death.

 Tissue mineralization does not necessarily involve the presence of grains or crystals. When we can observe them, for example with the use of microscopes, we are talking about overt mineralization.

However, tissues can also be affected by hidden mineralization. In that case, mineral concentrations are not visible, but the chemical analyzes of the tissues affected by such mineralization clearly show higher contents of various elements. It is similar to mineral water, which does not contain crystals and yet contains high levels of many elements.

In the case of hidden mineralization, the elements occur in the atomic structures of organic compounds that build tissues, where they replace various atoms. That usually happens in areas where tissues have been damaged for various reasons. These damaged sites may be affected by hidden mineralization – the first stage on the way to overt mineralization.     

This phenomenon, which includes the relationship between osteoporosis and tissue mineralization, is discussed in this article.   

Understanding osteoporosis is difficult without knowing the process of bone collagen mineralization.

 Collagen fibers present in bone trabeculae have centers of carbonate hydroxyapatite crystallization systematically arranged along their length. This mineral crystallizes in such crystallization centers from a chemical compound called alkaline phosphatase, which is generated in osteoblasts. Crystallization centers are endowed with electric charges that "trap" Ca2+ and PO43- ions from the flowing alkaline phosphatase, building lamellar hexagonal crystals in the bones. After apatite crystallizes, an aggregate of collagen fibers overgrown with mineral plaques is formed.

Osteoporosis develops in the bones in very different places. This is confirmed by densitometric examinations of the femoral head in its subsequent sections. Dark – "decalcified" – zones create fanciful shapes in the heads of the femur (Photo 1, A). Observations made with a binocular microscope confirm disappearance of bone trabeculae in these places (Photo 1, B).

The functioning of bone cells located in the pits in the bone trabeculae involves a number of life processes, including nutrition and excretion of metabolic products. Nutrition is, generally speaking, the breakdown of polysaccharides, as a result of which cells receive vital energy.

The products of metabolism are excreted outside the cells, from where they migrate to blood vessels based on the concentration gradient (Fig. 2, A).

In this process, carbon dioxide, a product of cellular metabolism, and other products are removed from the bone cells into the microvessels embedded in the Haversian canals. Penetrating into microvessels through the systems of microholes in their walls, they enter the bloodstream. In studies conducted with a scanning microscope, crystallizing substances were observed in those microvessels (Fig. 2B), which, apart from calcium phosphates, may also contain organic compounds that are difficult to identify (Fig. 3, C). Their crystallization causes clogging of the vessels and makes it difficult for the excess carbon dioxide to be removed through them. Another effect is the local formation of increased amounts of carbonic acid and then an acidified environment with a pH <7>

Bone apatite in collagen fibers is unstable in acidic environment and begins to dissolve. It also causes destruction of the structure of collagen fibers and, consequently, further deterioration of the bone trabeculae.

Since "clogging" of microvessels in bones is random and involves various microvessels, osteoporosis develops in an irregular manner.

Diagram of the migration of Ca2+ and PO43- ions released as a result of osteoporosis through the circulatory system

 Calcium and phosphorus, removed from bone in the process of osteoporosis, enter the venous blood (Fig. 4, A). Because of the CO2 supplied to the blood, weak, easily dissociating carbonic acid is formed, which slightly acidifies the venous blood. As phosphates, including apatite, crystallize in alkaline environment, in venous blood of pH <7>

Crystallization centers are found not only in arteries, but also in other tissues. If the tissue environment is slightly alkaline and there is any damage (crystallization centers), biomineralization may also develop here – not only with calcium phosphates, but also with organic substances (cholesterol, fats, etc.).

Biomineralization of arteries

It begins with hidden biomineralization and may or may not evolve to overt mineralization, which can be observed with the naked eye.
Initial biomineralization is only manifested by an increased content of mainly calcium and phosphorus, which can only be detected with sensitive chemical analysis methods. As it progresses further on the endothelium, but also in the artery wall, phosphate microcrystals appear (Fig. 4, A, B), which grow, typically together with cholesterol, to form the so-called “atherosclerotic plaque”. Its presence in the artery reduces its internal diameter (cross-section). A consequence of this phenomenon is an increase of blood pressure.

Biomineralization of the heart valves

A special case of biomineralization is the so-called calcification of the heart valves. They affect both the aortic and mitral valve (Figs. 6A, B), and often also artificial valves (Fig. 6, A, B). They are usually located in places where valve tissues are damaged, which may result from various phenomena.

The phosphate concentrations can take the form of irregular grains (Fig. 7, A) or even beautiful idiomorphic crystals whose edges are so sharp that they can easily cut and damage the valve tissues made of collagen fibers. The damaged valve leaflets also show mixed phosphate-cholesterol crystals (Fig. 7, B).

Cartilage biomineralization

More recent studies also indicate biomineralization of cartilage located in various parts of the body. A special case is the mineralization of spinal discs (Fig. 8, A). When it is hidden, it causes hardening of the disc cartilage, which may contribute to disc deformation. When overt mineralization occurs, visible within the discs, cauliflower-shaped micro-grains are observed, which may even form specific colonies (Fig. 8, B). This type of mineralization may apply to joints where, additionally, idiomorphic crystals, e.g. oxalates, may be present in the synovial fluid.

Tendon biomineralization

Mineral grains were also found in the areas of tendon damage (Fig. 9, A). Their size, observed microscopically, does not exceed several dozen micrometers (Fig. 9, B). The presence of this type of grains – most often phosphate – undoubtedly hinders the proper functioning of the tendons and thus the mobility of the whole body.