Bone matrix: what it is, components and characteristics.

A summary of the characteristics and parts of the bone matrix in humans.

Bones are rigid organs that form the endoskeleton of vertebrate animals. Their main function is to provide living beings with protection, movement, support, production of Blood cells (hematopoiesis) and storage and release of minerals based on the physiological demands of the organism.

Together with muscles and tendons, the bones together form what is known in humans as the "locomotor apparatus". The striated musculature contracts at will based on the brain's directives, and since it is attached to the bones, it acts as a lever to execute the movements we wish to perform at any given moment.

Humans have a total of 206 bones. 80 of them are part of the axial skeleton, the central axis of the body, and 126 make up our limbs. Beyond the basic osteology already explored in other opportunities, today we bring you a tissue section of bone of enormous anatomical and clinical interest: know all about the bone matrix..

Bone tissue in general

Bone tissue, together with bone marrow and other connective tissues, gives structure and function to bones.. Bone tissue is defined as a tissue organization of a conjunctive nature composed of cells and non-living calcified elements, which form what we know as extracellular bone matrix.

Because we see them as hard and resistant, we believe that bones are physiologically rigid and that they do not undergo changes over time: nothing could be further from the truth. Bone tissue is hard, but also extremely plastic, as it has a highly complex resorption and repair metabolism mediated by ions, cells, hormones, proteins and nutritional factors. There are cases in which injuries and trauma are too aggressive for the bone to repair, but, in most cases, bone tissue is capable of healing after a fracture.

Bone tissue is composed of 2% cells and 98% extracellular substances, i.e. bone matrix.i.e. bone matrix. Although we will dedicate some final notes to learn about bone cells, we will focus on what surrounds them.

What is the bone matrix?

The bone matrix is the characteristic component of bone, as it gives it its anatomical and physiological qualities.. It is composed of 65-70% inorganic salts (minerals) and 30-35% organic substances. We will tell you the particularities of each element in the following lines.

1. Inorganic portion

The mineral part of the matrix is what gives bones their capacity for storage, resistance and protection. The inorganic material encompassed in this section consists of calcium phosphate deposits, in the form of a substance known as calcium phosphate.in the form of a substance known as hydroxyapatite. Apatite crystals are about 40 nanometers long and have a hexagonal prism shape, often with highly developed pyramidal faces. As you may already know, they have a whitish-yellowish color.

As we have already mentioned, the inorganic matrix represents approximately 70% of the dry weight of the bone. 99% of the calcium, 85% of the phosphorus and 40-60% of the sodium and magnesium needed by the body is stored entirely in our skeleton. Without going any further, thanks to our bones, we humans "store" 1-1.2 kilograms of pure calcium in our bodies.. This mineral is essential for muscle contraction, transmission of nerve signals, absorption of vitamin B12, stimulation of hormone secretions and much more.

2. Organic portion

It accounts for approximately 30% of the bone matrix. This organic section is mostly represented by proteins, more specifically, by different types of collagen. Type I collagen has the greatest presence (95% of the total organic portion), but there are also traces of type IV collagen (5%) and, on occasions, type III. Depending on the orientation of the collagen fibers, the bone matrix can be of a lamellar, non-lamellar and osteonic or concentric lamellar nature..

In addition to collagen, other proteins are also found in the organic part of the bone matrix, although in much smaller proportions. Below is a list of the most important ones:

• Osteocalcin: a protein of a hormonal nature (hormone) whose presence in the blood correlates with the bone formation coefficient.
• Osteonectin: a phosphoprotein that interacts with collagen and the inorganic salts of the bone matrix.
• Thrombospondin: a secretion protein with anti-angiogenic capabilities, i.e., it inhibits the formation of new blood vessels from existing ones.
• Alkaline phosphatase: responsible for the process of dephosphorylation at the molecular level.

The cellular part of the bone

We leave the bone matrix to briefly mention the 2% of cellular matter that makes up the bone tissue. In the first place we have the osteoprogenitor cells.which differentiate, during their development, into other cell bodies useful for bone production.

Also of great interest are the osteoblasts osteoblasts, the bone cells in charge of secreting the bone matrix we have just described. They are capable of synthesizing hydroxyapatite crystals which, as we have already said, are made up of calcium and phosphate. It is curious to know that the osteoblasts, like almost any other type of cell, preserve the capacity to replicate themselves, but this is not possible over time because they remain "locked" in the solid matrix which they themselves synthesize.

This is where osteocytes come into play, representing more than 90% of the cells in bone tissue.. They are formed from osteoblasts (which in turn derive from osteoprogenitor cells) and their task is to maintain the bone matrix, either by synthesizing new materials or by resorbing them. Due to their capacity to store or "release" bone calcium in the blood, they are considered to be actively involved in the metabolism and mineral balance of the organism.

Lastly, we have osteoclasts, multinucleated cells that degrade and resorb bone tissue.. It may seem counterproductive to destroy bone material (and it really is), but when minerals essential for certain physiological processes are lacking in the body, sometimes there is no other option.

The bone matrix is not watertight

As previously mentioned, the bone matrix is constantly changing. Normally, the process of matrix synthesis and resorption is balanced, and is mediated by substances such as parathyroid hormone (PTH), calcitonin, estrogens, vitamin D, various cytokines and other local factors (prostaglandins).

Peak bone mass in the human species is around 30 years of age.The bone growth occurs up to this point, or, in other words, a higher rate of matrix synthesis than resorption. From this point on, a physiological "plateau" is generated that lasts about 10 years, but as we age, more and more bone matrix is destroyed and less is synthesized.

From the age of 40 onwards, 0.3-0.5% of the bone matrix is lost each year.. In women the condition accelerates exorbitantly during menopause, due to the drastic drop in sex hormones, which results in a rate of bone loss of up to 5% per year (although it then stabilizes again).

As you may already suspect, this bone aging results in a pathology that is more than familiar to any elderly person: osteoporosis. This condition is common in the aging population, but it is undoubtedly a clinical event with a much higher incidence in women, due to the exaggerated bone resorption that occurs in the 5-7 years following menopause.

It is estimated that 80% of women over 80 years of age suffer from osteoporosis, while the prevalence in men is much higher in women.while the prevalence in elderly men over 70 years of age is estimated at 11.3%. It is not surprising that most hip fractures and other serious injuries occur in the elderly: when bones are fragile, any fall can seriously compromise the integrity of the individual.

Summary

We do not want to end on a sour note, because the development of the process is as important as its outcome. The bone matrix harbors an infinite number of secrets and is undoubtedly a truly fascinating conglomerate of substances from an anatomical and physiological point of view. How could we suspect, for example, that calcium deficiency in the body could cause the bone matrix to be actively degraded by bone cells?

As you have seen, nothing in the human being is immovable, however rigid and indestructible it may seem. Like all the tissues that define us, bones undergo drastic changes based on individual development, environmental conditions, internal homeostasis and many other factors.

Bibliographic references:

• Bone matrix and consolidation, Medwave. Retrieved February 25 from https://www.medwave.cl/link.cgi/Medwave/Revisiones/RevisionClinica/4155.
• Module on anatomical and physiological basis of sport: bones. Retrieved on February 25 from http://www.edvillajunco.es/doc/1_los_huesos.pdf
• Journal of Osteoporosis and Mineral Metabolism: osteoporosis. Definition. Epidemiology. Retrieved February 25 from http://revistadeosteoporosisymetabolismomineral.com/2017/07/11/osteoporosis-definicion-epidemiologia/#:~:text=En%20Espa%C3%B1a%20se%20calcula%20que,%2D29%2C57%25)9.