Bone Matrix: What It Is, Components And Characteristics
Bones are rigid organs that form the endoskeleton of vertebrate animals. Its main function is to provide living beings with protection, movement capacity, 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 set of bones forms what we know as the “locomotor system” in humans. The striated muscles contract at will based on the brain’s guidelines, and as they are attached to the bones, they act as a lever to execute the movements that we want to perform at any given moment.
Human beings 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 on other occasions, today we bring you a tissue section of the bone of enormous interest at an anatomical and clinical level: know everything about the bone matrix
Bone tissue, along with bone marrow and other connective tissues, gives structure and functionality to bones For its part, this is defined as a tissue organization of a conjunctive nature composed of cells and calcified non-living elements, which form what we know as extracellular bone matrix.
Since we see them as hard and resistant, we believe that bones are physiologically rigid and do not undergo changes over time: nothing could be further from the truth. Bone tissue is hard, but also extremely plastic, since 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 replacing itself after a fracture.
Bone tissue is composed of 2% cells and 98% extracellular substances, that is, bone matrix. Although we will dedicate some final notes to knowing the bone cells, this time we focus on what surrounds them.
The bone matrix It is the characteristic component of bone, since it gives it its anatomical and physiological qualities It is composed of 65-70% inorganic salts (minerals) and 30-35% organic substances. We tell you the particularities of each element in the following lines.
1. Inorganic portion
The mineral part of the matrix is what gives the bones their storage capacity, resistance and protection. The inorganic material included in this section consists of calcium phosphate deposits, in the form of a substance known as hydroxyapatite. Apatite crystals are about 40 nanometers long and have a hexagonal prism shape, often with great development on the pyramid faces. As you already know, they have a whitish-yellowish color.
As we have already said, 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 that the body needs are stored entirely in our skeleton. Without going further, Thanks to our bones, human beings “keep” 1-1.2 kilograms of pure calcium inside us This mineral is essential for muscle contraction, transmission of nerve signals, absorption of vitamin B12, stimulation of hormonal secretions and many other things.
2. Organic portion
It represents approximately 30% of the bone matrix. This organic section is represented mainly by proteins, more specifically, by different types of collagen. Type I collagen is the one with the greatest presence (95% of the total organic portion), but traces of type IV collagen (5%) and, sometimes, type III are also observed. Depending on the orientation of the collagen fibers, The bone matrix can be lamellar, non-lamellar and osteonic or concentric lamellar in nature
In addition to collagen, in the organic part of the bone matrix we also find other proteins, although in a much smaller proportion. Below, we present a list of the most important ones:
We go beyond the bone matrix to briefly name that 2% of cellular matter that makes up bone tissue. First we have the osteoprogenitor cells which differentiate, during their development, into other cell bodies useful for bone production.
Of great interest are also osteoblasts, the bone cells responsible for secreting the bone matrix we 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 osteoblasts, like almost any other type of cell, preserve the ability to replicate themselves, but this is not possible over time because they become “enclosed” in the solid matrix that 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 job is to maintain the bone matrix, either through the synthesis of new materials or their resorption. Due to their ability to store or “release” bone calcium in the blood, they are considered to actively participate in the metabolism and mineral balance of the body.
Lastly we have osteoclasts, multinucleated cells that degrade and resorb bone tissue It may seem like a counterproductive task to destroy bone material (and it really is), but when the body lacks minerals essential for certain physiological processes, sometimes there is no other option.
The bone matrix is not watertight
As we have previously said, 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 occurs in the human species at around 30 years of age, because up to this point bone growth occurs, or what is the same, a higher rate of matrix synthesis than resorption. From here, 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, 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 translates into a rate of bone loss of up to 5% annually (although it later stabilizes again).
As you may already suspect, this bone aging translates into a pathology more than known to any elderly person: osteoporosis. This condition is common in the aging population, but without a doubt, it is a clinical event with a much higher incidence in women, due to the exaggerated bone resorption that occurs in the 5-7 years before menopause.
It is estimated that 80% of women over 80 years of age suffer from osteoporosis, while the prevalence in elderly men over 70 years of age ranges from 11.3%. It is not surprising that the majority of hip fractures and other serious injuries occur in elderly people: when the bones are fragile, any fall can seriously compromise the integrity of the individual.
Summary
We do not want to end on a bitter note, since the development of the process is as important as its outcome. The bone matrix harbors an infinite number of secrets and, without a doubt, it is 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, no matter how 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.
