Satellite cells: what they are, characteristics and function

Let's see what satellite cells are and how they function, involved in various processes in the body.

The cell is the functional anatomical unit of all living beings. For an organic entity to be considered living, it must have at least one cell body. Based on this premise, we can describe all terrestrial biodiversity: from microorganisms as evolutionarily "simple" as bacteria (prokaryotes, a single cell) to the human being, with about 30 trillion cells, divided according to their structure and functionality into different organs and systems.

The cells present in our body are divided into 2 large blocks: somatic and sexual. Somatic cells are those that form each and every one of the tissues of our body, characterized by their diploid nature (2 sets of chromosomes in the nucleus, one from the father and one from the mother) and their division by mitosis. While some cells form tissues that are in constant replacement, others are generated during development and do not regenerate again, as could be the case of the hair cells of the ear.

Thus, functionality and regenerative capacity are characteristics that allow us to group cells into different very heterogeneous blocks: neurons, adipocytes, hepatocytes, osteocytes, myocytes and a very long list. Even so, what would you think if we told you that there are cell groups within our body that are not "nothing" and respond to the needs of the organism? In these lines we will see what they are satellite cellsWe explain this crazy premise in the following lines.

What are satellite cells?

Satellite cells are a group of cell bodies that are undifferentiated, i.e. they lack tissue specialization, like most cells in our body.. However, these enigmatic structures can develop with the appropriate stimulus into a differentiated cell, in this case a Muscle cell. These curious cell lines were discovered by the biophysicist Alexander Mauro more than 50 years ago (1961), when he observed a group of undifferentiated mononucleated cells in the periphery of human skeletal muscle fibers.

The direct juxtaposition of satellite cells with muscle fibers immediately made this researcher think that they must have something to do with the repair and growth of muscle tissues. Indeed, satellite cells are the precursors of the cell bodies that form skeletal (voluntary) muscle, but they are also capable of adding additional nuclei to their parent cells (remember that skeletal muscle fibers are the same cells that form skeletal muscle).(remember that muscle fibers have several nuclei) and enter quiescent states when they are not needed.

Before continuing with the fascinating world of satellite cells, we think it would be interesting to clarify a series of general terms about the musculature, so that you can easily understand the following lines. We won't take too long.

About muscle cells

The striated or skeletal muscle forms what we know as the muscular system, that is, the grouping of more than 600 muscles that we move voluntarily to perform movements, adopt postures, transmit gestural information, and many other things. and much more. The basal cell of striated muscle tissue is the muscle fiber or skeletal myocyte, a multinucleated, cylindrical cell type with a marked contractile capacity.

Rather than typical cells, skeletal myocytes are actually syncytia, cytoplasms surrounded by membrane with a multitude of nuclei included. They have an atypical shape, since they are very elongated (several centimeters long) and, in addition, are characterized by a highly developed cytoskeleton, which allows the shortening of the cell and, therefore, the contraction of the musculature.

The nuclei of the muscle fibers are located in the cell periphery, just below a membrane, which allows for cell shortening and, therefore, muscle contraction.The nuclei of the muscle fibers are located at the cell periphery, just below a membrane called the sarcolemma. The central content of these cell bodies is dominated by actin and myosin II filaments, in addition to many mitochondria, which are necessary for the production of energy demanded by this tissue during contraction and relaxation movements.

Activation of satellite cells

The activation of satellite cells depends on the muscle niche, the surrounding microvasculature and local inflammatory responses.. Some specific factors, such as hepatic growth factor (HGF), nitric oxide synthase (ONS) and fibroblast growth factors (FGF) seem to play an essential role in this process, but the exact mechanism that makes satellite cells act is not yet known.

On the other hand, there is some research indicating that satellite cells are negatively regulated by a protein known as myostatin.. This myokine directly and indirectly inhibits muscle growth in humans, but again, in order to establish inalienable causalities it is necessary to continue studying this enigmatic cell lineage.

The functioning of this cell type

By definition, undifferentiated human stem cells must be able to replicate themselves and, when the time is right, give rise to differentiated functional progeny. Satellite cells fulfill both requirements, since they are activated and begin to replicate when they receive signals from a damaged surrounding tissue..

After emerging from quiescence, this cell group begins to proliferate, through a process known as "satellite cell activation". Moreover, it should be noted that this reparative action is not limited only to the injured site: when the satellite cells are activated in that section of tissue, others present in different parts of the myocyte are activated and migrate to the site, with the aim of "healing" the compromised section as soon as possible.

In addition to dividing, there is evidence that these cells are able to fuse with existing structures in order to facilitate growth.in order to facilitate growth and repair at the tissue level. However, it should be noted that this repair process is incomplete when extensive damage occurs and fibroblasts deposit scar tissue. If the function of satellite cells were 100% effective in all scenarios, muscular dystrophies would not exist.

Satellite cells and exercise

It is impossible not to wonder how all these mechanisms are linked to the performance of physical activities, as it is clear that various muscle groups can be damaged with a poorly performed exercise or a particularly nasty fall.

It has been postulated that exercise promotes the release of molecules of an inflammatory nature, cytokines and growth factors (such as the aforementioned HGF), which would activate the quiescence output of satellite cells While some specific factors are responsible for "waking up" the satellite cells, others, just as essential, will promote differentiation, but always with a specific purpose: to repair and improve the body's musculature. Thus, the very performance of physical activities would alert the satellite cells to be prepared in case something goes wrong.

Beyond this curious mechanism, studies have shown that endurance training results in an increased number of satellite cells in the skeletal muscle of athletes.. This could be an excellent type of response to counteract the action of age, as it seems that the proportion of satellite cells available in our body also decreases with aging.

Summary

All of the above should be taken with an important degree of critical thinking, since there is still a great deal to be known about these cell types and, therefore, it would be a mistake to attribute a series of miraculous properties to them. Everything seems to indicate that exercise and physical activity promote the expression and differentiation of satellite cells but, of course, a serious injury always promotes the formation of scar tissue, which translates into a reduction of muscle functionality.

We stress an idea already mentioned above: if the action of satellite cells were unequivocal and applicable in all scenarios, there would be no irreparable muscle lesions in the human body. In any case, this does not mean that they are without utility: understanding their functionality and their activation pathways can be a great tool for physiological and medical knowledge, as it could help us to elucidate the particularities of certain pathologies of the locomotor system, to date practically unknown.

Bibliographical references:

• Dumont, N. A., Bentzinger, C. F., Sincennes, M. C., & Rudnicki, M. A. (2011). Satellite cells and skeletal muscle regeneration. Comprehensive Physiology, 5(3), 1027-1059.
• Hawke, T. J., & Garry, D. J. (2001). Myogenic satellite cells: physiology to molecular biology. Journal of applied physiology.
• Mauro, A. (1961). Satellite cell of skeletal muscle fibers. The Journal of Cell Biology, 9(2), 493-495.
• Montarras, D., Morgan, J., Collins, C., Relaix, F., Zaffran, S., Cumano, A., ... & Buckingham, M. (2005). Direct isolation of satellite cells for skeletal muscle regeneration. Science, 309(5743), 2064-2067.
• Morgan, J. E., & Partridge, T. A. (2003). Muscle satellite cells. The international journal of biochemistry & cell biology, 35(8), 1151-1156.
• Moss, F. P., & Leblond, C. P. (1971). Satellite cells as the source of nuclei in muscles of growing rats. The Anatomical Record, 170(4): 421-435.
• Schultz, E., & McCormick, K. M. (1994). Skeletal muscle satellite cells. Reviews of Physiology, Biochemistry and Pharmacology, Volume 123, 213 - 257.
• Yin, H., Price, F., & Rudnicki, M. A. (2013). Satellite cells and the muscle stem cell niche. Physiological reviews, 93(1): 23 - 67.