Satellite cells: what they are, characteristics and operation

The cell is the functional anatomical unit of all living things. For an organic entity to be considered as 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 billion 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 its diploid nature (2 sets of chromosomes in the nucleus, one from the father and one from the mother) and its division by mitosis. While some cells form tissues that are constantly changing, others are generated during development and do not regenerate again, as might be the case with 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 list very long Even so, what would you think if we told you that there are cell groups within our body that are "nothing" and respond to the needs of the organism? In these lines we will see what they consist of

satellite cellsWell, we explain this crazy premise in the following lines.

• Related article: "Main Cell Types of the Human Body"

What are satellite cells?

satellite cells are a group of cell bodies that are undifferentiated, that is, they lack tissue specialization, like most cells in our body. In any case, these enigmatic structures can be converted with the appropriate stimulus into a differentiated cell, in this case muscle. These curious cell lines were discovered by the biophysicist Alexander Mauro more than 50 years ago (1961), when he observed a group of undifferentiated mononuclear cells at the periphery of skeletal muscle fibers human.

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

Before continuing with the fascinating world of satellite cells, we find it interesting to clarify a series of general terms about the musculature, so that you can easily understand the lines to come. We don't take long.

About muscle cells

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. 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 normal cells, skeletal myocytes are actually syncytia, membrane-enclosed cytoplasms with a multitude of included nuclei. They have an atypical shape, since they are very elongated (with a length of several centimeters) and, in addition, they are characterized by having a highly developed cytoskeleton, which allows the shortening of the cell and, therefore, the contraction of the muscles.

The nuclei of 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, necessary for the production of energy that this tissue demands during movements of contraction and relaxation.

• You may be interested in: "Muscle fiber: what it is, parts and functions"

Activation of satellite cells

Satellite cell activation depends on the muscle niche, the surrounding microvasculature, and local inflammatory responses.. Some specific factors, such as liver growth factor (HGF), nitric oxide synthase (ONS) and growth factors fibroblast (FGF) appear to play an essential role in this process, but the exact mechanism that makes cells satellite act.

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, to establish inalienable causalities it is necessary to continue studying this enigmatic cell line

The functioning of this type of cells

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

After coming out of its quiescent state, this cell group begins to proliferate, through a process known as “satellite cell activation”. In addition, it should be noted that this reparative action is not limited only to the injured place: when the satellite cells are activated in said section of the tissue, others present in different parts of the myocyte are launched and migrate to the place, in order to "cure" the section as soon as possible. engaged.

In addition to dividing there is evidence that these cells are capable of fusing with existing structures, in order to facilitate growth and repair at the tissue level. In any case, it should be noted that this repair process is incomplete when extensive damage occurs and fibroblasts deposit scar tissue. If satellite cell function were 100% efficient 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, since it is It is clear that various muscle groups can be damaged with a poorly done exercise or a fall especially disgusting.

It has been postulated that exercise promotes the release of molecules of an inflammatory nature, cytokines and growth factors (such as the HGF mentioned above), which would activate the exit of quiescence of the cells satelite and encourage them to start working. While some specific factors are responsible for "awakening" the satellite cells, others just as essential will promote differentiation, but always with a specific purpose: to repair and improve the muscles of the organism. Thus, the very performance of physical activities would alert the satellite cells that they must prepare themselves, in case something goes wrong.

Beyond this curious mechanism, studies have shown that physical resistance training reports a greater number of satellite cells in the skeletal muscle of athletes. This could be an excellent type of response to counteract the action of age, since It seems that the proportion of satellite cells available in our body also decreases with age. old age.

Summary

Everything exposed here should be taken with a significant degree of critical thinking, since there is still a lot to be done. Knowing about these cell types and, therefore, assigning them a series of miraculous properties would be a mistake. Everything seems to indicate that exercise and physical activity promote the expression and differentiation of satellite cells but, since then, a serious injury always promotes the formation of scar tissue, which translates into a reduction in functionality muscular.

We emphasize an idea already mentioned previously: if the action of the satellite cells were unequivocal and applicable in all scenarios, there would be no irreparable muscle injuries in the body human. However, this does not mean that they are useless: understanding their functionality and their activation paths 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, today practically unknown.

Bibliographic references:

• Dumont, N. A., Bentzinger, C. F., Sincenes, M. C., & Rudnicki, M. TO. (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. TO. (2003). Muscle satellite cells. The international journal of biochemistry & cell biology, 35(8), 1151-1156.
• Moss, F. P., & Leblond, C. Q. (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. TO. (2013). Satellite cells and the muscle stem cell niche. Physiological reviews, 93(1): 23-67.