Muscle growth is an extremely complicated process that in some aspects still needs to be clarified. The volume of our muscles is in fact regulated by numerous factors such as genes, hormones, enzymes, cells, macro and micronutrients, receptors, etc.
The term universally accepted to describe the phenomenon of muscle growth is "hypertrophy".
One of the most fascinating research in the sector was the one that led to the discovery of satellite cells in 1961. The most interesting feature of these mononuclear cells lies in their ability to unite to generate new muscle cells. Unlike satellite cells, the latter do not possess this characteristic and, although subject to continuous turnover, can only increase in size (hypertrophy) but not in number (hyperplasia).
Muscle hypertrophy
Under normal conditions, satellite cells do not participate in muscle development. They are in fact in a state of quiescence and become active only in particular circumstances (especially in response to strong hormonal stimuli or following a strong muscle trauma). These cells therefore possess a powerful regenerative action.
After entering the satellite cells begin to divide and multiply, giving rise to myoblasts (embryonic progenitor cells of muscle ones). This first phase is called "satellite cell proliferation".
The newly formed myoblasts merge with the damaged muscle cells giving them their nuclei (differentiation phase). Polynuclear muscle cells are the result of this union, and their name derives from the presence of more than one nucleus within the same cell.
The increase in the number of nuclei allows these cells to significantly increase protein synthesis by producing, among other things, even more contractile proteins (actin and myosin) and more receptors for androgens (hormones with anabolic effect).
The combination of all these processes, called muscular hypertrophy, leads to an overall increase in the size of the muscle cell.
Muscle hyperplasia
Myoblasts also have the ability to fuse together and thus generate new muscle cells. This process, called hyperplasia, plays a marginal role in muscle growth, which is mainly regulated by hypertrophy.
It is important to stress that muscle trauma can also be caused by particularly intense and exhausting training. Weight training and downhill running (eccentric muscle contraction) therefore represent a powerful stimulus for the activation of satellite cells.
Activation of satellite cells
As mentioned at the beginning of the article, satellite cells are normally inactive. Their proliferation can be triggered by hormonal factors or an important muscle trauma.
Hormones able to activate satellite cells are different and work together to perform a common action (testosterone, insulin, HGH, IGF-1 and other growth factors such as MGF *, FGF ** and HGF ***). For this reason, the intake of anabolic steroids, combined with a high-protein diet and adequate training, increases muscle mass by stimulating hypertrophy and to a lesser extent the formation of new muscle cells (hyperplasia).
However, not all anabolics work in the same way. From this point of view the best anabolic effects are attributable to hormones with strong androgenic and / or aromatizable activity. These two aspects are however responsible for most of the most dangerous side effects related to steroids (prostatic hypertrophy, acne, hair loss, aggression, gynecomastia and water retention).
The activation of satellite cells is regulated not only by hormones but also by numerous other factors. Among these we report myostatin which performs an inhibitory activity on the proliferation of satellite cells limiting muscle growth in development and in adult life.
* MGF or mechanical growth factor : it is an isoform of IGF-1 and, besides stimulating the growth of the muscle, it also favors the repair in case of injury. It is produced in the muscle and has an autocrine and paracrine action (it does not circulate in the blood and acts on the cells present in the immediate vicinity). Both of these activities are mediated by interaction with satellite cells. The MGF is predominantly produced under stimulation in resistance exercises and is less responsive to GH than to IGF-1 of liver origin. Experiments conducted on laboratory animals have attributed to the MGF decidedly superior anabolic properties compared to IGF-1. These results, still awaiting confirmation, represent one of the last frontiers in the field of genetic doping.
** FGF (Fibroblast Growth Factor) promotes the capilarization of muscle fiber through the formation of new microvessels (angiogenesis).
*** HGF Hepatic Growth Factor: it is produced by a variety of tissues, including the liver where it stimulates cell proliferation in vitro and hepatic regeneration in vivo.
