Beyond Satellite Cells: Eccentric Training Bullies Bystanding Stem Cells, Destined to Become Bone, Cartilage, Adipose or Nerve Tissue into Contributing to Skeletal Muscle Growth

Image 1: Apart from their epistemic value, these stained tissue samples from the Valero study are actually pretty aesthetic.The arrows, by the way, mark NG2 monocytes, which "coerce" the non-myogenic stem cells (perycites) to get going ;-)

I guess, the term "satellite cells" has been mentioned so often in the course of the past couple of weeks that it actually would not be necessary to tell you (once again) that these myogenic precursor cells are a necessary prerequisite for the repair and long-term growth of your pecs, legs, back, biceps, triceps, delts and the rest of the skeletal muscle tissue in your body. Those of you, who have read all the latest installments of the Intermittent Thougths will also be familiar with the notion that both the local expression of MGF-1 and inflammatory cytokines, as well as systemic hormones, such as testosterone and estrogen play an important role not only in the recruitement and migration of satellite cells into the muscle, but also in their regeneration and maintenance (cf. "Are You Serming Away Your Growth Potential"). A group of scientists from the University of Illinois must however have figured that this is not yet complicated enough and began digging even deeper into the (re-)generation of (new) muscle tissue... and let me tell you, what they found is intriguing.

Eccentric training is "numero uno" for satellite cell recruitment

I guess you will be aware that it is a relatively well-established fact that of all real-world physical activities eccentric, or lengthening, contractions of skeletal muscle appears to constitute the most potent inducer of both "productive" exercise induced muscle damage, as well as subsequent increases in satellite cell recruitment.

Figure 1: Changes in intramuscular architecture in young and old subjects in response to 6 sets of eccentric leg extensions (data calculated based on Dreyer. 2006)

Back in 2006 Dreyer et al. published the results of a study which compared the satellite cell response to an eccentric exercise regimen (1x12 + 5x16 eccentric-only reps on leg-extension machine) in young and old subjects (Dreyer. 2006). As the data in figure 1 goes to show, this unquestionably exhaustive bout of exercise produced a quite remarkable (>150%) increase in satellite cell volume per muscle fiber and a likewise highly significant (>100%) increase in the number of satellite cells compared to the total number of cells in the sublaminar compartment. It is also evident from the data in figure 1 that the preparatory accumulation of satellite cells 24h after the eccentric exercise-bout was profoundly blunted in the older subjects.

Figure 2: Satellite cell count per myonucleus before and 8d after 100reps of eccentric leg extensions in untrained young subjects (data adapted from Mikkelson. 2009)

Note: As you can see in figure 2, age is yet not the only factor that can compromise the adaptive response to eccentric lengthening contraction of skeletal muscle. Mikkelson et al. who had the (young) participants of their study perform 100 eccentric reps on a similar leg-extension machine (Mikkelson. 2009), for example, found that an indomethacin (NSAID; COX-inhibitor) infusion for 7.5 h during the exercise day did not only blunt the increase in pax-7+/myonuclei, or in "layman's terms", satellite cells 8 days after the exercise session, the combination of exercise and NSAID actually led to a slight and statistically obviously non-significant reduction in satellite cells per myonucleus. This does support the findings, I discussed in one of the previous installments of the Intermittent Thoughts, which dealt with the importance of "inflammation" as a vital constituent of both the repair and hypertrophy response to exercise induced muscle-damage (cf. "IGF-1, IL-15 & Co").

The age-related decline in satellite cell activity unquestionably raises a question Lerner addressed in one of his insightful comments on my post on the important role of estrogen for the maintenance of the satellite cell pool, which is whether or not the latter would be limited... I mean, when the whole craze about stem-cell therapy began, the general accepted notion appeared to be that the average adult human being has a very limited / if any of those "super cells".

If you have been following recent publications, you will yet be aware that as of late researchers (interestingly also from the University, I work at ;-) have made quite some progress in "producing" and "reprogramming" stem cells from all sorts of human and animal tissue. Assuming that you have also read everything about how testosterone works its "muscle building and fat burning magic" (cf. "Understanding the Big T"), you should also be aware that the latter, i.e. the "fat burning" effect is at least partly mediated by the reprogramming of stem cells which are actually supposed to become fat cells into "satellite cells". All that being said, it should actually not surprise you that the main finding of the initially mentioned study is that eccentric contractions of skeletal muscle have a very similar effect on "non-myogenic" stem cells, which "happen to be in the vicinity of the exercised muscle fibers".

A study on You, Wolverine and the Ultimate Hard Gainer

Unfortunately, the design of the study by M. Carmen Valero and her colleagues from the Department of Kinesiology and Community Health at the University of Illinois is... well, let's say not exactly straight-forward, or easy to understand (Valero. 2012). Basically, the scientists took 3 types of muscle fibers from rodents,

Figure 3: Localization of stem cell antigen-1 (Sca-1) (arrows, TRITC-red) positive mononuclear cells and a-7 integrin (FITC-green) in the different muscle tissue before (SED) or 24 hr post-exercise (Ex) at 20x magnification (adapted from Valero. 2012)

• wild type, as a normal control (that would be your muscle ;-)
   
• alpha-7 integrin transgenic (a7TG), which is resistant to injury, but still responsive to strain (I guess that is either the "Wolverine" or "Unbreakable" type of muscle tissue ;-), and
   
• alpha-7 (-/-), which is the "ultimate hard gainer muscle" that does not respond with alpha-7 integrin expression to overload

Now, even if you don't understand a word of what I am talking about here, I guess that you will be able to see the significant color-differences in the immuno-stained (=marked by antibodies) tissue samples in figure 3. If you just focus on the arrows, the red and green areas, that should suffice to grasp the idea that the both the stretch induced alpha-7 integrin response, as well as the number (arrows) and area (red staining) of stem cells in are maximal in the Wolverines, "normal" in guys and gals like you and minimal to non-existant in the "ultimate hard gainer".

The first take-home message of this study is thusly that the strain that is induced by eccentric training activates "dormant" stem cells via alpha-7 integrin. Why this is the case, becomes clear when you look at the structure of those heterodimers, which transverse the cell membranes of regular muscles and adhere the extracellular matrix to the cytoskeletal network. If you strain the muscle, this will obviously affect the integrin system, which thusly functions as a "sensor" for mechanical signals.

"No satellite cell available? Well I guess we just take this one, then..."

Upon closer analysis of the "satellite cells" that actually responded to the alpha-7 integrin signals, the scientists did yet realize that the resource from which the majority of the new muscle cells were recruited were actually mesenchymal stem cells, which are usually destined to become osteoblasts (bone), chrondocytes (cartilage) and adipocytes (fat cells):

In this study, we provide the first demonstration that muscle resident mesenchymal-like stem cells (mMSCs), predominantly pericytes, are increased in muscle in an a-7 integrin dependent
manner following an acute bout of eccentric exercise. mMSCs maximally appeared in a7BX2 transgenic muscle resistant to injury following eccentric exercise and were rarely present in
muscle lacking the a-7 integrin, suggesting that factors other than injury or inflammation are primary regulators of mMSC accumulation in skeletal muscle.

In view of the fact that a preliminary experiment in which the scientists transplanted exogenous mMSCs into the muscle of living mice, led to a very localized, but distinctly measurable increase in new fibers opens a whole new venue for research into the artificial enhancement of skeletal muscle hypertrophy... for you as an avid (and probably "natural") trainee, however, the main take home message is that exercise, in this case, the strain (not the damage!) that is induced by eccentric lengthening contractions of your muscles still appears to be the major determinant of all aspects of skeletal muscle hypertrophy. Don't forget that, when you are about to invest the next few hundred bucks into whatever "myogenic" supplement the guy at GNC is trying to persuade you into buying... without hard and consistent work at the gym those powders and caps are pretty useless.