1. Asymmetric Division in Muscle Stem Cells
Christian Elabd, Ph.D.
Joey Pham, B.A.
This presentation was put together by the efforts of Dr. Elabd and Mr. Pham at the University of California, Berkeley.
Notes are provided for each slide as a guide to assist the presenter in explaining each slide. Please take note that this presentation serves only to provide a basic understanding of asymmetric and symmetric division. The ideas presented in the following slides are by no means exhaustive, and must be understood to be true only within certain contexts.

2. Muscle System intact injured muscle muscle regenerated muscle
- The muscle system is a model for quiescence (resting) and activity
- Satellite cells, or muscle stem cells, can regenerate repeatedly
- Satellite cells give rise to myoblasts (mostly), and fibroblasts, adipocytes, chondrocytes.
intact
muscle
injured
muscle
regenerated
muscle
myonuclei
As you may know, adult stem cells represent a pool of non-specialized cells within our organs, which upon tissue damage or signaling, can respond and begin repair mechanisms. Adult stem cells are multipotent and can differentiate in culture into many different specialized cells such as skin, muscle, neural or bone cells. Muscle stem cells, also called satellite cells, are quiescent (non active) in absence of tissue damage. Upon muscle injury, for example after exercising or a car accident, satellite cells become activated. They start to proliferate to increase their number and differentiate into muscle cells to regenerate the muscle.
In this slide:
Left panel: muscle fibers are multinucleated cells, if you look closely you can see multiple nuclei in the single fiber. Satellite cells are located on the outside face of a myofiber plasma membrane waiting for activation signals, such as growth factors. Top right panel: low magnification picture (5X) of a muscle section after injury showing morphological differences between intact (right) and damage (left) muscle. The injured part of the muscle is functionally altered (loss of contraction potential).
Bottom right: higher magnification picture (10X) of a muscle section 2 weeks after injury showing regeneration of the damaged site. Regenerated (right) and intact (left) muscle areas are morphologically and functionally pretty similar (recovery of muscle contraction potential).
intact
muscle
satellite
cells
myofiber
Histology 2 weeks after injury
Hematoxylin and eosin cross section
Pictures from MJ Conboy, UC Berkeley

3. Quiescent Muscle Stem Cell
Muscle Regeneration
Quiescent Muscle Stem Cell
Myotrauma
Resting muscle Fiber
1) Activation
Multi-nucleation
Self
Renewal
Return to resting state
This slide describes the process of muscle regeneration. On the top left part of the slide is represented a resting muscle fiber that has a muscle stem cell laying on the outside face of the plasma membrane. Muscle regeneration can be summarized as a 3 steps process.
Activation: once signaled by trauma or damage, satellite cells becomes activated, turning on a number of genes among which genes necessary for cellular proliferation.
Proliferation: activated satellite cells then perform many rounds of cellular division (=proliferation) to increase their number. At that point a cell fate decision has to be taken and proliferating cells can either return to a quiescent stem cell state, this is called self-renewal, or start to differentiate into muscle progenitors (called myoblasts).
Differentiation: In a last step, myoblasts (mononucleated cells) differentiate and fuse with each other to form new muscle fibers (multinucleated cells) or with pre-existing myofibers and regenerate muscle.
Fusion to repair damaged muscle fibers
2) Proliferation
Fusion to produce new muscle fibers
3) Regeneration

4. Symmetric versus Asymmetric Division
Symmetry Asymmetry
Stem
Stem
Stem
Stem
Diff
The difference between symmetric and asymmetric division is simple. Symmetric division occurs when one cell divide and give rise to two fully equivalent daughter cells. Equivalent daughter cells should have the same DNA, RNA, and protein content thus leading them to a similar cell fate and future function. During asymmetric division, some molecules (DNA, RNA, and/or Proteins) present in the mother cells are not randomly and equally distributed to the two resulting daughter cells. As a direct consequence, two non-equivalent daughter cells will have different cell fate and therefore different functions in the future.
Example of symmetric versus asymmetric cell division. In the center is schematically represented a stem cell labeled “Stem” that contains factors, shown by red dots, necessary to maintain the stem cell as a pluripotent cell (a means to continue the stem cell lineage).
Left-side: if symmetric division occurs, both daughter cells contain an equivalent number of these stem cell factors and, thus, both of the daughter cells become stem cells.
Right-side: if asymmetric division occurs, only one daughter cell inherit most of the stem cell factors (top daughter cell) and become a stem cell. The other one that lacks stem cell factors (bottom daughter cell) will differentiate into a specialized cell labeled “Diff”.
self renewal
differentiation
Asymmetric Division = Segregation of DNA, RNA or Proteins in one daughter cell

5. Symmetric versus Asymmetric Division in the Context of Muscle+ =
Satellite
Satellite
Option 1 + =
Option 2
Myoblast
Myoblast
Satellite
Dividing Satellite +
Proliferating satellite cells in response to injury (slide 3, step 2) can divide symmetrically or asymmetrically. As shown in the diagram, one could observe different outcomes for dividing satellite cells depending on their cell fate. During symmetric division, the resulting daughter cells can be both undifferentiated cells or stem cells (option 1) leading to amplification of the stem cell pool (=self renewal) or both differentiated (option 2) leading to an increased number of muscle regenerating cells. Asymmetric division of satellite cells will lead to two un-equivalent daughter cells, one self-renewing stem cell and one differentiating cells. That particular case allows muscle regeneration without exhaustion of the stem cell pool in order to respond to multiple injuries. =
Option 3
Satellite
Myoblast

6. Asymmetric Division in the Context of Muscle
Hoechst
Desmin
Sca-1
Merge
This slide shows asymmetric division of proliferating satellite cells and therefore represent an example of the schematic view slide 5 option 3. This figure was taken from Conboy MJ et al. PLoS Biol Jul;5(7):e182
Activated satellite cells were extracted from mouse muscle 3 days after injury and plated on glass slides to finish their division in vitro. Asymmetry was evidenced using the immunostaining technique, which basically allow the detection of proteins present in the cells, each protein being represented by one color. These four pictures represent the same two daughter cells.
In blue (left picture) nuclei are evidenced by Hoechst staining (labeling the DNA). Red and green colors represent respectively Desmin and Sca-1 proteins. Desmin protein, which is a muscle differentiation marker, is present in the bottom daughter cell at much higher quantity than in the top daughter cell. Sca-1 protein, which is a stem cell marker (a more immature cell marker), is more present in the top daughter cell than in the bottom one. These results support that satellite cells undergo asymmetric division, the top cell being more stem like and the bottom one being more differentiated. On the right is shown a merged picture superimposing the three different pictures (DNA, Desmin and Sca-1).
Differentiated cells are Desmin positive.
Non-differentiated cells are Sca-1 positive.
Conboy MJ et al. PLoS Biol Jul;5(7):e182

7. Summary of Asymmetric Division
Hoechst
While most cells in the body divide strictly symmetrically, stem cells have the ability to divide asymmetrically as well.
In the context of muscle regeneration, asymmetric division may allow for muscle regeneration without exhausting the pool of satellite cells (or muscle stem cells).
Understanding the mechanism of asymmetric division is essential to advancing the control of stem cell self-renewal versus differentiation. Such control is necessary for the use of stem cells in regenerative medicine.
Desmin
Sca-1
Merge