RESEARCH ARTICLE SUMMARY
Asymmetric division of clonal muscle
stem cells coordinates muscle
regeneration in vivo
David B. Gurevich, Phong Dang Nguyen, Ashley L. Siegel, Ophelia V. Ehrlich,
Carmen Sonntag, Jennifer M. N. Phan, Silke Berger, Dhanushika Ratnayake,
Lucy Hersey, Joachim Berger, Heather Verkade, Thomas E. Hall, Peter D. Currie*
INTRODUCTION: Mammalian skeletal muscle
harbors tissue-specific stem cells that are triggered
to replace damaged fibers after injury. Genetic
ablation of satellite cells in the mouse results in
a failure to regenerate muscle, which indicates
that these cells are the major (and possibly only)
mediators for repair of skeletal muscle. Further
evidence for the central role of satellite cells
in muscle regeneration comes from transplan-
tation experiments with genetically marked cells,
which demonstrate that satellite cells are highly
proliferative myogenic precursors capable of self‐
renewal and the resumption of quiescence, prop-
erties deemed important in a cell population
responsible for muscle repair. Considerable in
vitro evidence, derived from cultured fibers and
myoblasts, is suggestive of a role for asymmetric
division in generating both a self-renewing “im-
mortal” stem cell and a differentiation-competent
progenitor cell that proliferates and ultimately
replaces damaged muscle. However, asymmetric
division of satellite cells has not been documented
in vivo. Furthermore, considerable doubt re-
mains over how accurately in vitro studies can
model satellite cell behavior, given that the iso-
lation and culture of individual muscle fibers
and cells stimulates satellite cell proliferation.
Finally, it is not clear whether the environment
an activated satellite cell encounters in a single
fiber explant, or in culture, mimics the molecular and biophysical architecture of a regenerating muscle injury in vivo. Consequently, what
role, if any, the wound environment itself plays
in regeneration and self-renewal is difficult to
address in these systems.
RATIONALE: Using the optical clarity and genetic tractability of the zebrafish system, we
developed tools to track and image the regeneration of living muscle tissue after injury. Marking
muscle stem and progenitor cells with transgenes
and using long-term imaging and lineage-tracing
modalities enabled us to visualize cell movements
and behaviors during regeneration in vivo.
RESULTS: In vivo cell tracking permitted high-resolution imaging of the entire process of muscle regeneration, from injury to fiber replacement.
Using this approach, we were able to determine
the morphological, cellular, and genetic basis for
zebrafish muscle regeneration. Our analysis identified a stem cell niche in the zebrafish myotome
that is equivalent to the mammalian satellite cell
system, revealing that this evolutionarily ancient
stem cell is probably present throughout the vertebrate phylogeny. Complex interactions were observed between satellite cells and both injured and
uninjured fibers within the wound environment.
Among the most notable of these was the identification of filopodia-like projections, emanating
adhere to and “lasso” the activated satellite cell to guide
ittothewoundedge.Further-more, we documented the
in vivo occurrence of asymmetric satellite cell division,
a process that drives both self-renewal and regeneration via a clonally restricted progenitor pool.
CONCLUSION: Asymmetric divisions occur
during in vivo muscle regeneration to generate
clonally related progenitors required for muscle
repair. This finding resolves a long-term debate
surrounding the existence of this mechanism of
stem cell self-renewal and muscle repair in vivo.
Our results also reveal the highly dynamic nature
of the wound environment, where uninjured fibers
at the wound edge play a crucial role in directing
differentiating progenitors to regions of the wound
that are most in need of new fiber addition. ▪
The list of author affiliations is available in the full article online.
*Corresponding author. Email: firstname.lastname@example.org
Cite this article as D. B. Gurevich et al., Science 353, aad9969
(2016). DOI: 10.1126/science.aad9969
Mechanism of in vivo muscle repair. (A to C) Muscle regeneration is clonal. Regenerating fibers (outlined in white) express the same color after
fluorescent lineage tracing, indicating clonal derivation from a single stem cell. Sagittal, transverse, and coronal sections are shown in (A) to (C), respectively.
(D) Regeneration dynamics in vivo. Quiescent satellite cells, activated upon injury, undergo asymmetric division, which results in self-renewing or proliferating
cells. Proliferative cells undergo myogenesis to generate de novo immature fibers.
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