J Appl Physiol
91: 534–551, 2001.
invited review
Myogenic satellite cells: physiology
to molecular biology
THOMAS J. HAWKE1 AND DANIEL J. GARRY1,2
1
Department of Internal Medicine and 2Department of Molecular Biology,
University of Texas Southwestern Medical Center, Dallas, Texas 75390
skeletal muscle; stem cells; regeneration; aging; transgenic models
mammalian species exhibit a
remarkable capacity to adapt to physiological demands
such as growth, training, and injury. The processes by
which these adaptations occur are largely attributed to
a small population of cells that are resident in adult
skeletal muscle and are referred to as satellite cells.
After their initial identification in 1961 (98), Mauro
(117) described a cell closely associated with the periphery of the frog myofiber and termed it a satellite
cell based on its location. Quiescent satellite cells are
physically distinct from the adult myofiber as they
reside in indentations between the sarcolemma and
the basal lamina (130). Adult skeletal muscle fibers are
terminally differentiated such that muscle growth and
regeneration are accomplished by satellite cells. In the
unperturbed state, these cells remain in a nonproliferative, quiescent state. However, in response to stimuli such as myotrauma, satellite cells become activated, proliferate, and express myogenic markers
SKELETAL MUSCLES OF ADULT
Address for reprint requests and other correspondence: D. J.
Garry, UT Southwestern Medical Center at Dallas, 5323 Harry
Hines Blvd., NB11.200, Dallas, TX 75390-8573 (E-mail:
[email protected]).
534
(satellite cells expressing myogenic markers are also
termed myoblasts). Ultimately, these cells fuse to existing muscle fibers or fuse together to form new myofibers during regeneration of damaged skeletal muscle
(12, 167).
Since the original description of the myogenic satellite cell, considerable interest and research efforts
have focused on myogenic satellite cell biology.
These research efforts have enhanced our understanding of muscle growth, remodeling, and regeneration. In addition, new paradigms have been proposed regarding the regenerative capacity and the
plasticity of the myogenic satellite cell population
(108, 119, 139, 168, 169). These paradigms suggest
that the satellite cell population not only has a
remarkable capacity for muscle regeneration but
may also contribute to alternative muscle and nonmuscle lineages and may have clinical applications
in the treatment of devastating and deadly diseases
such as muscular dystrophy.
The current review attempts to integrate the anatomic, physiological, biochemical, and molecular properties that regulate the myogenic satellite cell population. We begin with a brief overview of vertebrate
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Hawke, Thomas J., and Daniel J. Garry. Myogenic satellite cells:
physiology to molecular biology. J Appl Physiol 91: 534–551,
2001.—Adult skeletal muscle has a remarkable ability to regenerate
following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a
small population of resident cells termed satellite cells. Although this
population of cells was identified 40 years ago, little is known regarding
the molecular phenotype or regulation of the satellite cell. The use of cell
culture techniques and transgenic animal models has improved our
understanding of this unique cell population; however, the capacity and
potential of these cells remain ill-defined. This review will highlight the
origin and unique markers of the satellite cell population, the regulation
by growth factors, and the response to physiological and pathological
stimuli. We conclude by highlighting the potential therapeutic uses of
satellite cells and identifying future research goals for the study of
satellite cell biology.
535
INVITED REVIEW
myogenesis, highlighting the populations of myoblast
precursor cells that contribute to muscle development,
and outline a well-described genetic hierarchy that is
important in muscle specification. We describe the limited gene expression profile and the distinguishing
morphological characteristics of the satellite cell population. We describe the inductive signals that regulate the satellite cell in vitro and in well-described
physiological models. Finally, we will review the stem
cell-like features of the satellite cells with emphasis on
the novel strategies that may be pursued in the future
for the treatment of debilitating myopathies.
Importantly, myogenic satellite cell biology remains an emerging field of scientific inquiry, such
that satisfying and complete answers to the most
fundamental questions are currently unavailable. In
this review, we will provide a summary of the current knowledge in this area and highlight fertile
areas for future research.
BUILDING OF MUSCLE WITH WAVES OF PRECURSOR
CELLS
Anatomic and molecular mechanisms during muscle
regeneration have been postulated to recapitulate
muscle development. Although current evidence suggests the regenerative process may be more complex,
an understanding of muscle development is important
to appreciate the anatomic and molecular network
associated with muscle regeneration.
During embryogenesis, the head, trunk, and limb
skeletal muscles develop as separate lineages. Of the
three germ layers in the early embryo, the paraxial
mesoderm gives rise to the somite (Fig. 1A). The somite
is subdivided into the dorsomedial (epaxial) domain,
which generates the muscles of the back, and the
ventrolateral (hypaxial) domain, which gives rise to the
abdominal, intercostal, and limb musculature (see
Refs. 123 and 140 for review).
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Fig. 1. Derivation of muscle precursor
cells during mouse embryogenesis. A:
precursor cells from the epaxial region
of the somite migrate to form the back
musculature. Precursor cells from the
hypaxial region of the somite migrate
to the newly formed limb buds. Note
the migrating limb precursor cells form
dorsal and ventral masses, which will
later become the extensor and flexor
muscle groups of the limb. Surrounding structures such as the neural tube,
notochord, dorsal aorta, and overlying
ectoderm potentially provide signals
regulating precursor cell movements
and fate. B: members of the MyoD family play an integral role in skeletal
muscle myogenesis. MyoD and my f 5
expression is involved in determination
of precursor cells to a myogenic fate,
whereas myogenin and MRF4 expression is associated with terminal
differentiation.
536
INVITED REVIEW
J Appl Physiol • VOL
MUSCLE PRECURSOR CELLS AND LIMB
DEVELOPMENT
Whereas the cells associated with the epaxial region
of the somite contribute to the primary myocytes of the
myotome, the hypaxial region of the somite (at the level
of the limb) contributes to the migratory limb muscle
precursor cells (141). Specification, migration, and differentiation of the myogenic precursor cells to their
distal targets are complex processes involving intrinsic
and extrinsic cues of which little is known. Recent
studies have shown that these myogenic precursor
cells express the paired domain transcription factor
Pax-3 (181), the tyrosine kinase receptor c-Met (205),
the homeodomain transcriptional repressor msx1 (11,
189), and the homeodomain transcription factor Lbx 1
(17, 71) but lack expression of the myogenic regulatory
factors of the MyoD family. After migration to the
developing limb, these precursor cells coalesce into the
dorsal and ventral premuscle masses, which will be the
future flexor and extensor compartments of the forelimb and now express members of the MyoD family (19,
137, 141).
Targeted gene disruption studies (i.e., knockout
mouse models) have begun to provide insight into the
molecular regulation of limb development. In mice
lacking either Pax-3 (16, 60) or c-Met (50), limb precursor cells fail to migrate into the limb, resulting in
the complete loss of limb muscles. Combinatorial
knockout experiments crossing the Pax-3 mutant
mouse (Splotch) into the myf 5 null background result
in a further perturbation of myogenesis and an absence
of both limb and body wall muscle (181). Additional
studies support a genetic hierarchy where Pax-3 mediates the activation of other myogenic regulatory factors
(myf 5 and MyoD) and functions as a key regulator of
somitic myogenesis (113, 181). In addition, Pax-3 functions in the specification of the limb precursor cells and
is upstream of both c-Met and Lbx 1 (17, 50, 60, 181).
Inactivation of the Lbx 1 locus by homologous recombination results in an extensive loss of limb muscles,
although residual muscle groups are still present. This
finding suggests that Lbx 1 is not required for the
specification of limb muscles but may function in the
determination of which migratory highway the precursors should pursue (17, 71).
Proliferating limb myoblasts coalesce into the ventral and dorsal premuscle masses, withdraw from the
cell cycle, and form multinucleated primary myofibers
at approximately embryonic day 13 postcoitum (E13)
in the mouse embryo. In a process that is less clearly
defined, secondary myofibers form parallel to the primary fibers and constitute the predominant multinucleated myofibers during the latter stages of embryogenesis (E15–E16 in the mouse) and in postnatal
skeletal muscle (56, 123). Studies suggest that two
distinct lineages generate primary myofibers (i.e., embryonic myoblasts) and the secondary myofibers (i.e.,
fetal myoblasts). Furthermore, primary myofibers differ from secondary myofibers in their temporal devel-
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During somitogenesis, cross talk involving growth
factors (Wnt proteins, Sonic hedge hog, bone morphogenic proteins, and so forth) and transcription factors
(myf 5, MyoD, Pax-3, and so forth) occurs between the
developing somite and the anatomically adjacent structures, including the overlying ectoderm, the ventromedial neurotube and notochord, and the vascular structures including the aorta (38, 113, 123, 131, 145). The
constellation of these positional cues (i.e., growth and
transcription factors) results in the specification of
muscle through the regulation of a distinct molecular
(hierarchical) cascade.
Since the discovery of MyoD in 1987, the role of the
myogenic basic helix-loop-helix (bHLH) transcription factors in skeletal myogenesis has been defined
in elegant detail by several groups (19, 126, 137, 147,
148, 192, 200, 207). This subset of the bHLH family
includes MyoD, my f 5, myogenin, and MRF4. Each of
these myogenic bHLH proteins forms heterodimeric
DNA binding complexes that include other bHLH
proteins of the E2 gene family (E12 and E47) and
bind a canonical DNA sequence, CANNTG (E-box),
within enhancer elements of genes that encode terminal differentiation markers of the skeletal muscle
lineage (41, 103). MyoD family members share the
ability to activate skeletal muscle differentiation
when expressed ectopically in nonmuscle cells (115,
193). The essential role played by bHLH proteins in
skeletal myogenesis has been demonstrated unambiguously by gene disruption experiments (78, 138,
142, 147, 148, 157, 185). The results of these gene
knockout experiments support a role for my f 5 and
MyoD in the determination of the myogenic cell fate
and the formation of myoblasts during embryogenesis (Fig. 1B). Myogenin and MRF4 appear to function
in activation of muscle differentiation (149, 175).
Although a number of biochemical and transgenic
studies support an integral role for the myogenic
bHLH proteins during development, it is also clear
that the MyoD family members interact in a combinatorial fashion with known transcription factors
such as members of the MADS box family (i.e., myocyte enhancer factor 2 or MEF-2; Refs. 41, 103), cell
cycle regulatory proteins, and currently undefined
factors to regulate myogenesis (see Refs. 126 and 175
for review). Although the role of MyoD family members during embryogenesis has been defined in great
detail, the functional role of these family members in
established postnatal skeletal muscle remains unclear. Studies support the localization of MyoD and
myogenin in fast-twitch and slow-twitch adult myofibers, respectively, suggesting a function for these
myogenic regulatory factors in fiber-specific contractile protein gene expression (84, 85, 171). Future
studies utilizing transgenic technologies such as
conditional or tissue-restricted knockout strategies
of these myogenic regulatory factors will be useful
in the definition of their role in adult skeletal
muscle.
537
INVITED REVIEW
opment, size, number, and expression of myosin heavychain isoforms (123, 175).
The discovery of satellite cells in the early 1960s
demonstrated the existence of yet an additional population of proliferative cells that contributed to postnatal growth, the maintenance of adult skeletal muscle,
and the repair of damaged myofibers. Myogenic satellite cells are present in the limbs of midgestational
mouse embryos after E15 (35). After birth, the satellite
cell population accounts for ⬃30% of sublaminar muscle nuclei in neonatal hindlimb skeletal muscle (12).
These neonatal satellite cells fuse to growing myofibers
to contribute additional nuclei during postnatal growth
of skeletal muscle.
SOMITIC VS. NONSOMITIC ORIGIN OF SATELLITE
CELLS
J Appl Physiol • VOL
SATELLITE CELL IDENTIFICATION
Anatomic identification. Resident within adult skeletal muscle is a pool of undifferentiated mononuclear
cells termed satellite cells because of their anatomic
location at the periphery of the mature, multinucleated
myotube. The defining characteristic of the satellite
cell is that the basal lamina that surrounds the satellite cell and the associated myofiber is continuous
(167). As shown in Fig. 2, the identification of this cell
population has historically utilized ultrastructural
techniques (66, 161). Other distinguishing morphological features of the satellite cell population include a
relatively high nuclear-to-cytoplasmic ratio with few
organelles, a smaller nuclear size compared with the
adjacent nucleus of the myotube, and an increase in
the amount of nuclear heterochromatin compared with
that of the myonucleus (167). These morphological
features are consistent with the finding that satellite
cells are relatively quiescent and transcriptionally less
active. These distinguishing features are absent following activation or proliferation of the satellite cells in
response to growth, remodeling, or muscle injury. After
activation, the satellite cells are more easily identified
as they appear as a swelling on the myofiber with
cytoplasmic processes that extend from one or both
poles of the cell (Fig. 2; Ref. 167). Associated with the
increase in mitotic activity, there is a reduction in
heterochromatin, an increase in cytoplasmic-to-nuclear ratio, and an increase in the number of intracellular organelles (167).
Satellite cell markers. The profile of gene expression
of the quiescent satellite cell as well as their activated
and proliferating progeny is largely unknown. The
quiescent satellite cells do not express myogenic regulatory factors of the MyoD or MEF2 families or other
known markers of terminal differentiation (33, 119,
201). Through the identification of satellite cell markers, biologists will be able to address issues related to
the developmental origin of the satellite cell, the cell
cycle control, and the molecular regulation of this
unique cell population during growth and regeneration. Several satellite cell markers have been identified
and are restricted to either the quiescent, activated, or
proliferative state or are expressed more broadly (Table 1).
We have previously determined that myocyte nuclear factor (MNF or Foxk1), a member of the winged
helix transcription factor family, is localized to the
quiescent satellite cell in adult skeletal muscle (64).
We have identified two alternatively spliced isoforms
for MNF and termed them MNF-␣ and MNF-␤ (64,
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Previous studies support the hypothesis that muscle
precursor cells, including the myogenic satellite cell
population, originate from the multipotential mesodermal cells of the somite (58, 141, 167). The support for
this hypothesis is primarily derived from chimeric or
interspecies grafting experiments that have been performed in avian models. These fate-mapping studies
involved the transplantation (or exchange) of embryonic somites from donor quail embryos into host chick
embryos (27, 105). The transplanted quail cells have
distinguishing morphological characteristics and were
observed to migrate from the somite and contribute to
both the limb muscles and the satellite cell population
in postnatal chick skeletal muscle. Satellite cells have
been isolated from the fetal skeletal muscle from an
E15 or older mouse embryo, suggesting that satellite
cells populate the developing limb during the latter
stages of embryogenesis (34, 35, 36). Whether the satellite cells migrate from the somite as a distinct lineage
or whether they originate from a preexisting lineage
(i.e., embryonic or fetal myoblasts) in the developing
limb is unclear. Nevertheless, the underlying concept
was that each of the myoblast precursor cells (i.e.,
embryonic myoblasts, fetal myoblasts, and satellite
cells) was a derivative of the somite.
This paradigm has recently been challenged, as studies have suggested that multipotential cells of nonsomitic origin may be the precursors of the satellite cell
(45, 139). De Angelis et al. (45) reported that cells
isolated from the embryonic dorsal aorta had a similar
morphological appearance and a similar profile of gene
expression to that of satellite cells. Furthermore,
transplantation of the aorta-derived myogenic cells
into newborn mice revealed that this cell population
participated in postnatal muscle growth, regeneration,
and fusion with resident satellite cells. The authors
proposed that satellite cells may be derived from endothelial cells or a precursor common to both the satellite
cell and the endothelial cell.
Derivation of the satellite cell from the somite or a
nonsomitic source need not be mutually exclusive. Conceivably, both lineages may contribute under physiological or pathological states to the satellite cell popu-
lation. Additional fate-mapping strategies will be
needed to further define and dissect the lineage derivation of all the satellite cells that reside in adult
skeletal muscle. These studies will be important in
defining whether there is a common lineage source for
the entire satellite cell population and a common lineage source for cells that have regenerative capacities
in both muscle and nonmuscle tissues.
538
INVITED REVIEW
Fig. 2. Satellite cells occupy a sublaminar position in adult skeletal muscle.
In the uninjured muscle fiber, the satellite cell is quiescent and rests in an
indentation in the adult muscle fiber.
The satellite cells can be distinguished
from the myonuclei by a surrounding
basal lamina and more abundant heterochromatin. When the fiber becomes
injured, the satellite cells become activated and increase their cytoplasmic
content. The cytoplasmic processes allow for chemotaxis of the satellite cell
along the myofiber. Bar ⫽ 1 ␮m.
Table 1. Expression patterns of satellite cell markers
in adult skeletal muscle
Molecular Marker
MNF
Pax7
c-Met
M-cadherin
NCAM
VCAM-1
Desmin
myf5
MyoD
BrdU
PCNA
[3H]thymidine
Expression Observed in the Adult
Quiescent, activated, and proliferating
satellite cells
Quiescent, activated, and proliferating
satellite cells
Quiescent, activated, and proliferating
satellite cells
Quiescent, activated, and proliferating
satellite cells
Quiescent, activated, and proliferating
satellite cells; synaptic junctions in
adult myofibers
Quiescent, activated, and proliferating
satellite cells
Activated and proliferating satellite
cells
Activated and proliferating satellite
cells
Activated and proliferating satellite
cells
Proliferating cells
Proliferating cells
Proliferating cells
Reference
64
169
33
33
39
89
15
33
33, 124
163
90
128
Expression of selected molecular markers used to identify the
satellite cell population in adult skeletal muscle is outlined. MNF,
myocyte nuclear factor; NCAM and VCAM, neural cell and vascular
adhesion molecule; BrdU, bromodeoxyuridine; PCNA, proliferating
cell nuclear antigen.
J Appl Physiol • VOL
growth deficit, a marked impairment in muscle regeneration (63), and a decreased number of satellite cells
in adult MNF mutant skeletal muscle (Hawke and
Garry, unpublished observations). Additional winged
helix family members have also been identified in stem
cells or regenerating cells including Genesis (Foxd3;
Ref. 83), which is expressed selectively in embryonic
stem cells, and a protein related to hepatocyte nuclear
factor-3 (HNF3/forkhead homolog 11), which has been
identified in regenerating hepatocytes (206).
Utilizing single cell RT-PCR analysis, Cornelison
and Wold (33) characterized the satellite cells as a
heterogeneous population based on their profile of
gene expression. Additionally, they identified c-Met,
the receptor for hepatocyte growth factor (HGF), as a
marker of quiescent satellite cells. HGF is a potent
mitogen for satellite cells and has been shown to be
important in the migration of the myogenic precursor cells from the somite to the developing limb (5,
14). Moreover, c-Met deficient embryos fail to form
limb skeletal muscle due to a lack of myogenic precursor cells (14, 112).
Irintchev and colleagues (87) identified M-cadherin,
a calcium-dependent cell adhesion molecule, as a
unique marker of the satellite cell pool. M-cadherin is
only expressed in a subpopulation of the quiescent cell
pool; however, its expression is increased when the
satellite cells become activated in response to a stimulus (9, 33). Recent studies suggest that other cell
adhesion molecules, neural cell adhesion molecule
(NCAM) and vascular adhesion molecule-1 (VCAM-1),
are also potential markers of quiescent satellite cells
(39, 89). The role of these adhesion molecules is unclear
but collectively (NCAM, VCAM-1, and M-cadherin)
may function in the adhesion of the satellite cell to the
basal lamina of the myofiber and may participate in
the migratory capacity of this cell population in response to stimuli. NCAM is expressed in both myofibers and satellite cells, whereas VCAM-1 is broadly
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203, 204). These two alternatively spliced isoforms are
reciprocally expressed during myogenesis and during
muscle regeneration, suggesting that the two isoforms
of MNF may exert opposing effects on target genes at
discrete steps during muscle regeneration or in renewal of the satellite cell population (63). Using a
RT-PCR assay, we have shown that MNF-␤ is the
principal form expressed in quiescent satellite cells,
whereas MNF-␣ predominates in proliferating satellite
cells following muscle injury. Disruption of the MNF
locus, mutating both isoforms, resulted in a severe
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INVITED REVIEW
SATELLITE CELL QUANTITATION AND DISTRIBUTION
Quantitation of the satellite cell population in adult
skeletal muscle has been possible primarily through
the use of ultrastructural techniques. More recently,
immunohistochemical techniques have been utilized
for the identification of the satellite cell pool. Although
a limited number of markers for the quiescent satellite
cell population exist, proliferating satellite cells, as
measured by [3H]thymidine or bromodeoxyuridine
(BrdU) incorporation, can be identified immunohistochemically for the coexpression of the MyoD family
members or the intermediate filament protein, desmin
(15, 104, 123). MyoD expression occurs early during
the activation of the satellite cell population (within 6 h
following muscle injury) (63, 74, 100, 119, 134). In
addition, several nonselective markers of cellular proliferation have been used to characterize the proliferating satellite cell pool. These markers of cellular proliferation include proliferating cell nuclear antigen
(90), BrdU (163), and [3H]thymidine (128).
Satellite cell number is dependent on the species,
age, and muscle fiber type (Table 2; see Ref. 167 for
review). Satellite cells constitute ⬃30% of the muscle
nuclei in the neonate and decrease with age to ⬃4% in
the adult and ⬃2% in the senile (29–30 mo) mouse
(176). The decrease in the percentage of satellite cells
with aging (i.e., senile rodent) is the result of an increase in myonuclei (oxidative and glycolytic myofibers) and a decrease in total number of satellite cells
(glycolytic myofibers) (12, 66, 167).
The satellite cell distribution between muscle groups
is a result of the heterogeneity in satellite cell content
Table 2. Satellite cell content in skeletal muscle
Model
Rat
Rat
Rat
Rat
Rat
Mouse
Mouse
Quail
Muscle
TA
Soleus
Diaphragm
LD
Soleus
Soleus
Soleus
EDL
EDL
EDL
EDL
EDL
EDL
Levator ani
Levator ani
Soleus
Soleus
Gastroc
Gastroc
LD
LD
Human
Human
Human
Pig
Lizard
Trapezius
Trapezius
Biceps brachii
Biceps brachii
Sartorius
PL
Tail
Tail
Age and Protocol
%SC
7–9 wk
7–9 wk
7–9 wk
adult
1 mo
1 yr
2 yr
1 mo
1 yr
2 yr
4 mo
Hypothyroid
Hypothyroid; chronic stimulation
4 mo
32 mo
8 mo
30 mo
7–10 days
Pax 7⫺/⫺
6 wk
Stretched
Control patients
DMD patients
Control (38 yr)
Resistance trained
⬍30 mo
Werdnig-Hoffman infants
64 wk
64 wk
Lygosoma species
Anolis species
4
11
8
4.5
9.6
6.6
4.7
7.0
2.9
1.9
3.8
3.8
7.9–13.8
1.9
1.2
4.6
2.4
25
0
15.6
16.7
15
25
3.7
5.4
8.4
14.4
1.1
4.3
7.5
4.8
SC#/muscle
5.2 ⫻ 105
7.3 ⫻ 105
5.4 ⫻ 105
3.1 ⫻ 105
2.1 ⫻ 105
1.3 ⫻ 105
Reference
161
161
161
2
66
66
66
66
66
66
146
146
146
135
135
176
176
169
169
198
198
190
190
94
94
158
158
21
21
95
95
Satellite cell (SC) content in various species under varying physiological and pathological conditions. TA, tibialis anterior; LD, latissimus
dorsi; Gastroc, gastrocnemius; EDL, extensor digitorum longus; PL, peroneus longus; DMD, Duchenne muscular dystrophy.
J Appl Physiol • VOL
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expressed during embryogenesis but limited to satellite cells in adult muscle (39). Furthermore, VCAM-1
has been shown to mediate satellite cell interaction
with leukocytes following injury (89).
Recent work from the Rudnicki laboratory (169)
identified the paired box transcription factor, Pax7,
expressed selectively in quiescent and proliferating
satellite cells. Analysis of the Pax7 mutant skeletal
muscle revealed a complete absence of satellite cells.
This novel finding supports the hypothesis that Pax7 is
essential for the specification of the satellite cell population. Future studies will be important in the definition of Pax7 downstream target genes in the satellite
cell population and may provide insight into the regulation of this cellular pool.
Identification of other satellite cell markers is the
focus of current research efforts. Emerging candidates
include the cell surface antigen Sca-1 (stem cell antigen-1; Refs. 88, 177), the glycoprotein Leu-19 (94, 162),
the anti-apoptotic factor Bcl-2 (106, 123), CD34 (9), and
interferon regulatory factor-2, which is a transcription
factor that mediates VCAM-1 expression in skeletal
muscle (89).
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INVITED REVIEW
between muscle fiber types. An increase in satellite cell
density has been demonstrated in association with the
proximity of capillaries (161), myonuclei (18, 161), and
motoneuron junctions (199). The proximity of satellite
cells to these anatomic structures suggests a permissive role in the regulation of the cellular pool. In support of this hypothesis, oxidative fibers (characterized
by increased capillary and motoneuron density compared with glycolytic myofibers) demonstrate a five to
six times greater satellite cell content (65, 161).
GROWTH FACTORS AS REGULATORS OF THE
SATELLITE CELL POPULATION
Growth Factor
Chemotactic
Ability
Proliferation Differentiation Reference
1
1
1
FGF
NE
1
1
HGF
IGF-I
IGF-II
TGF-␤
1
1
Macrophages
NE
Crushed muscle/
plateletderived
extract
LIF
IL-6
PDGFAA/AB
PDGFBB
EGF
1
1
NE
1
NE
NE
2
1
1
1
1
1
1
79
3
54
13
152
13
4
124
182
79
3
54
26(t)
79
54
79
152
3
13
208
152
121
13
86(t)
1
2
79
1
1
NE
1
NE
NE
1
2
13
7
152
7
152
54
152
13
13
*54
1
1
1
1
1
1
1
1
1
1
1
2
1*
2
2
1
1
2
2
A number of factors affect satellite cell proliferation, differentiation, and chemotaxis. All studies were performed using cell culture
techniques except those denoted with a (t), which were performed
using skeletal muscle tissue. NE, no effect. FBF, HGF, IGF, TGF,
PDGF, and EGF, fibroblast, hepatocyte, insulin-like, transforming,
platelet-derived, and endothelial growth factor, respectively. LIF,
leukemia-inhibitory factor. * Stimulation of satellite cell proliferation in the presence of serum but no effect in serum-free medium. 1
and 2, Increase or decrease in effect, respectively.
proliferation (25, 32, 170). IGF-I-stimulated satellite
cell differentiation appears to be mediated through the
PI-3K pathway (32). Additional studies, utilizing genetic mouse models (i.e., transgenic overexpression or
knockout models), may further define the regulation
and signaling pathways of the IGFs and satellite cell
biology (8, 25, 132).
Hepatocyte growth factor. Hepatocyte growth factor
(HGF) is a multifunctional cytokine initially described
as a mitogen in mature hepatocytes (122). Recently,
HGF and its receptor c-Met have been localized to
satellite cells and adjacent myofibers but are absent in
the adjacent fibroblasts. In addition, HGF expression is
proportional to the degree of muscle injury (33, 174,
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The process of muscle regeneration requires the influence of growth factors and a sequence of cellular
events, which results in the regulation of the satellite
cell population (Table 3, Fig. 3; Refs. 73, 168). Many of
the studies that have examined the effect of growth
factors on satellite cell biology have utilized satellite
cell cultures. These studies have defined the effect of
growth factors alone or in combination and have provided valuable insight into the regulation of the satellite cell. Admittedly, in vitro studies are limited due to
the lack of permissive and repressive factors that are
present in vivo and may influence cellular activity.
Currently, most satellite cell cultures are derived from
neonatal skeletal muscle due to the abundance of satellite cells in these tissues compared with older animals (⬎30% in young animals vs. ⬃5% in older animals; Ref. 143). The population and age of the satellite
cell is an important consideration in cell culture preparations as the response of aged, quiescent satellite
cells to growth factor stimulation differs compared
with young, proliferating satellite cells (182). This section will provide a brief introduction of growth factors
that are important in the regulation of satellite cell
proliferation, differentiation, and motility (for review,
see Table 3).
Insulin-like growth factors. Skeletal muscle secretes
insulin-like growth factors I and II (IGF-I and IGF-II),
which are known to be important in the regulation of
insulin metabolism (3, 109, 186). In addition, these
growth factors are important in the regulation of skeletal muscle regeneration. IGF-I and IGF-II increase
satellite cell proliferation and differentiation in vitro
(Table 3). The importance of these growth factors was
demonstrated with the intramuscular administration
of IGF-I into older, injured animals. In this study,
IGF-I administration (using an osmotic minipump) resulted in enhanced satellite cell proliferation and increased muscle mass (26). Moreover, skeletal muscle
overload or eccentric exercise results in elevated IGF-I
levels, increased DNA content (suggesting an increase
in satellite cell proliferation), and a compensatory hypertrophy of skeletal muscle (1, 202).
IGF-I appears to utilize multiple signaling pathways
in the regulation of the satellite cell pool. The calcineurin/NFAT, mitogen-activated protein (MAP) kinase, and phosphatidylinositol-3-OH kinase (PI-3K)
pathways have all been implicated in satellite cell
J Appl Physiol • VOL
Table 3. Factors affecting satellite cell activity
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Fig. 3. Factors modulate satellite cell activity.
Growth factors and hormones are released from
a number of tissues and modulate satellite cell
activity (chemotaxis, proliferation, and differentiation). These factors utilize signaling pathways in the regulation of the satellite cell. LIF,
leukemia inhibitory factor; IL-6, interleukin-6.
EGF, FGF, HGF, IGF, PDGF, and TGF, endothelial-derived, fibroblast, hepatocyte, insulinlike, platelet-derived, and transforming growth
factors, respectively.
J Appl Physiol • VOL
injury. In contrast, Fiore et al. (57) pursued a similar
targeting strategy to mutate the FGF-6 locus and observed an absence of defects in response to either a
crush injury or chemically induced injury (notexin).
Consequently, the functional role(s) of FGF-6 during
muscle regeneration remains unclear. Nevertheless,
these studies underscore the ability of redundant family members to compensate for one another and result
in preserved function under pathological conditions
(i.e., mouse knockout models).
The release of FGF-2 from the damaged myofibers,
like HGF, is proportional to the degree of injury (29).
FGF levels are coordinated with FGF receptor expression. When receptor expression is increased, satellite
cells propagated in culture demonstrate an increased
proliferation and decreased differentiation (160). Conversely, when receptor expression is diminished, proliferation is decreased and there is a concomitant increase in satellite cell differentiation. Interestingly,
during the period of satellite cell activation and proliferation (0–48 h after injury), FGF receptor (FGF-R1)
mRNA is increased fivefold, and this increase is further
enhanced in the presence of HGF (173).
The signaling pathway(s) that transduces the FGF
signal has recently been investigated with the use of
both transgenic techniques and pharmacological inhibitors (92). These studies revealed that the MAP kinase
pathway is important in transducing the FGF-induced
increase in satellite cell proliferation; however, the
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182). Multiple roles for HGF have been proposed for
the regulation of the satellite cell, including a role as a
potent chemotactic factor, an activator of the satellite
cell, and an inhibitor of myoblast differentiation (Table
3). HGF is capable of activating and selectively promoting satellite cell proliferation (4). Furthermore, HGF
administration attenuates satellite cell differentiation
through the transcriptional inhibition of the myogenic
regulatory factors (i.e., MyoD and myogenin) (62).
Fibroblast growth factors. Fibroblast growth factor
(FGF) has nine different isoforms (FGF-1 to FGF-9).
Although many of the FGF isoforms are broadly expressed, FGF-6 is restricted to skeletal muscle (59).
Sheehan and Allen (173) investigated in detail the role
of the FGF family on satellite cell proliferation in
culture. In these studies, it was demonstrated that
FGF-1, -2, -4, -6, and -9 stimulated cellular proliferation, whereas FGF-5, -7, and -8 had no mitogenic activity. The investigators further observed that addition
of HGF to either FGF-2, -4, -6, or -9 resulted in a
synergistic increase in satellite cell proliferation. In
addition to an increase in satellite cell proliferation,
the FGF family has also been observed to attenuate
satellite cell differentiation to myofibers (30, 91, 173,
178).
Floss et al. (59) reported that mice deficient for
FGF-6 (i.e., knockout mice at the FGF-6 locus) have
impaired satellite cell proliferation and a subsequent
defect in muscle regeneration in response to a crush
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J Appl Physiol • VOL
dosages did not appreciably affect muscle regeneration
(125). Future studies that combine cell culture methodologies and overexpression or loss of function models
using molecular technologies will be helpful in the
definition of the role of growth factors in satellite cell
biology.
FUNCTIONAL RESPONSES OF SATELLITE CELLS
TO PHYSIOLOGICAL STIMULI
Hypertrophic stimuli. Load-induced hypertrophy
(chronic stretch, agonist ablation, tenotomy) and resistance training are physiological challenges that promote a hypertrophic response in both human and animal models (154–156, 197). Hypertrophic growth of
skeletal muscle is stimulated by short bursts of muscle
activity against high resistance. Resistance training
induces muscle hypertrophy through a process of satellite cell activation, proliferation, chemotaxis, and fusion to existing myofibers to contribute to muscle
growth (Fig. 4; Ref. 167). The migratory capacity (chemotaxis) of satellite cells is dependent on the integrity
of the basal lamina. After the rupture or interruption
of the basal lamina in response to myotrauma, satellite
cells may migrate to adjacent myofibers utilizing tissue
bridges (164, 191). In response to limited myotrauma,
where no rupture of the basal lamina occurs, satellite
cells migrate from the proximal intact portion of the
myofiber, under the basal lamina, to the site of injury
to participate in the repair process (165, 167).
Exercise-induced myotrauma initiates an immune
response, resulting in the influx of macrophages into
the damaged region. After the acute insult, macrophage infiltration peaks within 48 h (186). Initially, the
role of these blood-borne macrophages was believed to
be limited to phagocytosis and the digestion of myonecrotic fibers. However, additional roles for macrophages during the early stages of muscle repair are
emerging. Macrophages are essential in the orchestration of the repair process as they secrete a collection of
cytokine factors that regulate the satellite cell pool
(133). Importantly, in the absence of a macrophage
response, muscle regeneration is absent; in the presence of an enhanced macrophage response, there is an
increase in satellite cell proliferation and differentiation (110).
In response to resistance training, myotrauma results in the release of growth factors that will, in part,
regulate the satellite cell population during regeneration (Fig. 3 and Table 3). For example, IGF-I is upregulated in response to hypertrophic signals in skeletal
muscle and promotes proliferation and fusion of the
satellite cell pool (1, 26, 186). As outlined in the previous section, additional growth factors and/or cytokines
including LIF and members of the TGF-␤ family may
play a role in the signaling or commissioning of the
satellite cells to participate in the hypertrophic remodeling response. Although a number of questions remain
regarding the role of the satellite cell in muscle remodeling, the primary physiological consequence of the
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MAP kinase signaling pathway did not mediate the
FGF-mediated repression of satellite cell differentiation.
Transforming growth factors. Transforming growth
factor-␤ (TGF-␤) is the prototypical family member of
cytokines that includes bone morphogenic protein and
growth-differentiation factors. The TGF-␤ family of
cytokines transduces their signal through the SMAD
family of proteins (see Ref. 196 for review). Generally,
the TGF-␤ family members function to inhibit muscle
proliferation and differentiation (3, 70, 97, 99, 208) by
silencing the transcriptional activation of the MyoD
family members (115). This inhibition of differentiation by TGF-␤, like FGF, persists even in nonmuscle
cell lines engineered to ectopically express members of
the MyoD family (97, 115, 184). The combination of
IGF-I or FGF with TGF-␤ was unable to alter the
TGF-␤-induced attenuation of satellite cell differentiation; however, TGF-␤ action had little effect on IGF-Ior FGF-mediated increases in proliferation (70). During muscle regeneration, TGF-␤ receptor levels (TGF-␤
RII) and TGF ligand are reciprocally expressed, resulting in the initial promotion of cellular proliferation
followed by enhanced muscle differentiation (159).
Interleukin-6 cytokines. Leukemia inhibitory factor
(LIF) and interleukin-6 (IL-6) are members of the IL-6
family of cytokines produced by many different cells,
including myoblasts and macrophages. These cytokines share a common receptor component, and their
actions are mediated through the same signaling pathways (81, 144). Skeletal muscle regeneration after injury in LIF mutant mice is attenuated, whereas exogenous administration of LIF increased the regenerative
process and produced enlarged myofibers (101). The permissive effect of LIF was associated only with the muscle
lineage and had no effect on nonmuscle cells in skeletal
muscle (101). IL-6 promotes the degradation of necrotic
tissue, synchronizes the cell cycle of satellite cells, and
induces apoptosis of macrophages following muscle injury (22). Unlike LIF, however, IL-6 expression in injured
muscle does not increase satellite cell proliferation (96).
Collectively, this family of growth factors appears to play
an integral role in skeletal muscle regeneration.
Many other factors may be involved in the regulation
of the satellite cell in adult skeletal muscle. Nitric
oxide, platelet-derived growth factor, endothelial-derived growth factor, and testosterone have been shown
to mediate satellite cell activity (6, 93, 133). Obviously,
the regulation of satellite cells is orchestrated by numerous factors in a temporal and concentration-dependent fashion during regeneration.
Few animal studies have examined the effect of
growth factors in vivo. Chakravarthy et al. (26) observed that local IGF-I administration to atrophied
muscle increased satellite cell proliferation and muscle
mass within 2 wk. Unlike the observations with IGF-I,
the intramuscular injection of HGF, at specified intervals following skeletal muscle injury, increased satellite cell proliferation and either had no effect or impaired the rate of regeneration (124). Similarly,
administration of FGF at timed intervals and selected
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Fig. 4. Satellite cell response to myotrauma.
*Skeletal muscle trauma or injury may be minor
(e.g., resistance training) or may be more extensive (e.g., toxin injection, Duchenne muscular
dystrophy). In response to an injury, satellite
cells become activated and proliferate. Some of
the satellite cells will reestablish a quiescent
satellite cell pool through a process of self-renewal. Satellite cells will migrate to the damaged
region and, depending on the severity of the
injury, fuse to the existing myofiber or align and
fuse to produce a new myofiber. In the regenerated myofiber, the newly fused satellite cell nuclei will initially be centralized but will later
migrate to assume a more peripheral location.
J Appl Physiol • VOL
dition, there are an increased number of satellite cells
associated with the neuromuscular junction (199). It is
conceivable that neurotrophic factors are important in
satellite cell homeostasis. A number of laboratories
have reported that the percentage of satellite cells
increase (from 3 to 9%) during the initial period following denervation (118, 187). However, a prolonged period of denervation results in a significant decrease in
the percentage of satellite cells (3 to 1% following an
18-mo period of denervation). Viguie et al. (187) hypothesize that the progressive decline in the satellite
cell pool may be the result of satellite cell apoptosis.
Alternatively, denervation may result in a lack of neurotrophic input (including growth factors) that negatively impacts satellite cell function and content.
Long-term denervation has considerable clinical implications. Denervation for periods of 6–18 mo results
in the inability of skeletal muscle to reestablish a
preinjury functional capacity even if neuronal sprouting and regeneration occurs (180). The mechanisms for
this phenomenon are unclear, but considerable data
support the conclusion that the intact neuromuscular
junction and the denervation model mediate positive
and repressive influences, respectively, on the satellite
cell pool (187).
Aging. Recent advances have allowed biologists to
interrogate the proliferative history of a cellular population. With each cell replication, there is ⬃100 bp lost
from the ends of eukaryotic chromosomes (47, 48, 151,
172). The ends of the chromosomes contain TTAGGG
repeats termed telomeres. The length of these telomeres reflects the number of replications of a particular cell and its proliferative capacity. Using this technology, investigators now have the ability to analyze
the proliferative history and future capacity of the
satellite cell, providing valuable insight into the effects
of aging and disease in this cell population.
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hypertrophic response is to produce a muscle with a
greater capacity for peak force generation.
Atrophic stimuli. Atrophy of skeletal muscle results
in a reduction in myonuclei number and can be induced
by numerous factors including denervation, hindlimb
suspension, and malnutrition (73). Atrophy and remodeling that result from muscle disuse can be produced in laboratory rodents physiologically by tail/
hindlimb suspension or immobilization of specific
muscle groups in plaster casts or pathologically
through denervation. The response of the satellite cells
appears to be pleiotropic and dependent on the atrophic stimulus.
In adolescent rats, hindlimb suspension results in an
irreversible remodeling process, including decreased
satellite cell content and an impaired proliferative capacity within 3 days of unloading in both the oxidative
slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles (44, 129). Thus the atrophic
stimulus in the adolescent animal may irreversibly
alter the developmental program for myofibers to accrue nuclei even with the resumption of weight bearing. A similar reduction in the satellite cell population
was observed in adult rat hindlimbs using an immobilization model as an alternative atrophic stimulus
(188). In contrast to adolescent animals, remobilization
of the hindlimb was accompanied by increased myofiber regeneration, supporting the hypothesis that, following completion of the developmental program, adult
satellite cells are capable of activation and proliferation to repopulate atrophied muscle (129, 188).
Unlike the other forms of atrophy, denervation is a
pathological rather than physiological stimulus. Denervation produces a form of disuse atrophy that includes myofiber degeneration and is accompanied by
distinctive changes in the myonuclei and quiescent
satellite cells (102, 111, 153). In the unperturbed con-
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This question is applicable to the young population as
well. If satellite cells have a limited proliferative capacity (⬃60 doublings), does a lifetime of intense exercise have a negative influence on their ability to regenerate skeletal muscle as aging progresses?
FUNCTIONAL RESPONSES TO DISEASE STATES
Most myopathies have a molecular mutation that
affects the structural or cytoskeletal proteins in skeletal muscle. Duchenne muscular dystrophy (DMD) is
the most common and the most devastating of the
muscular dystrophies (20, 55, 76, 82, 127). Disease
progression and death are ultimately due to a failure of
the myogenic satellite cells to maintain muscle regeneration (36, 80). DMD is a recessive X-linked disease
that results in a null mutation at the dystrophin locus
(20, 82, 127). The absence of this cytoskeletal protein
renders the muscle fiber extremely fragile. In response
to mechanical stress associated with repeated contraction, there is widespread degeneration. The satellite
cells respond to the injury by repopulating the injured
skeletal muscle with defective myofibers lacking dystrophin. This process results in continuous degeneration-regeneration cycles and ultimately exhausts the
satellite cell pool (36, 80).
Clinical symptoms are apparent by 4–5 yr of age in
boys with DMD (10, 20, 127). DMD patients (4–5 yr of
age) have been shown to undergo more skeletal muscle
regeneration than that measured in a total of six normal patients over 60 yr of age (46). These results were
confirmed by Renault et al. (151), who demonstrated
that the proliferative life span of satellite cells derived
from a 9-yr-old DMD patient was approximately onethird of an age-matched control. Proliferative fatigue
or senescence of the satellite cell population and the
milieu of the DMD skeletal muscle may collectively
impair the proliferative or regenerative capacity of this
cell population. In the DMD patient, increased levels of
IGF binding proteins (IGFBP) are released by fibroblasts. The elevated IGFBP sequesters IGF-I, limiting
its bioavailability for satellite cells and ultimately resulting in increased skeletal muscle fibrosis (120). The
evidence to date demonstrates the tremendous strain
and ultimate failure of the satellite cell population to
adequately compensate for the persistent degeneration-regeneration process that is occurring in the DMD
skeletal muscle.
GENETIC MOUSE MODELS OF MYOPATHY
A number of gene knockout mouse models and experimental methods are available for studies of muscle
regeneration and satellite cell biology. Although the
mdx mouse, which lacks dystrophin, has provided important insights into the pathophysiology of DMD, the
myopathy in these animals does not represent the
myopathic process of DMD in humans (75). The mdx
mice have a normal life span, a temporally restricted
myofiber degeneration, and adapt to muscle degeneration with an expansion of the satellite cell pool and
muscle hypertrophy, thereby avoiding the compro-
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As age progresses, there is an impairment of skeletal
muscle regeneration following injury (see Ref. 72 for
review). A decrease in satellite cell number and/or
proliferative capacity has been used to explain this
phenomena. Support for this hypothesis is observed in
rodent models, as increasing age is associated with a
decrease in satellite cell number and a reduced proliferative capacity (52, 66, 166). In contrast to the rodent
model, a decrease in human satellite cell population
and proliferative ability is observed only during the
childhood years. For example, neonatal (5-day-old) and
infant (5-mo-old) satellite cells are capable of ⬃60 and
45 replications, respectively, whereas 9-yr-old and
ⱖ60-yr-old humans are both capable of 20–30 replications (151). However, as aging progresses, satellite
cells (ⱖ60-yr-old skeletal muscle) fuse to form thinner,
more fragile myotubes (151). Thus, despite a normal
ability to proliferate, the satellite cells of older humans
have a reduced capacity to repopulate the myofiber
population.
The impaired regenerative response that is observed
with aging in humans thus appears to be much more
complex than satellite cell senescence alone. Results
from cross-transplantation experiments suggest that
the host environment is a critical factor in the ability of
older skeletal muscle to regenerate (23, 24). Cross
transplantation of EDL muscles between 4-mo-old and
24-mo-old rats demonstrated that mature skeletal
muscle was as capable as young skeletal muscle in
recovering from transplant and from toxin-induced injury. However, young (4-mo-old) nerve-muscle autografts functioned significantly better than old (24mo-old) nerve-muscle autografts, as determined by the
measurement of mass and maximum isometric force,
suggesting that the ability for neuronal regeneration
may be a critical factor in activating the satellite cell
response and, ultimately, regenerating the damaged
muscle (23, 24).
Other factors within the host environment affect the
efficiency of skeletal muscle regeneration as aging
progresses. A thickening of the basal lamina (176),
increased fibrosis within skeletal muscle (114), and
reduced capillary density (31) may also contribute to
impaired regeneration. Inflammatory factors (macrophages and associated cytokines) are essential for the
normal satellite cell response to injury. Aging negatively impacts the immune response, resulting in a
decrease of the inflammatory factors and macrophages
(43). In association with an impaired immune response, there are reduced serum levels of growth factors including IGF-I in aged rats and humans (150,
183). After multiple cycles of atrophy, there is no restoration of muscle mass or satellite cell proliferation in
aged rats even after 9 wk of recovery. However, local
IGF-I administration to the atrophied muscle resulted
in significant increases in mass and satellite cell proliferation within 2 wk (26).
An unresolved question with regard to satellite cells
and the aging process is whether repeated exhaustive
and/or resistance training exercise programs have a
negative impact on the long-term satellite cell content?
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mised muscle function that afflicts humans who lack
dystrophin (51, 69, 116, 179). In addition to the spontaneous dystrophin mutation in the mdx mouse, myopathic mouse models include genetically engineered
mouse strains with knockouts of utrophin (49, 69),
MyoD (119), or MNF (63) crossed into the mdx background. Each of these double mutant mouse models
exhibit features that more closely resemble DMD in
humans, including a severe myopathy, an impaired
regenerative capacity, and a decreased life span.
MUSCLE REGENERATION MODELS
An alternative strategy that has been successfully
used for the study of satellite cell activation, proliferation, regeneration, and self-renewal is to experimentally produce a controlled skeletal muscle injury. Strategies including crush (101), freeze (40), or chemically
induced injury (42, 63) have all been successfully used
to study satellite cell biology. Perhaps the most extensive and reproducible muscle injury is the delivery of
cardiotoxin (purified from the venom of the Naja nigricollis snake) into the hindlimb skeletal muscle of the
mouse (53, 63, 134). The intramuscular injection of 100
␮l of 10 ␮M cardiotoxin into the gastrocnemius muscle
results in 80–90% muscle degeneration (Fig. 5). After
cardiotoxin-induced injury, satellite cells become activated within 6 h of injury (Garry, unpublished observations). In response to locally released growth factors
from injured myotubes and macrophages, the satellite
cells proliferate extensively within 2–3 days of injury
(63, 64). Approximately 5 days after injury, the satellite cells withdraw from the cell cycle and either selfrenew or form differentiated myotubes that contain a
central nucleus (63, 64). With the use of this cardiotoxin-induced injury protocol, the architecture of the
injured muscle is largely restored within 10 days after
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Fig. 5. Skeletal muscle response to toxin-induced injury. A: transverse section
of the adult gastrocnemius muscle
stained with hematoxylin and eosin at
defined intervals following cardiotoxininduced injury. After cardiotoxin delivery, there is evidence of extensive myonecrosis and edema of the myofibers
(12 h; denoted by a). A hypercellular
response (proliferating satellite cells
and inflammatory cells) is observed
within 2 days of injury. Muscle regeneration is evident within 5 days of injury. At this time, newly regenerated
myofibers are evident as small, basophilic, central-nucleated myofibers (denoted by b). The architecture of the
muscle is largely restored within ⬃10
days following injury. The newly regenerated myofiber displays numerous
centrally aligned nuclei, demonstrating the fusion of many satellite cells to
form a single myofiber (hyperplasia) as
denoted by the myofiber designated c.
B: schematic diagram emphasizing the
temporal pattern of satellite cell proliferation and muscle differentiation following a chemically induced injury of
adult mouse skeletal muscle. Bar ⫽
100 ␮m.
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injury (Fig. 5). The complete profile of intrinsic and
extrinsic cues that regulate the satellite cell population
during muscle regeneration remains unclear. Therefore, the use of reproducible experimental injuries such
as cardiotoxin-induced injury will be important to evaluate and define the regulation of the satellite cell
population in molecular and physiological myopathic
models. Additional myonecrotic agents such as notexin
have been used with similar success (107), resulting in
a well-characterized regenerative response in skeletal
muscle.
SATELLITE CELLS AS MUSCLE PRECURSOR CELLS OR
STEM CELLS
SIDE POPULATION CELLS
Recent studies have identified a population of pluripotent stem cells from adult skeletal muscle termed
side population (SP) cells. Skeletal muscle- and bone
marrow-derived SP cells are isolated using a DNAbinding dye (Hoechst 33342) and dual-wavelength flow
cytometric analysis (67, 68, 77). This method, which
relies on the differential ability of the SP cells to efflux
the Hoechst dye, defines a small and homogeneous
population of cells that can adopt alternative fates in
permissive environments (77, 88). For example, Gussoni et al. (77) demonstrated that SP cells isolated from
adult bone marrow were able to reconstitute the irradiated mdx mouse bone marrow, and later these cells
were recruited from the bone marrow to participate in
muscle repair. Furthermore, Jackson et al. (88) utilized
the same protocol to isolate muscle SP cells from adult
mice and observed that as few as 100 SP cells could
reconstitute the entire bone marrow of a lethally irradiated mouse. These results demonstrate that adult
skeletal muscle contains a stem cell population that
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FUTURE PROSPECTS
Since the initial description of the satellite cell approximately 40 years ago, a number of anatomic and
physiological studies have established the importance
of this cellular pool in the growth, remodeling, and
regeneration of adult skeletal muscle. The profile of
gene expression that regulates the cell cycle progression of the satellite cell from a quiescent to an activated
and proliferating state remains ill-defined. Despite its
complexity, future challenges for this field will include
the definition of the development, maintenance, selfrenewal, and potentiality of the satellite cell population. These challenges will require an integration of
each of the biological disciplines, including the use of
increasingly powerful molecular biological tools.
The authors recognize the research efforts of those included and
those omitted (due only to space considerations) who have contributed to this field of study. Special thanks to Drs. M. I. Lindinger, P.
Mammen, and R. S. Williams for critical review of this manuscript.
The authors’ work is supported by grants from the Muscular
Dystrophy Association, March of Dimes, Doris Duke Charitable
Foundation, Texas Advanced Research (Technology) Program, and
the Donald W. Reynolds Foundation.
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Myogenic satellite cells have a tremendous proliferative capacity and are capable of self-renewal. These
tissue-specific progenitor cells or satellite cells are important in the maintenance and regeneration of skeletal muscle. Progenitor cells that are resident in other
adult tissues have stem cell characteristics, as they are
capable of self-renewal and multilineage differentiation along a specified molecular pathway (61, 194, 195).
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developing embryo. These results suggest that adult
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cell pool in adult skeletal muscle remains incompletely
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can be purified based on the exclusion of Hoechst dye
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exposed to the homeobox-containing transcriptional repressor, msx1. Furthermore, in response to appropriate cues (i.e., conditioned media), these dedifferentiated myoblasts have the capacity to adopt alternative
phenotypes. Collectively, these studies are challenging
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molecular regulation of this dedifferentiation process
but clearly have applications for the treatment of myopathies including DMD.
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