ARTICLE |
Correspondence to: Tetsuro Tamaki, Dept. of Physiology, Div. of Human Structure and Function, Tokai U. School of Medicine, Bohseidai, Isehara, Kanagawa 259-1193 Japan. E-mail: tamaki@is.icc.u-tokai.ac.jp
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Summary |
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The purpose of this study was to determine whether fiber hyperplasia occurs in the rat plantaris muscle during postnatal weeks 320. Total muscle fiber number, obtained via the nitric acid digestion method, increased by 28% during the early postnatal rapid growth phase (310 weeks), whereas the number of branched fibers was consistently low. Whole-muscle mitotic activity and amino acid uptake levels showed an inverse relationship to the increase in total fiber number. The expression of MyoD mRNA (RT-PCR) levels decreased from 3 to 20 weeks of age, as did the detection of anti-BrdU- and MyoD-positive cells in histological sections. Immunohistochemical staining patterns for MyoD, myogenin, or developmental myosin heavy chain on sections stained for laminin (identification of the basal lamina) and electron micrographs clearly indicate that de novo fiber formation occurred in the interstitial spaces. Myogenic cells in the interstitial spaces were negative for the reliable specific satellite cell marker M-cadherin. In contrast, CD34 (an established marker for hematopoietic stem cells)-positive cells were located only in the interstitial spaces, and their frequency and location were similar to those of MyoD- and/or myogenin-positive cells. These findings are consistent with fiber hyperplasia occurring in the interstitial spaces of the rat plantaris muscle during the rapid postnatal growth phase. Furthermore, these data suggest that the new fibers may be formed from myogenic cells in the interstitial spaces of skeletal muscle and may express CD34 that is distinct from satellite cells. (J Histochem Cytochem 50:10971111, 2002)
Key Words: skeletal muscle, myogenic cell, de novo fiber, interstitial space, MyoD, myogenin, M-cadherin, developmental MHC, hyperplasia, growing rat
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Introduction |
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THE POSTNATAL GROWTH CURVE (body mass) for rats is sigmoid and shows 3 phases (
It generally has been accepted that in growing animals muscle fibers increase in length by the addition of new sarcomeres and increase in diameter by the addition of myofibrils. These two factors are believed to account for the postnatal increases in total muscle mass (
After birth, satellite cells located between the basal lamina and the plasma membrane of myofibers in growing and mature muscles are considered to be the only source of myogenic precursors in skeletal muscles (30%) during postnatal weeks 310 (
An important limitation to many of the earlier studies reporting satellite cell activity has been the lack of specific markers for satellite cells. In the past decade, however, several useful markers have been identified. MyoD is a myogenic determination gene that encodes transcription factor (
The primary purpose of the present study was to determine the role of satellite cells in fiber hyperplasia in the plantaris muscle of rapidly growing rats. Our hypothesis was that satellite cells provided the nuclear material for the formation of "new" fibers during this developmental period. Changes in muscle mass and the number of muscle fibers were compared to the expression of the myogenic transcription factors MyoD and the changes in the mitotic activity and amino acid uptake levels in whole muscles of 320-week-old rats. We also examined new muscle fiber formation by electron microscopy and myogenin, developmental MHC, M-cadherin, and CD34 immunohistochemistry (IHC). Our results confirm that muscle hyperplasia occurs in the plantaris muscle of growing rats and suggest that new fiber formation in the interstitial spaces may be derived from a myogenic cell population that primarily resides in the interstitial spaces of the muscle.
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Materials and Methods |
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Animals
Wistar male rats (specific pathogen-free, 320 weeks old, n = 137) were used in this study. The rats were housed in standard cages and provided food and water ad libitum. The room temperature (RT) was kept at 23 ± 1C and a 12:12-hr light:dark cycle was maintained throughout the experiment. All experimental procedures were conducted in accordance with the Japanese Physiological Society Guide for the Care and Use of Laboratory Animals as approved by the Tokai University School of Medicine Committee on Animal Care and Use and followed the American Physiological Society Animal Care Guidelines. The numbers of rats used at each time point and for each analysis are summarized in Table 1. For the histological and IHC analyses and the muscle fiber counts, the rats were sacrificed with an overdose of sodium pentobarbital (60 mg/kg IP). The plantaris muscle was removed bilaterally, trimmed of excess fat and connective tissue, wet-weighed, and then immediately frozen in isopentane pre-cooled by liquid nitrogen and/or prepared for further analysis (see below). Frozen samples were stored at -80C. For the remaining analyses, the methods for sample preparation are described below.
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Determination of Muscle Fiber Number
The total number of fibers and the number of branched fibers in the plantaris muscle were determined using a modified nitric acid digestion method (
Determination of Mitotic Activity and Amino Acid Uptake
Our previous studies indicate that in vivo [3H]thymidine and [14C]leucine labeling are useful methods to detect the mitotic activity and amino acid uptake in muscles, respectively (
Determination of Myosin Synthesis
Determination of the synthesis of the contractile component was performed at the same time points and for the same muscles as for the analysis of mitotic activity and amino acid uptake. The contractile component (mostly myosin) was extracted with 0.6 M KCl solution (50 ml) from a 1-ml homogenate for 15 min at 4C and filtered with three sheets of gauze. The extracted KCl solution was diluted with cool distilled water (1:20), which resulted in the reappearance of myosin deposits. The diluted solution was passed through an omnipore non-dissolving membrane filter (10 µm aperture and 47 mm diameter; Nihon Millipore, Yonezawa, Japan). The membrane containing the deposits was dried, cut into several pieces, and soaked in a dissolving solution overnight at 45C. The radioactivity was counted using the same procedures employed for the mitotic activity and protein synthesis analyses. Values were expressed as dpm/mg protein.
Histological and Immunohistochemical Analyses
Histological and IHC analyses were performed in 320-week-old rats (Table 1). 5-Bromo-2'-deoxyuridine (BrdU; Takeda Chemical, Osaka, Japan), a non-radioactive marker for DNA synthesis, was injected (100 mg/kg IP) 1 hr before sampling. This procedure labels proliferating cells in muscle (100 fibers and whole-muscle cross-sectional area (used for the comparison of the number of BrdU- and MyoD-positive cells/mm2) was determined from a cross-section of each muscle at each time point. The data are expressed as mean ± SE.
For the remaining six to ten sections, IHC staining was performed using BrdU (1:50, monoclonal anti-BrdU; BectonDickinson, San Jose, CA) or MyoD (1:50, monoclonal anti-MyoD1, 5.8A; Dako, Carpinteria, CA), antibodies to identify proliferating (BrdU) and myogenic (MyoD) cells. The numbers of cells reacting positively for anti-BrdU and MyoD per whole-muscle cross-section were counted under a light microscope and reported as mean ± SE/mm2. Monoclonal anti-myogenin (1:100, F5D; Dako) and monoclonal anti-developmental myosin heavy chain (1:40, NCL-MHCd; Novocastra, Newcastle, UK) antibodies were used to determine whether myogenic cells had differentiated into myotubes or myofibers. Monoclonal anti-laminin (1:100; Chemicon International, Temecula, CA) antibody was used to identify the basal lamina and to determine whether the myogenic cells were located inside and/or outside of the basal lamina of the parent fiber. Goat polyclonal anti-M-cadherin (1:200, N-19; Santa Cruz Biotechnology, Santa Cruz, CA) and goat polyclonal CD34 (1:50, C-18; Santa Cruz Biotechnology) antibodies were used to determine whether the myogenic cells corresponded to satellite cells, as has been reported previously (
Electron Microscopy
Electron microscopic analyses of the plantaris muscle from 3-week-old rats were performed (n = 3; Table 1). The animals were anesthetized with sodium pentobarbital (60 mg/kg IP) and immediately perfused with PBS and 2% paraformaldehyde and 1% glutaraldehyde/0.05 M PB, pH 7.4. The plantaris muscles were carefully dissected and placed in the same fixative for 2 hr at 4C. Each muscle then was divided into three portions in the same manner as described for the IHC analyses. Each portion was divided into small pieces and fixed overnight in 2.5% glutaraldehyde/0.05 M PB at 4C. The samples were washed in 0.1 M PB and fixed in 1% osmium tetroxide/0.05 M PB for 3 hr at 4C. After fixation, the samples were dehydrated with serial graded ethanol and acetone and were prepared for electron microscopic analysis.
Reverse Transcriptase-polymerase Chain Reaction (RT-PCR) Analyses
Expression of MyoD mRNA was analyzed in 3-, 6-, 8-, 10-, 15-, and 20-week-old rats (two rats at each time point, n = 12; Table 1). The plantaris muscles stored at -80C were equilibrated at -20C and 200300 sections (15 µm thick) were cut from each muscle with a cryostat. Sections were collected in microtubes under RNase-free conditions and total RNA was extracted with Trizol reagent (Life Tech; Boston, MA). RNA quantity was determined by an optical density measurement at 260 nm. Primer pairs and the nucleotide position used for PCR amplification of MyoD mRNA were as follows: MyoD-forward, 5'-GTGCAAGCGCAAGACCACTAA-3' and MyoD-reverse, 5'-TGCAGACCTTCAATGTAGCGG-3' (182 bp). RT-PCR was performed using an RNA-PCR kit (PerkinElmer; Norwalk, CT). To enable a comparison among the six age groups, 25 and 28 PCR cycles were chosen: ß-Actin-forward, 5'-TGCAGAAGGAGATTACTGCC-3' and ß-actin reverse, 5'-GCAGCTCAGTAACAGTCC-3' (211 bp) was used as a stably expressed reference mRNA (an internal control) (
Statistics
All data are presented as mean ± SE.
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Results |
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Changes in Body and Plantaris Mass
Both the body and plantaris muscle masses rapidly increased until 10 weeks of age (10-fold increase, Fig 1). For the next 10 weeks the growth rate was slower. The growth curves for the body and plantaris masses were parallel, with the plantaris mass consistently being
0.1% of the body mass. The numbers of rats used for each of the following analyses are listed in Table 1.
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Total Number of Fibers and Branched Fibers and Mean CSA of the Fibers in the Plantaris Muscle
The mean total number of fibers in the plantaris muscle increased by 28% from 3 (8737 ± 162) to 10 (11,215 ± 451) weeks of age. Total fiber number then was consistent (11,000 fibers) until 20 weeks of age (Fig 2A). The number of branched fibers was similar throughout the experimental period, i.e., an average of
30 branched fibers or
0.3% of the total fiber number (Fig 2B). The pattern of increase in fiber CSA was similar to that observed for body and plantaris mass, a rapid increase up to
10 weeks and a slower rate thereafter (compare Fig 1 and Fig 2C).
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Thymidine and Amino Acid Uptake Rates and Fraction of Amino Acid Uptake for Myosin
The uptake of [3H]-thymidine rapidly decreased from 4 to 8 weeks and was consistently low thereafter (Fig 2D). The uptake of [14C]-leucine (Fig 2E) and of the myosin fraction (Fig 2F) also decreased rapidly from 4 to 8 weeks, but both were slightly elevated during weeks 1013. The pattern of change for these parameters was almost inverse to the volumetric and numerical measurements, i.e. body and muscle mass, total fiber number and mean fiber CSA (compare Fig 2A and Fig 2C vs Fig 2D2F).
Changes in the Number of BrdU- and MyoD-positive Cells
Typical immunostaining for BrdU (Fig 3A, Fig 3C, and Fig 3E) and MyoD (Fig 3B, Fig 3D, and Fig 3F) from 3- (Fig 3A and Fig 3B), 6- (Fig 3C and Fig 3D) and 8- (Fig 3E and Fig 3F) week old rats is shown in Fig 3. All photographs were taken at the same magnification (x120) except for the insets (x230365) and clearly show the increase in fiber size from 3 to 8 weeks of age. The number of BrdU-positive cells was consistently higher than the number of MyoD positive cells up to 8 weeks of age. The numbers of anti-BrdU- and MyoD-positive cells rapidly decreased from 3 to 8 weeks of age and were undetectable at 20 weeks of age (Fig 4). The pattern of decrease was similar to that observed for [3H]-thymidine uptake (Fig 2D).
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Expression of MyoD mRNA
The expression of MyoD mRNA levels was consistent with the protein determinations (IHC), being highest at 3 weeks of age and then gradually reducing with age. The lowest levels were observed in 20-week-old rats (Fig 5).
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Localization of MyoD- and Myogenin-positive Cells
Relative to all of the ages being studied, the plantaris muscle of 3-week-old rats had the highest mitotic activity, amino acid uptake, and MyoD mRNA expression when expressed per unit of tissue or protein. Therefore, we used the muscles from 3-week-old rats to investigate the distribution of myogenic cells. MyoD- and myogenin-positive cells were identified in cross-sections double labeled for laminin and MyoD or for laminin and myogenin (Fig 6A6H). Approximately 70% of the MyoD-positive cells were located inside the basal lamina of the parent fiber, as shown in Fig 6D. The remaining 30% of the MyoD-positive cells were located in the interstitial spaces (Fig 6A6C). A similar distribution pattern was observed for myogenin-positive cells (Fig 6E6H), although the frequency of occurrence was somewhat lower than that for MyoD-positive cells (data not shown). Myogenin-positive cells also were observed inside the basal lamina of parent fibers (Fig 6E) and in the interstitial spaces (Fig 6F6H). Very little cytoplasm was observed in MyoD-positive cells (insets in Fig 6B and Fig 6C), while myogenin- positive cells had a small amount of cytoplasm (insets in Fig 6G and Fig 6H).
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We also detected about 1520 small fibers per cross-section that expressed developmental MHC (Fig 6I). A high-magnification photograph of a small fiber positive for developmental MHC is shown in Fig 6J. Serial sections show that this small fiber has myogenin-positive central nuclei and laminin-positive basal lamina (Fig 6K), and eosin-positive cytoplasm (Fig 6L). Myogenic cells also were identified clearly by electron microscopy (Fig 7A7D). Small fibers containing myofilaments and surrounded by a basal lamina were evident in the interstitial spaces, usually near capillaries (Fig 7A, Fig 7C, and insets). The location of these small fibers (Fig 7D) was consistent with the presence of small MyoD- and myogenin-positive cells identified by immunohistochemistry (Fig 6A, Fig 6C, Fig 6F6H). Activated (mitotic) satellite cells were evident in the electron micrographs (Fig 7B) and corresponded to the immunostaining of satellite cells (Fig 6D and Fig 6E).
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Characterization of Myogenic Cells
A large number of M-cadherin-positive reactions were observed in the muscles of 3-week-old rats (Fig 8A, black arrowheads) and this was similar to the number of MyoD-positive cells per entire muscle cross-section at this time point (Fig 4). The number of M-cadherin-positive reactions gradually decreased with age (Fig 8B) and only 1015 reaction products per cross-section were observed in the muscles of 8-week-old rats. Again, this was similar to the incidence of MyoD-positive cells at the same time point as shown in Fig 4. Double labeling with M-cadherin and laminin showed that M-cadherin-positive reactions were always located inside the basal lamina and stained most intensely on the side of the satellite cells facing the muscle fiber (Fig 8B, Fig 8C, and Fig 8E). No positive reactions were seen in the interstitial spaces. Some of the cells staining positive for M-cadherin also expressed MyoD (marked by black arrowheads and arrows in Fig 8C8F), whereas others did not (marked by black arrowheads only in Fig 8C8F). CD34-positive cells typically were located in the interstitial spaces outside the basal lamina (white arrowheads in Fig 8G and in insets 1 and 2). There were 3040 CD34-positive cells per entire muscle cross-section of 3-week-old rats, a number similar to that of MyoD-positive cells per section in the interstitial spaces (see above). However, we did not find any cells that co-expressed CD34 and MyoD.
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Discussion |
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Muscle Fiber Hyperplasia
The postnatal increase in the total number of fibers in the rat plantaris muscle from 8550 fibers at 3 weeks to
11,000 [consistent with that reported by
Whole-muscle Mitotic and Myogenic Activity
During postnatal weeks 310, the decrease in plantaris muscle mitotic activity, as measured by [3H]-thymidine uptake, was inversely related to the increase in total fiber number (compare Fig 2A and Fig 2D). In addition, the decrease in [3H]-thymidine uptake was paralleled by a decrease in the number of BrdU- and MyoD-positive cells per section (Fig 4) and a decrease in the levels of MyoD mRNA (RT-PCR; Fig 5), although the number of BrdU-positive cells was consistently higher than the number of MyoD-positive cells up to 8 weeks of age, most likely reflecting the proliferation of endothelial cells and/or fibroblasts that contribute to the formation of additional capillaries and connective tissues during these developmental stages (Fig 4). These data also suggest that the majority of mitotic activities during this period are related to the myogenic response.
The reduction of postnatal replication of myogenic precursor cells (mpc) in a range of species has been reviewed by 10 weeks postnatally.
Formation of De Novo Fibers in the Interstitial Spaces
Based on the large number of MyoD-positive cells at postnatal week 3 (Fig 3), we investigated the possibility of the formation of "new" fibers during the rapid growth phase of the plantaris muscle. Double labeling with MyoD and laminin showed MyoD-positive cells within the basal lamina, i.e., satellite cells (Fig 6D), and these cells comprised 70% of the total number of MyoD-positive cells. The remainder of the MyoD-positive cells were located in the interstitial spaces (Fig 6A6C). There was a tendency for myogenin positive cells to be distributed similarly (Fig 6E6H), although the frequency was somewhat lower than for MyoD-positive cells. These observations indicate that myogenic cells located either inside or outside of the basal lamina may differentiate to the stage of myotube formation. Developmental (neonatal) MHC-positive cells with a small diameter also were observed at postnatal week 3 (Fig 6I) and were located between large-diameter fibers (parent fibers). This observation indicates that myogenin-positive cells located in the interstitial spaces must have differentiated from a myotube to a myofiber, since these myogenin-positive cells had a small amount of cytoplasm (insets of Fig 6G and Fig 6H), stained for developmental MHC, and expressed myogenin (Fig 6J and Fig 6K). Furthermore, electron microscopic analyses clearly demonstrated small fibers with a sparse amount of myofilaments that were surrounded by basal lamina in the interstitial spaces and often associated with a capillary (Fig 7A, Fig 7C and insets). All of these observations are consistent with de novo fiber formation in the interstitial spaces in addition to the activation of satellite cells (mitosis) inside of the basal lamina sheath of parent fibers (Fig 7B) and probably reflect the addition of myonuclei associated with fiber hypertrophy of parent fibers.
Consideration must be given to the possibility that the small fibers observed in the interstitial spaces could be split and/or branched fibers (
Possible Sources of Myogenic Cells in the Interstitial Spaces
There are two possible sources for the myogenic cells associated with the observed de novo fiber formation in the interstitial spaces. First, activated satellite cells could have migrated from their normal location within the basal lamina into the interstitial spaces, then proliferated, fused, and formed new fibers. Second, the de novo fibers could be derived from unidentified myogenic cells existing in the interstitial spaces. Migration of satellite cells has been demonstrated both in vitro (
In the present study, we characterized the interstitial myogenic cells via MyoD, myogenin, M-cadherin and CD34 immunostaining, all reported possible markers for satellite cells. MyoD- and myogenin-positive cells were located both inside the basal lamina of the parent fiber and in the interstitial spaces in the plantaris muscle of 3-week-old rats. This staining indicates the presence of myogenic cells in both locations, with the former being satellite cells. M-cadherin is a specific marker for satellite cells and it has been reported that (a) the frequency of M-cadherin-labeled cells was higher in the soleus muscles of 2-week-old than of adult mice, (b) M-cadherin was expressed whether a satellite cell was quiescent, activated, or replicating, and (c) M-cadherin staining was more intense at the side of the satellite cells facing the muscle fiber (
It has recently been reported that CD34, an established cell surface marker for hematopoietic stem cells, is a useful marker for quiescent mouse satellite cells and C2C12 mouse myoblast cells (3040 per muscle cross-section) CD34-positive cells were observed in the interstitial spaces (Fig 8G and insets 1 and 2). Capillaries (red arrowheads, insets 1 and 2 in Fig 8G), muscle spindle capsules, and axons (data not shown) also were CD34-positive, consistent with previous observations (
A Suggested Myogenic Cell Population Distinct from the Satellite Cell
Several lines of evidence suggest the possibility for myogenic cell populations distinct from satellite cells. For example, the progeny of a single human satellite cell generate both differentiated myotubes and quiescent cells that are undifferentiated, but still myogenic, under differentiation-inducing culture conditions (
In the present study, we found an 28% increase in the mean total fiber number in the plantaris muscle between postnatal weeks 3 and 10. There were very few centrally nucleated fibers in the muscles from young adult (20-week-old) rats. Therefore, it is possible that the new fiber formation observed in the postnatal rapid growth phase was not induced by satellite cells but that myogenic cells similar to fetal myogenic cells remained in the interstitial spaces and formed new fibers. Furthermore, recent transplantation studies have identified a population of pluripotent stem cells, called side-population (SP) cells that are mostly CD34-negative, in adult mouse skeletal muscle whose nuclei can be incorporated into muscles of mdx mice and partially restore dystrophin expression in the affected muscle fibers (
On the basis of results of the present and related previous studies, it appears that (a) myogenic precursor cells distinct from satellite cells exist in the interstitial spaces of the plantaris muscle during the rapid postnatal growth phase, (b) these cells could be main contributors to new fiber formation during this growth phase, (c) the number of these cells gradually decrease with age, but a few of these cells continuously remain in mature, adult muscles, and (d) these cells may contribute to muscle hyperplasia in adult muscles if they are stimulated by factors such as heavy weight-lifting (
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Acknowledgments |
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Supported in part by Tokai University School of Medicine Research Aid and by a research grant by Sankyo Corporation.
We thank K. Nakane (Facilities for Radioisotope Research, Tokai University School of Medicine) for helpful assistance in animal care and use.
Received for publication May 23, 2001; accepted February 20, 2002.
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Literature Cited |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Baroffio A, Hamann M, Bernheim L, BochatonPiallat ML, Gabbiani G, Bader CR (1996) Identification of self-renewing myoblasts in the progeny of single human muscle satellite cells. Differentiation 60:47-57[Medline]
Baumhueter S, Dybdal N, Kyle C, Lasky LA (1994) Global vascular expression of murine CD34, a sialomucin-like endothelial ligand for L-selectin. Blood 84:2554-2565
Beauchamp JR, Heslop L, Yu DS, Tajbakhsh S, Kelly RG, Wernig A, Buckingham ME, Partridge TA, Zammit PS (2000) Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells. J Cell Biol 151:1221-1234
Beauchamp JR, Morgan JE, Pagel CN, Partridge TA (1999) Dynamics of myoblast transplantation reveal a discrete minority of precursors with stem cell-like properties as the myogenic source. J Cell Biol 144:1113-1122
Beilharz MW, Lareu RR, Garrett KL, Grounds MD, Fletcher S (1992) Quantitation of muscle precursor cell activity in skeletal muscle by northern analysis of MyoD and myogenin expression: application to dystrophic (mdx) mouse muscle. Mol Cell Neurosci 3:326-331
Bischoff R (1989) Analysis of muscle regeneration using single myofibers in culture. Med Sci Sports Exerc 21:S164-172[Medline]
Bischoff R (1990) Control of satellite cell proliferation. Adv Exp Med Biol 280:147-157[Medline]
Bischoff R, Heintz C (1994) Enhancement of skeletal muscle regeneration. Dev Dyn 201:41-54[Medline]
Chiakulas JJ, Pauly JE (1965) A study of postnatal growth of skeletal muscle in the rat. Anat Rec 152:55-62[Medline]
Cornelison DDW, Wold BJ (1997) Single-cell analysis of regulatory gene expression in quiescent and activated mouse skeletal muscle satellite cells. Dev Biol 191:270-283[Medline]
CusellaDe Angelis MG, Lyons G, Sonnino C, De Angelis L, Vivarelli E, Farmer K et al. (1992) MyoD, myogenin independent differentiation of primordial myoblasts in mouse somites. J Cell Biol 116:1243-1255[Abstract]
di Maso NA, Caiozzo VJ, Baldwin KM (2000) Single-fiber myosin heavy chain polymorphism during postnatal development: modulation by hypothyroidism. Am J Physiol 278:R1099-1106
Edgerton VR (1970) Morphology and histochemistry of the soleus muscle from normal and exercised rats. Am J Anat 127:81-88[Medline]
Ellender G, Feik SA, Carach BJ (1988) Periosteal structure and development in a rat caudal vertebra. J Anat 158:173-187[Medline]
Enesco M, Puddy D (1964) Increase in number of nuclei and weight in skeletal muscle of rats of various ages. Am J Anat 114:235-244[Medline]
Ferrari G, CusellaDe Angelis G, Coletta M, Paolucci E, Stornaiuolo A, Cossu G, Mavilio F (1998) Muscle regeneration by bone marrow derived myogenic progenitors. Science 279:1528-1530
Gollnick PD, Timson BF, Moore RL, Riedy M (1981) Muscular enlargement and number of fibers in skeletal muscles of rats. J Appl Physiol 50:936-943
Goss RJ (1978) The strategy of growth. In Goss RJ, ed. The Physiology of Growth. New York, Academic Press, 1-8
Grounds M, Partridge TA, Sloper JC (1980) The contribution of exogenous cells to regenerating skeletal muscle: an isoenzyme study of muscle allografts in mice. J Pathol 132:325-341[Medline]
Grounds M, Yablonka-Reuveni Z (1993) Molecular and cell biology of skeletal muscle regeneration. In Partridge TA, ed. Molecular and Cell Biology of Human Diseases. London, Chapman & Hall, 210-256
Gussoni E, Soneoka Y, Strickland CD, Buzney EA, Khan MK, Flint AF, Kunkel LM et al. (1999) Dystrophin expression in the mdx mouse restored by stem cell transplantation. Nature 401:390-394[Medline]
HallCraggs ECB (1970) The longitudial division of fibres in overloaded rat skeletal muscle. J Anat 107:459-470[Medline]
Hall-Craggs ECB (1972) The significance of longitudinal fibre division in skeletal muscle. J Neurol Sci 15:27-33[Medline]
Ho KW, Roy RR, Tweedle CD, Heusner WW, Van Huss WD, Carrow RE (1980) Skeletal muscle fiber splitting with weight-lifting exercise in rats. Am J Anat 157:433-440[Medline]
Hughes SM, Blau HM (1990) Migration of myoblasts across basal lamina during skeletal muscle development. Nature 345:350-353[Medline]
Irintchev A, Zeschnigk M, StarzinskiPowitz A, Wernig A (1994) Expression pattern of M-cadherin in normal, denervated, and regenerating mouse muscles. Dev Dyn 199:326-337[Medline]
Jackson KA, Mi T, Goodell MA (1999) Hematopoietic potential of stem cells isolated from murine skeletal muscle. Proc Natl Acad Sci USA 96:14482-14486
James NT (1973) Compensatory hypertrophy in the extensor digitorum longus muscle of the rat. J Anat 116:57-65[Medline]
Konigsberg UR, Lipton BH, Konigsberg IR (1975) The regenerative response of single mature muscle fibers isolated in vitro. Dev Biol 45:260-275[Medline]
Krause DS, Fackler MJ, Civin CI, May WS (1996) CD34: structure, biology, and clinical utility. Blood 87:1-13
Lee JY, QuPetersen Z, Cao B, Kimura S, Jankowski R, Cummins J et al. (2000) Clonal isolation of muscle-derived cells capable of enhancing muscle regeneration and bone healing. J Cell Biol 150:1085-1100
Lipton BH, Schultz E (1979) Developmental fate of skeletal muscle satellite cells. Science 205:1292-1294[Medline]
Mauro A (1961) Satellite cells of skeletal muscle fibers. J Biophys Biochem Cytol 9:493-494
Moss FP, Leblond CP (1971) Satellite cells as the source of nuclei in muscles of growing rats. Anat Rec 170:421-435[Medline]
Ontell M, Dunn RF (1978) Neonatal muscle growth: a quantitative study. Am J Anat 152:539-555[Medline]
Ontell M, Hughes D, Bourke D (1982) Secondary myogenesis of normal muscle produces abnormal myotubes. Anat Rec 204:199-207[Medline]
Ontell M, Kozeka K (1984) Organogenesis of the mouse extensor digitorum logus muscle: a quantitative study. Am J Anat 171:149-161[Medline]
Pastoret C, Sebille A (1995) Age-related differences in regeneration of dystrophic (mdx) and normal muscle in the mouse. Muscle Nerve 18:1147-1154[Medline]
Phillips GD, Hoffman JR, Knighton DR (1990) Migration of myogenic cells in the rat extensor digitorum longus muscle studied with a split autograft model. Cell Tissue Res 262:81-88[Medline]
Rayne J, Crawford GNC (1975) Increase in fibre numbers of the rat pterygoid muscles during postnatal growth. J Anat 119:347-357[Medline]
Ross JJ, Duxson MJ, Harris AJ (1987) Formation of primary and secondary myotubes in rat lumbrical muscles. Development 100:383-394[Abstract]
Roth D, Oron U (1985) Repair mechanisms involved in muscle regeneration following partial excision of the rat gastrocnemius muscle. Exp Cell Biol 53:107-114[Medline]
Schmalbruch H (1976) The morphology of regeneration of skeletal muscles in the rat. Tissue Cell 8:673-692[Medline]
Schultz E, Jaryszak DL, Gibson MC, Albright DJ (1986) Absence of exogenous satellite cell contribution to regeneration of frozen skeletal muscle. J Muscle Res Cell Motil 7:361-367[Medline]
Schultz E, Jaryszak DL, Valliere CR (1985) Response of satellite cells to focal skeletal muscle injury. Muscle Nerve 8:217-222[Medline]
Snow MH, Chortkoff S (1987) Frequency of bifurcated muscle fibers in hypertrophic rat soleus muscle. Muscle Nerve 10:312-317[Medline]
Suopanki J, Tyynela J, Baumann M, Haltia M (1999) The expression of palmitoyl-protein thioesterase is developmentally regulated in neural tissues but not in nonneural tissues. Mol Genet Metab 66:290-293[Medline]
Tamaki T, Akatsuka A (1994) Appearance of complex branched fibers following repetitive muscle trauma in normal rat skeletal muscle. Anat Rec 240:217-224[Medline]
Tamaki T, Akatsuka A, Itoh J, Nakano S (1989) A newly modified isolation method of single muscle fibers: especially useful in histological, histochemical and electron microscopic studies on branched fibers. Tokai J Exp Clin Med 14:211-218[Medline]
Tamaki T, Akatsuka A, Tokunaga M, Ishige K, Uchiyama S, Shiraishi T (1997) Morphological and biochemical evidence of muscle hyperplasia following weight-lifting exercise in rats. Am J Physiol 273:C246-256
Tamaki T, Akatsuka A, Tokunaga M, Uchiyama S, Shiraishi T (1996) Characteristics of compensatory hypertrophied muscle in the rat: I. Electron microscopic and immunohistochemical studies. Anat Rec 246:325-334[Medline]
Tamaki T, Sekine T, Akatsuka A, Uchiyama S, Nakano S (1992a) Detection of neuromuscular junctions on isolated branched muscle fibers: application of nitric acid digestion method for scanning electron microscopy. J Electron Microsc 41:76-81[Medline]
Tamaki T, Sekine T, Akatsuka A, Uchiyama S, Nakano S (1993) Three-dimensional cytoarchitecture of complex branched fibers in soleus muscle from mdx mutant mice. Anat Rec 237:338-344[Medline]
Tamaki T, Uchiyama S (1995) Absolute and relative growth of rat skeletal muscle. Physiol Behav 57:913-919[Medline]
Tamaki T, Uchiyama S, Nakano S (1992b) A weight-lifting exercise model for inducing hypertrophy in the hindlimb muscles of rats. Med Sci Sports Exerc 24:881-886[Medline]
Tamaki T, Uchiyama S, Uchiyama Y, Akatsuka A, Roy RR, Edgerton VR (2001) Anabolic steroids increase exercise tolerance. Am J Physiol 280:E973-981
Tamaki T, Uchiyama S, Uchiyama Y, Akatsuka A, Yoshimura S, Roy RR, Edgerton VR (2000) Limited myogenic response to a single bout of weight-lifting exercise in old rats. Am J Physiol 278:C1143-1152
Weintraub H (1993) The MyoD family and myogenesis: redundancy, networks, and thresholds. Cell 75:1241-1244[Medline]
Weintraub H, Dwarki VJ, Verma I, Davis R, Hollenberg S, Snider L, Lassar A et al. (1991) Muscle-specific transcriptional activation by MyoD. Genes Dev 5:1377-1386[Abstract]
YablonkaReuveni Z (1995) Development and postnatal regulation of adult myoblasts. Microsc Res Tech 30:366-380[Medline]
Young HE, Mancini ML, Wright RP, Smith JC, Black AC, Jr, Reagan CR et al. (1995) Mesenchymal stem cells reside within the connective tissue of many organs. Dev Dyn 202:137-144[Medline]
Yoshida N, Yoshida S, Koishi K, Masuda K, Nabeshima Y (1998) Cell heterogeneity upon myogenic differentiation: down-regulation of MyoD and Myf-5 generates reserve cells.. J Cell Sci 111:769-779