The Muscle Satellite Cell: A Review

INTERNATIONAL REVIEW OF CYTOLOGY. VOL. 87

The Muscle Satellite Cell: A Review DENNISR. CAMPION USDA-SE-ARS, Animal Physiology Unit? Richard B . Russell Agricultural Research Center, Athens, Georgia, and Department of Foods and Nutrition, University of Georgia, Arhens, Georgia I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Cell Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Skeletal Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Cardiac Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Gross Morphology.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Fine Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Situations Affecting Satellite Cell Content A. Normal Growth . . . . . . . . . . . . . . . . . B. Nutrition.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Muscle Regeneration and Compensatory Hypertrophy . . . . . . . . VI. Activation Stimulus.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... . VII. Summary References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225 226 226 231 231 233 243 244 245 246 247

I. Introduction During normal growth of skeletal muscle the total amount of DNA increases as muscle mass increases. Because of the constancy of the DNA content per nucleus within a species, it is reasonable to conclude that the number of myonuclei increases during growth. The fact notwithstanding that approximately 25% of the nuclei in human muscle (Cheek er al., 1971), 29-40% of the nuclei in rat muscle (Enesco and Puddy, 1964; Ontell, 1974; Schmalbmch and Hellhammer, 1977) and 41% of the nuclei in androgen-sensitive muscle (Venable, 1966) are extraplasmalemmal in location, considerable microscopic evidence is available to verify that the nuclear content of individual myofibers increases during normal growth (e.g., Enesco and Puddy, 1964; MacConnachie et a l ., 1964; Moss, 1968). Embryologically, mononucleated myoblasts fuse, forming a syncytium or myotube. The myotube ultimately develops into the mature myofiber. A point germane to the topic of this article is that investigations using a variety of techniques to study myogenesis in siru (e.g., Przybylski and Blumberg, 1966; Shafiq er al., 1968; Kelly and Zacks, 1969; Stockdale, 1970; Moss and Leblond, 1971; Allbrook et al., 1971; Cardasis and Cooper, 1975), and in virro (e.g., 225 ISBN 0-12-3644117-9

226

DENNIS R. CAMPION

Stockdale and Holtzer, 1961; Okazaki and Holtzer, 1966; Bischoff and Holtzer, 1969; Richler and Yaffe, 1970; Bischoff, 1974, 1975; Konigsberg et a l . , 1975; Yeoh and Holtzer, 1977; Pullman and Yeoh, 1978; Yeoh et a l . , 1978), have demonstrated that the nuclei of the myotube do not divide. Thus, nuclei contained within the myofiber, at least in the normal situation, are not capable of mitosis. In situations wherein there is a biological need to generate additional nuclei for normally growing myofibers, to generate new myofibers, or to repair damaged or diseased myofibers, the primary source of these nuclei is thought to be the satellite cell. This cell was first identified by name by Katz (1961) and Mauro (1961). Mauro (1961) originally suggested that satellite cells may be the source of nuclei added to myofibers during regeneration. In this article, the topic of the satellite cell in relation to muscle regeneration is limited as this area has been the subject of several recent reviews (Carlson, 1973; Reznik, 1976; Allbrook, 1981; Klishov and Danilov, 1981) and conferences (Mauro et al., 1970; Mauro, 1979). In the recent review of myogenic cell proliferation by Allen et al. (1979), particular attention was focused upon the importance of satellite cell and presumptive myoblast proliferation to meat animal production. Therefore, this latter relation is only minimally stressed in the present article. 11. Cell Location

A. SKELETAL MUSCLE The satellite cell is a mononucleated cell that lies under or embedded in the basal lamina of the myofiber. Frequently, the satellite cell lies within a groove that is parallel to the long axis of the myofiber. Some variation has been noted, however, as satellite cells were observed by scanning electron microscopy and by special lead staining techniques and light microscopy to be occasionally oriented obliquely or transversely to the long axis of myofibers of the frog sartorius muscle (Franzini-Armstrong, 1979; Larocque et al., 1980; Mazanet et al., 1982). A gap of 15-60 nm separates the opposing cell walls of the satellite cell and the myofiber. The wider spacings are more commonly found in immature muscle (Schultz, 1976). Generally, basal lamina material is not seen in the intervening gap. A notable exception, however, is Rana pipiens. Maruenda and FranziniArmstrong (1978) reported that in 21% of the satellite cell profiles in the sartorius muscle, basal lamina had penetrated a variable distance into the gap region. In addition, 6% of the satellite cell profiles was completely encapsulated by the basal lamina of the attendant myofiber. Other, less extensive exceptions have also been noted. Protrusion of basal lamina material into the gap region has

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227

been observed in the lumbrical muscle of growing mice (Schultz, 1976) and in the soleus muscle of aging mice (Snow, 1978). Kelly (1978a) reported insinuation of basal lamina between the opposed plasma membranes of perisynaptic satellite cells and myofibers in the soleus muscle of the mature rat. In the tail muscles of lizards, the basal lamina did not protrude into the gap region (Kahn and Simpson, 1974). Several lines of evidence exist to demonstrate that satellite cells are independent of the adjacent myofibers. Muir el al. (1965) observed that cut, web-muscle fibers that had been immersed in 2.5 M sucrose became distended. The density of the satellite cells, on the other hand, was increased, presumably due to extraction of water from the satellite cells. These authors concluded, therefore, that there was no continuity of satellite cell cytoplasm with myofiber cytoplasm. A similar conclusion was reached for the satellite cells of frog muscle wherein it was found that horseradish peroxidase injected into the myofiber never appeared in the cytoplasm of the satellite cell (Cull-Candy et al., 1980). In addition, Schmalbruch (1978), in the only freeze-fracture study of satellite cells, saw no membrane specializations between opposing cell walls of satellite cells and myofibers of the adult rat soleus muscle. However, desmosome-like specializations were reported in the craniovelar muscle of the Atlantic hagfish (Sandset and Korneliussen, 1978). These specializations were not noted in the parietal muscle of this species. Hess and Rosner (1970) observed thickening in the membranes of satellite cells and myofibers of the extraocular muscles of the guinea pig. But the two membranes were not joined at these thickenings. The significance of these membrane specializations is not known. Satellite cell nuclei often appear near myofiber nuclei when muscle cross sections are examined (Allbrook et al., 1971; Sandset and Korneliussen, 1978; Schmalbruch and Hellhammer, 1977; Ontell, 1974, 1977). In the sartorius muscle of the fetal pig (Campion et al., 1978) and in the soleus muscle of the adult mouse (Snow, 198l), satellite cells are uniformly distributed throughout the length of the muscle. Maruenda and Franzini-Armstrong (1978) reported a significantly higher incidence of satellite cells in the distal portion of the adult frog sartorius muscle. However, when the data of these two authors were recalculated by Snow (1981), it became evident that the mean percentage of satellite cells associated with the distal, belly, and proximal locations was similar. Satellite cells are frequently seen in close association with cross-sectional profiles of myoneural junctions of the soleus (Kelly, 1978a; Cardasis and Padykula, 1981; Snow, 1981; Gibson and Schultz, 1982) and diaphragm muscle of the rat (Cardasis, 1979), and with the nerve terminals on intrafusal fibers of frog sartorius muscle (Katz, 1961). In the adult rat soleus muscle, there is a nearly 20-fold higher incidence of satellite cell nuclei in the synaptic than in the nonsynaptic area (Kelly, 1978a). A 14-fold higher incidence of satellite cells was reported in the synaptic than in the nonsynaptic area of the adult mouse soleus

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DENNIS R. CAMPION

muscle (Snow, 1981). But, because the number of perisynaptic satellite cells constitutes a relatively small proportion of the total number of satellite cells, and because the frequency of sectioning through a nerve terminal is low, the effect of perisynaptic satellite cells on experimental quantitation of the percentage of nuclei within the basal lamina that are satellite cell nuclei is minimal (Snow, 1981). Interestingly, perisynaptic satellite cell clusters were not observed in the rat sternomastoid muscle (Mazanet, 1981). And their incidence was relatively low in the extensor digitorum longus muscle of the rat when compared to the soleus muscle (Kelly, 1978a; Gibson and Schultz, 1982). Thus, not all skeletal muscles within a species exhibit similar characteristics with respect to linear distribution of satellite cells. In anuran gastrocnemius muscle (Trupin, 1976), there is no obvious association between the satellite cell and the motor-end plate or vasculature. More than half of the satellite cells in rat muscle are associated with a capillary (Schmalbruch and Hellhammer, 1977). No difference in the incidence

INFLUENCE OF FIBERTYPEO N OF

Muscle Axolotl, trunk muscle "Young" Red Intermediate White Shark, axial muscle Red fibers White fibers Atlantic hagfish, parietal muscle Red fibers White fibers Atlantic hagfish, craniovelar muscle Red

TABLE I INCIDENCE OF NUCLEIIN TRANSVERSE SECTIONS MUSCLEFROM THE ADULT THE

Incidence of muscle fiber nuclei/fiber transected

(a) 47 35 59 I00

Incidence of satellite cell nuclei/fiber transected (%)

Percentage nuclei within basal lamina that are satellite cell nuclei (70)

Flood ( I97 I )

0 (58)a 0 (135p 11.8 (l53)<' 0 (220)l'

79 62

6.0 4.0

7.1 6. I

78 200

27 27

11.1

43

30

22.6

uPercentags based on incidence of cytoplasmic profiles

Reference

2.8

Kryvi and Eide (1977) Sandset and Korneliussen (1978) Sandset and Korneliussen (1978)

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229

of satellite cells was noted in the myotendinous or myoneural junction of web muscles in the fruit bat (Muir et al., 1965). Satellite cells, in addition to their presence on extrafusal muscle fibers, are also present on intrafusal muscle fibers. This latter relationship was reported in frog (Katz, 1961; Karlsson et al., 1966; Karlsson and Anderson-Cedergren, 1971), mouse (Rumpelt and Schmalbruch, 1969; Snow, 1977a), rat (Landon, 1966; Rumpelt and Schmalbruch, 1969; Maynard and Cooper, 1973; Anastasi et a / . , 1979), cat (Adal, 1969), man (Rumpelt and Schmalbruch, 1969), dog (Banker and Girvin, 1971), and snake (Fukami, 1982) muscle. Bird and Allbrook (1980) calculated that the ratio of satellite cell nuclei to myofiber nuclei was 1:9 in chain fibers and 1:5 in bag fibers in the juxta-equatorial region of rat lumbrical muscle. In the less complex spindle of amphibian muscle, satellite cells are commonly present in the sensory compact zones. They are seldom seen in the reticular or motor compact zones (Karlsson et al., 1966). Satellite cells exist in association with the various fiber types present in skeletal muscle (Tables I, 11, and 111; Takahama, 1983). Schmalbruch and Hellhammer (1976) reported that satellite cells were associated with the various fiber types present in human skeletal muscle. They did not attempt to segregate satellite cell content by fiber type. However, from the fiber type data given in Table I, no unique relation is evident between fiber type and the incidence of satellite cell nuclei. Gibson and Schultz (1982) examined in detail the relation between the incidence of satellite cells and fiber type to determine if differences in fiber type

INFLUENCEOF FIBERTYPEON OF

Muscle Rat muscles Soleus EDL

Rat muscles Soleus Diaphragm Tibialis anterior

TABLE I1 INCIDENCEOF NUCLEI I N TRANSVERSE SECTIONS MUSCLEFROM THE ADULT THE

Incidence of muscle fiber nuclei/fiber transected (%)

Incidence of satellite cell nucleilfiber transected (%j

2.7

5-6.4 I .7-2.9

1.o

(4.6 x 1 0 4 ~ (6.4 x 1 0 4 ) ~ (2.3 x lo4)"

"Number of nuclei per mm3 muscle

Percentage nuclei within basal lamina that are satellite cell nuclei (%j

(4920)" (5310)" (920)"

10.7 8.5 4.0

Reference Aloisi et a / . (1973) Kelly (1978b); Gibson and Schultz (1982) Schmalbruch and Hellhammer (1977)

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DENNIS R . CAMPION

TABLE I11 PERCENTAGE FIBER TYPECOMPOSITION IN THE GENERAL FIBERPOPULATION AND SATELLITE CELL FIBER POPULATION OF ADULTRAT MUSCLESU

IN THE

Satellite cell-fiber population

Satellite cell/general

Muscle

Fiber type

General fiber population

EDL

Type I1 A Type 1 Type I1 B

14.1 41.5 38.4

23.8 43.8 32.5

1.68 0.92 0.84

Soleus

Type I1 A Type I

14.9 85.1

4.0 96.0

0.26 1.12

“Adapted from Table 3, Gibson and Schultz (1982).

frequency could account for the higher satellite cell content of the rat soleus relative to the extensor digitorum longus muscle. The relative frequency of fiber types in the general population and in the population of fibers that contained satellite cell nuclei in muscle sections is given in Table 111 for the adult rat. The fiber type composition was significantly different between the general population and satellite cell-fiber population for both muscles. The satellite cell-fiber is the ratio of frequency of a given satellite cell-fiber type to the frequency of the corresponding fiber type in the general population. A ratio greater than 1 indicates that the particular fiber type has a relatively greater number of satellite cells per unit length. Since in the extensor digitorum longus, Type I1 B (glycolytic) fibers exhibited a satellite cell population which approximated that of Type I (red) fibers, it was concluded that the higher incidence of satellite cells in soleus muscle was not due to the higher proportion of red fibers present in this muscle when compared to the extensor digitorum longus muscle. A meaningful relation is established when the satellite cell content is compared with the myonuclear-cytoplasmic volume ratio (Table 11). Schmalbruch and Hellhammer (1977) observed that the concentration of satellite cells/mm3 muscle was positively associated with the concentration of myonuclei/mm3 muscle. Thus, a muscle (or fiber type) that possesses a high concentration of myonuclei will have a correspondingly high concentration of satellite cells. Expressing the data on a per unit volume basis eliminates the bias inherent in determining the incidence of satellite cell nuclei per fiber transected that results from variability in fiber diameter of the different fiber types. For example, in shark axial muscle (Table I), the incidence of satellite cell nuclei per fiber transected was 6% in the red fibers and 4% in the white fibers. But, because the red fibers were smaller in diameter and had a higher myonuclear-cytoplasmic volume ratio than the white fibers (Kryvi and Eide, 1977), the concentration of satellite cell/mm3 muscle must have been

THE MUSCLE SATELLITE CELL

23 1

greater in the red portion of the muscle. A similar association can be drawn between red and white muscle portions of the Atlantic hagfish as the red fibers of the parietal and craniovelar muscle were smaller in diameter than the white fibers (Korneliussen and Nicolaysen, 1973) of the parietal muscle. B. CARDIAC MUSCLE Satellite cells are present in the striated, skeletal muscle of almost all species of vertebrates (for representative listings see Muir, 1970; Midsukami, 1981). Possibly one exception to this rule exists as satellite cells are not evident in uninjured adult newt limb muscle (Hay, 1979; Popiela, 1976). Interestingly, satellite cells have not been found in vertebrate cardiac muscle (Mauro, 1961; Stenger and Spiro, 1961; Shafiq et al., 1968). But satellite cells have been identified in the cardiac muscle of a number of species in the order Decapoda (Midsukami, 1964, 1981; Aizu, 1973). In species of this order the cardiac satellite cells show close structural similarity to those cells described in other striated muscles. The cardiac satellite cells frequently extend long, thin processes into the cardiac fibers. These cells possess neither lipid droplets nor glycogen. Presumably their origin and function are similar to their skeletal muscle counterparts (Midsukami, 1981). 111. Gross Morphology

Muir (1970) described the satellite cell as fusiform in shape. However, considerable variation exists. For example, satellite cells on intrafusal muscle fibers of the frog were reported to ramify into smaller cell processes (Karlsson et al., 1966). When studied by freeze-fracture (Schmalbruch, 1978) lateral projections from the satellite cells of rat soleus muscle were seen to extend over the attendant myofiber. These projections resided in grooves on the surface of the myofibers as did the satellite cell proper. With scanning electron microscopy, tails were seen to eminate from central, longitudinally oriented fusiform bodies of satellite cells in frog sartorius muscle (Mazanet et al., 1982) and in rat sternomastoid muscle (Mazanet, 1981). The tails were generally paired and originated from either end of the cell body. When a third tail was evident, it originated either as a branch from an existing tail or from the cell body. In the frog sartorius muscle (Mazanet et al., 1982), the fusiform cell bodies were 7-15 pm long and 3-5 km wide. Tails extended up to 40 pm in length from either end of the cell bodies. While the satellite cell surface generally appeared smooth in the freeze-fracture study (Schmalbruch, 1978), use of scanning electron microscopy revealed that the cell body and tails of a few satellite cells were serrated. Serration, when evident, was more extensive in the tail than in the cell body. When serrations were present on

TABLE IV IN NORMAL MUSCLE SATELLITECELLDIMENSIONS Nuclear dimensions (pm)O Species

Muscle

Fruit bat Human, cat, and dog Mouse and rat Frog Fruit bat

Web Gastrocnemius Levator ani EDL (spindle) Web

Guinea pig Dog Axolotl Lizard Mouse Shark R . clamitans R . pipiens Rat Rat

Eye Gastrocnemius Trunk Tail Lumbrical Axial Gastrocnemius Gastrocnemius Hindlimb Soleus Tibialis anterior Diaphragm sartorius

R . pipiens Hagfish

R . pipiens

Parietal and craniovelar Sartorius and peroneus longus sartorius

Length

Width

Depth

Similar (8-12) 13 (16) 20 5.5-10.0 ( 10.0- 12.5)

(3) 4.5

(7)

Satellite cell length (pm) 25 30

2.8 (2.5) 50

25 20 10

2.8-3.5

600 <800 18.56 100

<2 (2-3)

Similar (12.6) 12 12.4 (23.1) 12.8 (24.0) 12.1 (12.6) 9.6 (13.4) 9.6 (16.0) 9.9 (11.5) 20.4 (22)

4.7 (5.1) 4.4 (4.9) 4.3 (5.9)

1.1 (2.6) 1.3 (2.4) 2.4 (2.8)

4.7 (4.6)

1.5 (1.2)

10-15

5

Similar (12.2)

OValues for myofiber nuclei are given in parentheses. Walculated from data in Tables 1 and 2 of Schultz (1974). CCell bodies 7-15 pm, tail extensions up to 40 pm long according to Mazanet er a / . (1982). dCalculated from data of Campion et nl. (1978, 1981a) for belly of muscle.

Reference Muir et al. (1965) Ishikawa (1966) Venable ( 1966) Karlsson et al. (1966) Church ( 1970) Hess and Rosner (1970) Banker and Girvin (1971) Flood (1971) Kahn and Simpson (1974) Schultz (1974) Kryvi (1975) Trupin (1976) Trupin (1976) Snow (1977a) Schmalbruch and Hellhammer (1977)

30-50

Maruenda and FranziniArmstrong (1978) Sandset and Komeliussen (1978)

24.4d

Campion er a/. (1979)

40-90

Larocque er al. (1980)

7-55c

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233

the cell body, they were also present on the tails. This was not always the case in the converse situation. Kryvi (1975) also proposed that the satellite cells of G . melastomus axial muscle possessed bifurcating extensions. Extensions of satellite cell cytoplasm into the myofiber have been observed. In the case of the shark, these extensions can penetrate up to 7 pm into the myofiber (Kryvi, 1975). While the distinction may be a qualitative one, cytoplasmic projections of the myofiber into the satellite cell have only been reported in the muscle of the Atlantic hagfish (Sandset and Komeliussen, 1978). The length of the satellite cell generally ranges between 18 and 50 pm (Table IV). Lizard tail muscle (Kahn and Simpson, 1974) and axolotl trunk muscle (Flood, 1971) possess satellite cells of unusual length. In axolotl trunk muscle, satellite cell nuclei are rarely transected, but cytoplasmic profiles are numerous. The incidence of satellite cells based on cytoplasmic processes may be overestimated if these processes bifurcate or branch. The linear dimensions of the satellite cell nucleus are similar to or slightly smaller than comparable measurements of the myofiber nucleus. Typically, satellite cell nuclei measure 10-15 pm in length, 2-5 pm in width, and 1.1-2.8 pm in depth.

IV. Fine Structure With few exceptions (Flood, 1964; Laguens, 1963; Midsukami, 1964; Muir et al., 1965), the satellite cell nucleus has been described as more heterochromatic than the myofiber nucleus (Fig. 1). The distinction, however, is not always apparent. Occasionally, a myofiber nucleus may exhibit a relatively high degree of heterochromaticity. Moss and Leblond (1971) suggested that such myonuclei were recently incorporated from satellite cells. An oval shape is characteristic of satellite cell nuclei transected in a plane perpendicular to the long axis. Some variation in shape exists, however, as indentations (Maruenda and Franzini-Armstrong, 1978; Takahama, 1983) and an irregular outline (Karlsson et al., 1966) of satellite cell nuclei were reported in frog skeletal muscle. Occasional in-foldings were seen in satellite cell nuclei in human skeletal muscle (Wakayama, 1976) and in the muscle of malnourished or recovering children (Hansen-Smith et al., 1979). However, this situation was not observed in other studies of human muscle (Laguens, 1963; Ishikawa, 1966; Reger and Craig, 1968) which suggests that the satellite cell nucleus is more characteristically oval in shape. Lipofusin is present in the cytoplasm of satellite cells of more mature animals. These inclusions were reported in frog (Trupin, 1976), Atlantic hagfish (Sandset and Korneliussen, 1978), shark (Kryvi, 1975), mouse (Schultz, 1976), and human (Conen and Bell, 1970) satellite cells. The significance of the presence of

234

DENNIS R. CAMPION

FIG. 1. Satellite cell in the sartorius muscle of an oblob mouse at 2 weeks of age.The satellite cell lies beneath the basal lamina (arrowheads) of the myofiber. The chromatin material is slightly more heterochromatic in the satellite cell nucleus than in the myofiber nucleus. The high nuclearlcytoplasmic volume and paucity of cytoplasmic organelles is indicative of a relatively inactive satellite cell. ~ 2 1 , 0 0 0 .

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235

lipofusion is not known. Specific mention of the presence of lysosomes has been made for satellite cells of the frog (Franzini-Armstrong, 1979), Atlantic hagfish (Sandset and Korneliussen, 1978), shark (Kryvi, 1975), and pig (Campion et al., 1978). The diversity of species within which lipofusin and lysosomes have been described suggests that they are normally occurring structures in satellite cells. The presence of glycogen, on the other hand, is restricted to the satellite cells in the axial muscles of G . melastomus (Kryvi, 1975). Using differential staining techniques to distinguish between small glycogen particles and free ribosomes, Schiaffino and Hanzlikova (1972) and Galavazi (1971) demonstrated the absence of glycogen particles in the satellite cells of young and adult rat skeletal muscle, respectively. The absence of glycogen particles in satellite cell cytoplasm has been reported in several other species, namely, human (Ishikawa, 1966; HansenSmith et al., 1979), cat, dog (Ishikawa, 1966), shark (Flood, 1964), mice and rats (Snow, 1977a), and pig (Campion et al., 1978). Glycogen particles are also not evident in satellite cells of Japanese quail (Coturnix coturnix japonica) and ob/ob mice (Campion, unpublished). Myofilaments are characteristically absent in satellite cells while the presence of microtubules and microfilaments is often observed. The mitochondria of satellite cells are few in number and smaller in size and exhibit fewer internal cristae when compared to myofiber mitochondria. Centrioles have been observed in the satellite cells of normal skeletal muscle of human, cat, dog (Ishikawa, 1966), pig (Campion et al., 1978), rat (Snow, 1977a), mouse (Muir et al., 1965; Shultz, 1976), fruit bat (Muir et al., 1965), frog (Franzini-Armstrong, 1979), and Atlantic hagfish (Sandset and Kornelieussen, 1978). Another characteristic observation is that pinocytotic vesicles are present at the satellite cell wall exposed to basal lamina as well as that exposed to the myofiber. Lamellar bodies were reported within endoplasmic reticulum cisternae and the nuclear envelope of satellite cells in the muscle of children and adults (Wakayama et al., 1981). The structures were most frequently observed in the space between the satellite cell and myofiber. Wakayama et al. (1981) proposed that lamellar bodies were part of a mechanism to transfer or exchange phospholipid material between satellite cell and fiber. As yet, there is no proof of this hypothesis. Bearing in mind that a spectrum of fine structural detail exists, satellite cells can be classified at the extremes as being either active or inactive based on fine structural analysis. The variation in ultrastructural detail was most eloquently described by Schultz (1976) for the mouse lumbrical muscle, by Snow (1977a) for the soleus and gastrocnemius muscles of the mouse and rat, and by HansenSmith et al. (1979) for the vastus lateralis muscle of humans. The physiological state of the muscle is basically reflected in the proportion of active to inactive satellite cells, e.g., in young, growing animals a high propor-

236

DENNIS R. CAMPION

tion of metabolically active cells is seen while the opposite is apparent at older ages. The ultrastructural characteristics described above are typical of satellite cells in general. But, in inactive cells the nuclear-cytoplasmic volume ratio is relatively greater, and the nucleus may be slightly more heterochromatic and nucleoli are very infrequent when compared to the activated state. If rough or granular endoplasmic reticulum is present, segments are short and fragmented. Kahn and Simpson (1974) stated that rough endoplasmic reticulum was not present in the satellite cells of lizard tail muscle. This latter situation is certainly unusual and represents the extreme. A variable number of free ribosomes are located in the cytoplasm. When observed, the Golgi apparatus possesses few folds and is otherwise poorly developed. Cilia are found in the satellite cells of healthy muscle from the fruit bat (Muir et al., 1965) and the mouse (Schultz, 1976) but their function is not known. The more dynamic state of the satellite cell is characterized by a lower nuclear-cytoplasmic volume ratio which is a result of the elaboration of cyto-

FIG. 2. Satellite cell in peroneus longus muscle of the pig at 95 days of gestation. The elaborate Golgi apparatus (arrowheads), and the presence of a centriole (arrow) in this satellite cell is indicative of a relatively more active cell. X 10,200.

FIG. 3. Satellite cell in the sartorius muscle of an oblob mouse at 2 weeks of age. Many pinocytotic vesicles (mows), elaborate channels of rough endoplasmic reticulum (arrowheads), and the presence of a relatively greater number of mitochondria indicate an active cell. Note that the nuclear chromatin is relatively similar to the inactive satellite cell depicted in Fig. 1 . The pronounced heterochromaticity of this active satellite cell is in contrast to the more nearly euchromatic appearance of the active satellite cell of Fig. 2. X21,OOO.

238

DENNIS R. CAMPION

plasmic organelles (Figs. 2 and 3). Nuclear chromatin may become more euchromatic and the nucleoli may become more prominent (Fig. 2). But a heterochromatic appearance of the nuclear chromatin is also commonly found (Fig. 3). The Golgi is well developed and more extensive. The rough endoplasmic reticulum is more elaborate and the channels appear longer and more torturous. Schultz (1976) suggested that basal lamina material might be produced by the satellite cell; but this hypothesis has not been tested. Satellite cells of mouse lumbrical muscle (Schultz, 1976) have polysomes of 5 to 6 units, while in the pig strands of 10 to 12 units are apparent (Campion et al., 1978). Based on the work of Heywood and Rich (1968) strands this length should be capable of producing proteins of 16,000 to about 35,000 daltons, respectively.

V. Situations Affecting Satellite Cell Content A. NORMALGROWTH The addition of myonuclei to growing fibers is brought about through mitotic activity (MacConnachie et al., 1964). That satellite cells undergo mitosis in vivo during normal growth and development is well documented. For example, satellite cells have been observed in metaphase after arrest with colchicine (Shafiq et al., 1968; Allbrook et al., 1971; Hellmuth and Allbrook, 1973). DNA synthesis and mitotic activity have also been demonstrated by the use of [3H]thymidine (Moss and Leblond, 1970, 1971; Allbrook el al., 1971; Hellmuth and Allbrook, 1973; Snow, 1977c; Kelly, 1978b). In addition, proliferation of satellite cells in cultures of isolated fiber preparations has been observed in mammalian (Bischoff, 1975) and in avian (Konigsberg et al., 1975) species. No evidence of mitosis in myofiber nuclei was found in any of these studies. From the original studies of Moss and Leblond (1970, 1971), the appearance of [3H]thymidinelabeled myonuclei was considered to be the result of incorporation of daughter satellite cell nuclei that were labeled during mitosis of the mother cell. Thus, nuclei added to muscle fibers during normal growth and development originate from satellite cell nuclei. Presumably, the daughter nuclei can be added to the growing myofibers without regard to location on the fiber (Snow, 1979). Satellite cells are first distinguishable morphologically when the basal lamina begins to envelope the individual muscle fibers (Church, 1969; Dorn, 1969; Kelly and Zacks, 1969; Conen and Bell, 1970; Ontell, 1974; Cardasis and Cooper, 1975; Ontell and Dunn, 1978). Usually by birth to 1-2 weeks postnatally in mammals and by the time of hatching in fowl, the basal lamina fully encompasses the individual myofibers. As the basal lamina forms it “entraps” myoblastic cells. These myoblastic cells, or satellite cells, become situ-

THE MUSCLE SATELLITE CELL

239

ated between the cell wall of the myofiber and the basement membrane. These cells remain throughout life. For example, Schmalbruch and Hellhammer (1976) observed a satellite cell in the extensor digitorum muscle of a 73-year-old man. Their presence has also been verified in the levator ani muscle of 3-year-old rats (Gutmann and Hanzlfkovi, 1972) and in the muscles of 29- to 30-month-old mice (Snow, 1977a; Allen et al., 1982) and rats (Schultz and Lipton, 1982). In the mouse, deposition of basal lamina happens about the nineteenth day of gestation (Cardasis and Cooper, 1975). However, newly deposited basal lamina material encompasses clusters of fetal myoblasts, myotubes, myofibers, satellite fibers, and other cell types (Kelly and Zacks, 1969; Ontell, 1977; Ontell and Dunn, 1978). Unless serial section is used at these early developmental stages to differentiate among the various cell types, identification of cells that contain no nucleus or myofibrils in cross section cannot be conclusively labeled as satellite cells or as early formed myotubes. In the early stages of development, when the incidence of satellite cells is high, many of these cells appear activated by the ultrastructural criterion outlined in Section IV. This morphology is qualitatively similar to that of the presumptive myoblast (Lipton, 1977; Przybylski, 1971). It is not known, however, if the postnatal satellite cell and the mononucleated myogenic cells of embryonic origin are unequivocally identical. On the one hand, Jones (1982) demonstrated that satellite cells isolated from regenerating muscle in adult rats and embryonic presumptive myoblasts from rats responded similarly in culture, i.e., each proliferated and ultimately fused to form myotubes. These cells did not differ in growth rate or fusion characteristics. The same increase in creatine kinase activity and shift in isozyme profile developed after fusion. Based on temporal and spatial changes in acetylcholinesterase activity in fetal rabbit muscle, Tennyson et al. (1973) concluded that the satellite cell was a remnant from embryonic development. Young ef al. (1979) cultured myogenic cells from mice less than 24 hours old and from 3-week and 7-week-old mice and measured protein synthesis in the myotubes. In this study, age did not affect quantitatively or qualitatively the ability of the cells to differentiate and to synthesize and assemble the myofibriller proteins in culture. On the other hand, Allen ef al. (1982) reported that myotubes which differentiated from neonatal muscle cells of the rat accumulated more than three times as much a-actin per myotube nucleus as myotubes differentiated from the satellite cells of older rats. Furthermore, Schultz and Lipton (1982) found that, as the age of normal donor rats increased from 6 days to 30 months of age, the satellite cells required a longer time to initiate proliferative activity in culture. Replicative capacity also decreased with age. Studies have not been conducted to determine if the satellite cells and embryonic myogenic cells isolated and cultured from the same species under the same conditions respond similarly to hormonal and other growth factors. Quantitative data (Table V) exist for several species to confirm that the satel-

AGE-ASSOCIATED CHANGES IN

THE

TABLE V RELATIVE AND ABSOLUTENUMBER OF SATELLITE CELLS Postnatal

Days gestation Species

Muscle

Rat

Subclavius

Mouse

Lumbrical

Mouse pig

Trait %SCN/TN" Number ( X lO5)d

%SCNe %SCN/TN Numberg Gastrocnemius %SCN/TN Numberh sartorius Peroneus Longus

%SCN %SCN/TN Numbern %SCN %SCN/TN Numbern

95

110

Days 1

7

31.7 1.06

25.36 1.496

32 50

18 28 23 27 55

14

21 17.4c 1.04'-

11

16 15 -20 48

10 15 15

28

Months

35

42 49 63

11.4 9.4 0.99 1.10

70

6

32

4.6 4.3 0.95 1.05

64

Reference Hellmuth and Allbrook (1973) Schultz (1974)

4f ( i f

7f 10 28

5 15

7 7 6 22 25 21

5 3 3-4 18 11 12-13 122 95 100-130 12 7 7-7 31 20 18-18 23 183 199-220

O%SCN/TN, percentage nuclei within basal lamina that are satellite cell nuclei. bTen day values. cFifteen day values. dLTsed N = PL/d, P = number of nuclei per cross section of muscle; L = muscle length, d = nuclear length. <%SCN, percentage fibers in cross section that exhibit satellite cell nuclei. mhirty day values. KCalculations of number of satellite cells by formula, N = PL/d, where P = %SCN, L = muscle length, d = nuclear length. hCalculations based on observations of isolated fibers.

4 6 450 5 8 800

Cardasis and Cooper (1975) I Campion er al. 1 (1979, 140 1981) 8 4 720

THE MUSCLE SATELLITE CELL

24 1

lite cell population is not constant throughout life. As more daughter nuclei are added to the growing muscle, the number of myonuclei increases out of proportion to any changes in the satellite cell population. Thus, the percentage of nuclei within the basal lamina that are satellite cell nuclei decreases until the mature muscle weight is attained. At very old ages this percentage will once again begin to decrease. Snow (1977a) suggested that this latter decrease might be due to passage of some satellite cells into the intestitial space during normal aging. In the mouse (Schultz, 1974; Cardasis and Cooper, 1975; Young et al., 1978), the apparent number of satellite cells decreased as age increased. This is the only species, however, wherein an inverse relation has been consistently shown. In the rat (Hellmuth and Allbrook, 1973), the apparent number of satellite cells in the subclavius muscle changed little between 1 day and 6 months, while apparent satellite cell number increased dramatically from 1 day to 32 weeks of age in two pig muscles (Campion et af., 1979, 1981b). Apparent satellite cell number also increased in the sartorius muscle of Japanese quail between 4 days and 4 weeks of age (Campion er af., 1982a). Between 1 month and 12 months of age, the number of satellite cells per unit length of muscle increased in the soleus muscle but decreased in the extensor digitorum longus muscle of the rat (Gibson and Schultz, 1982). Although muscle length was not determined in this study, it can be concluded that the apparent number of satellite cells increased in the soleus muscle over the age period studied because the satellite cell number per unit of muscle length increased. The variation seen in apparent satellite cell number suggests that each satellite cell mitosis does not necessarily result in the incorporation of one daughter nucleus into the myofiber and in the other daughter nucleus becoming a satellite cell. This is particularly shown by the increase in apparent satellite cell number in pig muscles between 1 and 32 weeks of age (Table IV) and in Japanese quail muscle between 4 days and 4 weeks of age. Conversely, the near twofold reduction in apparent satellite cell number between 14 and 28-30 days of age in mouse muscle indicates that either satellite cell nuclei can be incorporated directly into myofibers, or that both daughter nuclei can be incorporated. The higher incidence of satellite cells that characterizes the soleus muscle when compared to the extensor digitorum longus muscle of the rat is a distinction that appears early in myogenesis, even before histochemical differentiation is evident (Kelly, 1978b). In addition, the normal age-associated changes in fiber type composition in these muscles was not related to age changes in the distribution of the satellite cell-fiber population (Gibson and Schultz, 1982). In the extensor digitorum longus muscle, the concentration and rate of proliferation of myonuclei is lower than in the soleus muscle. This correlates with the finding of a relatively smaller satellite cell population and a more rapid decline with age in the satellite cell labeling index in the extensor digitorum longus muscle than in the soleus muscle (Kelly, 1978b).

242

DENNIS R. CAMPION

Satellite cells are present in anuran tadpole tail muscles during metamorphosis. From the hind-limb-bud stage to climax, the percentage of nuclei within the basal lamina that are satellite cell nuclei decreases from 19.2 to 2.1% in red fibers and from 11.9 to 0.4% in white fibers (Takahama, 1983). Based on morphological evidence, it seems that satellite cells migrate into the interstitium during the late stage of metamorphosis. A similar migration was suggested to occur during metamorphosis of several species of urodeles (Popiela, 1976). Popiela (1976) proposed that these cells were the progenitors of the “pericyte” found in adult salamander muscle. Satellite cells are not seen in adult salamander muscle (Hay, 1979). Whether or not the pericytes in salamanders are indeed myogenic stem cells that differ only in anatomical location has not been clearly established (Hay, 1979). At some stage of adulthood, the satellite cells become mitotically quiescent. This happens at least by 4 months of age in mouse tibialis anterior muscle (Schultz et al., 1978). At 30 months of age, mouse and rat muscle satellite cells were found in situ that contained centrioles, but no ultrastructural evidence of mitosis was seen (Snow, 1977a). Myonuclear proliferation has been investigated in animal models that vary in muscle mass and body composition. These models were employed to ascertain if the concentration of satellite cells and/or if the rate of mitosis and nuclear incorporation were related to differences in rate of muscle growth and in attainment of total muscle mass. In a study of muscle from Japanese quail selected for high body weight, the satellite cells in the muscle of growth strain quail when compared to the control strain (Campion et al., 1982a) exhibited a slightly higher mitotic rate. The muscles of the growth strain Japanese quail also possessed more muscle fibers (Fowler et al., 1980; Campion et al., 1982b). Since the incubation period for eggs is similar for the control and growth strain birds and mature myofiber number is established about the time of hatch, mitotic activity must have also been greater in the embryonic presumptive myoblasts. Therefore, selection for high body weight in this model appeared to result in a higher mitotic rate in the myogenic cells whether they be termed presumptive myoblasts or satellite cells. Penney et al. (1983) studied two strains of mice that differed in myofiber number and concluded that the primary difference in muscling between the two strains was due to a difference in the rate of proliferation of presumptive myoblasts before fusion. This conclusion is in general agreement with the results of the studies with Japanese quail. A question of considerable importance is whether the rate of mitotic activity and incorporation of nuclei into the myofiber can account for the increase in myofiber nuclei seen during normal growth and development. Moss and Leblond ( 197l), using autoradiographic and electron microscopic techniques, calculated the actual and relative incidence of labeled satellite cell nuclei and myofiber nuclei over a 72-hour period in 14- to 17-day-old rats. It was concluded that the

THE MUSCLE SATELLITE CELL

243

mitotic rate and incorporation rate was consistent with the hypothesis that satellite cells can account for all nuclei added to normally growing myofibers of the rat. B. NUTRITION Skeletal muscle is responsive to the nutritional status of the animal (Winick and Noble, 1966). While the mass and DNA content of muscle are both responsive to nutritional manipulation, there is limited knowledge on how the satellite cell might be affected. Hansen-Smith et al. (1978a) examined the effect of 10 weeks of protein restriction or protein-energy restriction on the fine structure of the quadriceps muscle of rats. Most of the myofibers of the treated groups were similar to age matched controls. In each of the dietary groups, a small percentage of the myofibers possessed satellite cells. Morphologically, dietary treatment did not influence satellite cell fine structure. Nuclei were not quantitated in this study. Nutritional restriction of rats during gestation and lactation had a transient effect on the incidence of satellite cells in the skeletal muscles of the progeny (Beermann et al., 1981; Beermann, 1983). In 1-day-old rats from restricted- and control-fed dams, counts of satellite cells and myonuclei were similar. Counts were markedly decreased in control rats at 21 days of age but remained unchanged or elevated in restricted rats. Ad libitum feeding of restricted and control rats from weaning to 175 days of age resulted in similar counts in the two groups of offspring at 175 days. It might be inferred from this study that the satellite cell population changes in concert with the weight of the muscle rather than with the age of the animal. Seerley er al. (1974) first suggested the possibility of feeding high levels of dietary fat to the sow immediately before farrowing to improve the energy status of the piglets at birth. Work in this area was recently reviewed by Pettigrew (1981). We examined the effect of feeding diets containing 0, 2, 10, or 30% fat to sows during late gestation on the cellularity of the fetal sartorius muscle (Campion et al., 1983). Maternal diet had no effect on muscle weight, on myofiber nuclear content, or on satellite cell nuclear content. In the only studies conducted on humans, Hansen-Smith et al. (1978b, 1979) examined the myonuclear content of the vastus lateralis muscle of malnourished children. The percentage nuclei within the basal lamina that were satellite cell nuclei was lower in the muscle of malnourished children than in the muscle of either their clinically recovered or well-nourished counterparts. From the limited work in this important area, it can be suggested that only when muscle weight is affected by nutritional manipulation is the population of satellite cells affected.

244

DENNIS R. CAMPION

C. MUSCLEREGENERATION AND COMPENSATORY HYPERTROPHY Muscle regeneration has long been studied (Bottcher, 1858; Waldeyer, 1865; Volkmann, 1893). From recent reviews of skeletal muscle regeneration (Carlson, 1973; Reznik, 1976; Allbrook, 1981), it has been concluded that the essential process of regeneration was similar regardless of the cause of the injury or disease. In addition, muscle regeneration appears to capitulate embryonic myogenesis-mononucleated myogenic cells (embryologically, presumptive myoblasts) proliferate, line up usually within the confines of the residual basal lamina (embryologically basal lamina is formed after initial myotube formation), and fuse to form myotubes. These myotubes mature into fibers if innervated and if the blood supply is adequate. The satellite cell is thought to be a source of these myogenic cells which impart to skeletal muscle the ability to regenerate. Several lines of evidence exist to support this hypothesis. Satellite cells have been harvested and cultured (where they proliferate and fuse to form myotubes) from the skeletal muscle of mice (Young et al., 1978) and rats (Allen et al., 1980; Jones, 1982) at various ages, up to 30 months of age in the rat (Allen et al., 1982), and from adult humans (Nag and Foster, 1981). In addition, explants of muscle tissue or fibers from human (Mendell et al., 1972; Askanas and Engel, 1975), rat (Bischoff, 1974, 1975), avian (Konigsberg et al., 1975), and other species yield mononucleated cells in culture that also proliferate and differentiate into myotubes. Bischoff and Konigsberg et al. concluded from their studies that satellite cells were a source of the proliferative myoblasts observed in culture. The autoradiographic-electron microscopic studies of Snow (1 977c, 1978), Lipton and Schultz (1979), and Trupin et al. (1982) lend further evidence to implicate the satellite cell as a source of the myogenic cells involved in regeneration. The effect of a lesion in the central nervous system on the muscle satellite cell population was reported by Campion et al. (1978). Removal of the influence of the brain by spinal cauterization of the fetal pig in early gestation resulted in a reduced incidence of satellite cells in the sartorius muscle of the near-term fetus. Decapitation of' the fetal pig, however, had no influence on the content of myofiber nuclei or satellite cells in the peroneus longus muscle (Campion et al., 1981a). A gross endocrine imbalance existed in the decapitated fetus (Martin et al., 1983). Whether or not the effect of the endocrine imbalance counteracted the loss of the influence of the brain is not known. Since different muscles were examined in the two fetal pig studies, the differences in treatment effects may alternatively have been the result of variation in muscle specific characteristics. In a variety of situations, the satellite cell has shown considerable motility. For example, neofiber formation has been observed in denervated muscle (Miledi and Slater, 1969; Ontell, 1975; McGeachie and Allbrook, 1978; Schultz, 1978), in minced muscle implants (Snow, 1977b), in muscle with experimentally in-

THE MUSCLE SATELLITE CELL

245

duced fibrillation (Salleo et al., 1979), in muscle grafts (Hansen-Smith and Carlson, 1979; Ontell et al., 1982), and in muscle undergoing compensatory hypertrophy (McGeachie and Allbrook, 1978; Salleo et al., 1980). Under these various conditions, the satellite cells migrate out into the interstitium, line up, and fuse to form new fibers. The evidence to support migration, however, is largely circumstantial. Many cell types are labeled when tracer studies using [3H]thymidine are used, making it impossible to accurately follow movement of the satellite cell in situ. But, the work of Bischoff (1974, 1975), Konigsberg et al. (1975), and Lipton and Schultz (1979) revealed that satellite cells could migrate through injured or intact basal lamina. Miledi and Slater (1969) suggested that satellite cells may have a role in fiber splitting. There is still no direct evidence to support this hypothesis. In fact, Schiaffino et al. (1979) argued that the term “fiber splitting” was misleading, as “splitting” fibers can correspond to satellite fibers or to branched fibers. Recent studies of muscle hypertrophy in rats (James and Cabric, 1981) and in mice (Atherton et al., 1981) suggest that fiber splitting does not happen in skeletal muscle undergoing hypertrophy as a result of overload. Lipton and Schultz (1979) demonstrated that satellite cells, labeled with [3H]thymidine in culture, underwent extensive migration away from the implant site when returned to the muscle of the original donor. In both rats and quail, most of the labeled cells penetrated the basal lamina of intact myofibers while a few appeared in satellite fibers. It was suggested that, after an injury, satellite cells from undamaged fibers may migrate to the site of the lesion and participate in the regenerative process. Schultz (1978) also postulated this mechanism in denervated mouse and rat muscle. In addition, Hall-Craggs and Lawrence (1970) reported seeing satellite cells forming a bridge between two myofibers in the soleus muscle of the rat 10 days after crushing. Apparent migration of satellite cells between myofibers was described in normal mouse lumbrical muscle (Schultz, 1976), in the gastrocnemius muscle of the human fetus (Ishikawa, 1966), and in shark muscle (Kryvi, 1975). In situations evaluated solely by morphological observation, however, the possibility that the bridging cell was an invasive cell rather than a satellite cell cannot be entirely ruled out (Trupin, 1979; Trupin and Hsu, 1979; Maruenda and Franzini-Armstrong, 1978).

VI. Activation Stimulus Certain hormones provide a mitogenic stimulus to myogenic cells (for recent reviews, see Allen et al., 1975; Allen, 1979). Little information is available, however, on the effect of hormones on the satellite cell population in situ. After 12 months of streptozotocin-induced diabetes mellitus, the incidence of satellite cells appeared to be increased in the red fibers of the diaphragm muscle when compared to control rats (Bestetti et al., 1981). Injection of glucocorticoids into

246

DENNIS R. CAMPION

rabbits from 1 to 9 days after birth caused retardation of muscle differentiation and satellite cell degeneration (Jirmanova et al., 1982). It is not known if the effects observed on the satellite cells in these studies were directly or indirectly related to the particular hormones. In culture, synthesis of muscle specific actin by fused rat satellite cells was not influenced by addition of growth hormone or testosterone (Allen er al., 1983). Myotube cultures from neonatal rat, embryonic chick, and Yaffe’s L-6 myogenic cell line did not alter rates of incorporation of a-amino isobutyric acid in the presence of near physiological concentrations of growth hormone (Ewton and Florini, 1980). While more careful comparisons are needed, the results from these two studies suggest that myotubes derived from presumptive myoblasts and satellite cells do not respond to growth hormone. Passive stretch of chicken wing muscles results in an increase in total muscle DNA content (Bamett et al., 1980; Holly er al., 1980). In this situation, neuromuscular junction activity is not altered. Although fiber damage (and subsequent release of mitogens) cannot be ruled out, it seems possible that mechanical stretch of the satellite cell, per se, may initiate activation of the satellite cell. Satellite cells were activated in the bulbi rectus superior muscle of the rat by light compression (Teravainen, 1970). That no discernible injury occurred further suggests that activation can result by a purely physical mechanism. Furthermore, denervation atrophy was inhibited when denervation was combined with synergistic incapacitation (Schiaffino and Hanzlikovi, 1970). Murray and Robbins (1982a,b) suggested that denervation of muscle causes tissue disruption that results in release of mitogen(s) which enhance proliferation of connective tissue cells and satellite cells. They cited altered surface membrane proteins, altered ionic permeabilities, increased release of enzymes or degradation products, and appearance of proteolytic enzyme activity as potential mitogenic signals. These “signals” exist in denervated mammalian muscle for several weeks. Selective stimulation of satellite cell proliferation may result from release of a muscle-cell specific mitogen during injury (Bischoff, 1981). Salleo et al. (1979) postulated that fibrillation may be the common link whereby denervated muscle and muscle undergoing compensatory hypertrophy exhibit similar characteristics in terms of satellite cell activation and neofiber formation. And Anastasi et al. (1979) hypothesized that satellite cell division could be stimulated by motor activity. As yet there is no unified hypothesis to explain activation of satellite cells and how the activation process is regulated.

VII. Summary Since the first reports of satellite cells in 1961, considerable knowledge has accumulated concerning their phylogenetic distribution and their location, mor-

THE MUSCLE SATELLITE CELL

247

phology, and function. There is no doubt that satellite cells are capable of undergoing mitosis and that they have considerable motility. These cells function as the progenitors of the myofiber nuclei that must be added during normal (postnatal) growth of muscle. In muscle undergoing or attempting to undergo regeneration, the satellite cell functions as a myogenic stem cell to produce myoblasts that line up and fuse within the scaffolding of the remnant basal lamina or migrate into the interstitium to produce neofibers. A number of problems remain to be solved concerning the regulation of satellite cell function. At this time it is equivocable whether or not the presumptive myoblast and the satellite cell are functionally identical and at the same stage of myogenic differentiation. Apparently there is species variation in terms of the ability of myotubes from embryonic myogenic cells and satellite cells to synthesize protein. The mechanism(s) by which a wide variety of stimuli activate satellite cells is not known, nor is the mechanism(s) by which satellite cells become inactive during the latter stages of growth and adulthood known. Mitogenic factors are present in damaged muscle; but the specific characteristics of these factors and their mechanism of activation are also unknown. Hormones are certainly involved in the regulation of proliferation and differentiation of myogenic cells, but whether presumptive myoblasts and satellite cells or their myotubes respond similarly to hormones in culture has not been adequately examined. Greater understanding of these mechanisms will increase the possibility of total muscle recovery from severe injury or disease. Such knowledge would also have particular application to the production of meat animals and to a greater understanding of the growth process in general.

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