Muscle fiber necrosis and regeneration induced by prolonged weight-lifting exercise in the cat.код для вставкиСкачать
THE ANATOMICAL RECORD 211:133-141(1985) Muscle Fiber Necrosis and Regeneration Induced by Prolonged Weight-Lifting Exercise in the Cat C.J. GIDDINGS, W.B. NEAVES, AND W.J. GONYEA University of Texas Health Science Center, Department of Cell Biology, Dallas, TX 75235 For periods ranging from 26 to 87 weeks, the morphological charABSTRACT acteristics of the flexor carpi radialis (FCR) muscle were examined in four cats trained to perform weight-lifting exercise. Four untrained, sex and weight-matched cats served as controls. The right FCR from each cat was surgically isolated, attached to a tension transducer, and set at its optimal 1ength.The forelimb was perfused with 2% glutaraldehyde in 0.1 M cacodylate buffer. Small bundles of fibers were teased from their origin and insertion tendons and embedded in Epon. Spaced serial sections were used to examine the morphological features of the fibers for trained and control animals. Ultrastructural examination revealed muscle fiber degenerative changes, such as pyknotic nuclei, disruption of the sarcolemma, vacuolation, and disorganization of myofilaments. Such changes were observed a t a higher frequency in trained muscle than in control muscle. Spaced serial sections of fiber bundles showed that the degree of degeneration varied along the length of the fiber. Fiber area measurements showed that trained muscle had both larger and smaller fibers than control samples. The very small fibers observed in the trained muscle were considered to be regenerating or “new” fibers since they had not undergone degenerative changes. “Satellite-like” cells were observed in trained muscle. Such cells resembled satellite cells but also contained developing myofilaments. Since evidence of degeneration-regeneration was observed in control samples, but at a lower frequency, it was postulated that weight-lifting exercise accelerates muscle fiber turnover in the cat FCR. It is well known that skeletal muscle adapts differently to different forms of exercise. High resistance exercise such as weight-lifting induces increases in muscle mass (hypertrophy) whereas endurance training usually does not. Muscle hypertrophy has generally been attributed only to increases in muscle fiber size. However, it has also been suggested that high resistance exercise may increase fiber number (hyperplasia) (cf. Gonyea, 1980). Muscle biopsy studies in humans support the concept of hyperplasia. Biopsies from elite (competition class) bodybuilders and powerlifters showed no difference in fiber size when compared to control samples in spite of large differences in elbow extension strength and arm girth (MacDougall et al., 1982). Similar observations were reported by Tesch and Larsson (1982) in a study comparing competition class bodybuilders to nonstrength-trained physical education students. Mean fiber area did not differ between the two groups in spite of greater limb circumference and muscle mass shown by the bodybuilders. These observations indicate that hypertrophy of muscle fibers cannot account for the increased strength or limb circumference and that the bodybuilders may possess a greater number of fibers than control subjects. However, the concept of hyperplasia still remains controversial (Gollnick et al., 1981). Recently, studies have shown that muscle fiber damage occurs with intense exercise. Acute eccentric exercise in rats running downhill resulted in necrosis of muscle fibers (Armstrong et al., 1983).Disruption of the 0 1985 ALAN R.LISS, INC. normal banding pattern occurred in some fibers. Later, accumulations of monocytes and macrophages were observed in muscle fibers and in the interstitium. Necrosis of muscle fibers also occurred in response to marathon running in humans (Hikida et al., 1983).Ultrastructural changes, most evident 1-3 days after a marathon, consisted of empty basal lamina tubes representing muscle fiber remnants, disrupted sarcolemma, and infiltration by phagocytic cells. These observations suggest inflammation may be a contributing factor in a n acute response to intense exercise. Less is known about the long-term effects of exercise on muscle morphology. A higher percentage of muscle fibers from elite bodybuilders, who had trained intensively for a n average of 7 years, showed abnormalities when compared to control individuals (MacDougall et al., 1982). Central nuclei were present more frequently in muscle fibers from the elite group. Enlarged cytoplasmic spaces and atrophied fibers were found only in samples from the elite bodybuilders. The atrophied fibers exhibited no degenerative changes except that they were reduced in size when compared to other normal fibers in the biopsy sample. The presence of central nuclei and small size strongly suggests fibers undergoing regeneration (Carlson, 1973; Carlson and Faulkner, 1983). Received November 4, 1983; accepted September 11, 1984. 134 C.J. GIDDINGS, W.B. NEAVES, AND W.J. GONYEA Preliminary data from cats trained to perform weightlifting exercise indicated that long-term high resistance exercise may induce degenerative changes in muscle along with regeneration which may provide a mechanism for hyperplasia (Gonyea et al., 1982). This study was undertaken to examine the morphological characteristics of skeletal muscle after prolonged high resistance exercise (weight-lifting) in cats and to determine the relationship of fiber size to morphological changes in the exercised muscle. METHODS Four adult cats (three female and one male) were trained to perform weight-lifting exercise for a food reward according to the method of Gonyea and Ericson (1976). Four sex- and weight-matched untrained adult cats served as control animals. The trained cats performed one arm curls by grasping a bar with their right forelimb and moving the bar a specific distance. Weights attached to the bar via a pulley were lifted as the bar was moved. Additional weight was added weekly throughout the training period which ranged from 2687 weeks (average 51 wks). The length of the training period was determined by the ability of the cat to lift increasing amounts of weight. When the cat had achieved its maximum level and could not lift the next increment of weight, it was anesthetized with sodium pentobarbital (35 mgkg). The long insertion tendon of the right flexor carpi radialis (FCR) muscle was surgically isolated from the forelimb of the animal and the tendon was attached to a tension transducer. The muscle was set at its optimal length following the procedure of Gonyea and Bonde-Petersen (1977). The brachial artery was cannulated and the forelimb was perfused with 50 ml minimal essential medium (MEM) followed by 50 ml 2% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.3). The FCR was then removed from the animal, weighed, and stored in fresh fixative a t 4°C overnight. The right FCR muscles from the untrained cats were perfused in the same manner. Muscle fiber bundles of 200-500 fibers were teased from their origin and insertion tendons of the muscle using microsurgical technique under a dissecting microscope. The bundles were postfixed in 1%osmium tetroxide, stained in uranyl acetate overnight, dehydrated, and embedded in Epon 812. A modification of the technique used by Ontel (1977) of spaced serial ultrathin sections was employed to examine the structural characteristics of the muscle fibers throughout their length. The embedded bundles were cut transversely into 1-mm segments and mounted on blank Epon blocks. Serial transverse sections were cut a t 0.5 pm and 1.5 pm from each block. The 0.5 pm sections were stained with toluidine blue and used for orientation to follow individual fibers through their length. The 1.5 pm sections were stained with p-phenylene diamine (Korneliussen, 1972) and were used for fiber area measurements. Fiber areas were determined from single cross sections near the midregion of the fiber bundle. Photomicrographs ( x 25) of the sections were taken and a Summagraphics digitizing tablet on line with a DEC 10 computer was used to determine muscle fiber area. Spaced serial ultrathin sections were taken from areas of interest observed during light microscopic evaluation of the 0.5 pm and 1.5 pm sections. The ultrathin sections were stained with lead citrate and viewed using a Zeiss electron microscope. The data were analyzed statistically using Student’s t test unless Bartlett’s test for equal variances demonstrated nonparametric analysis should be used, in which case the Mann-Whitney test was used (Zar, 1974).Means are expressed one standard error of the mean. RESULTS Mean body weight for trained and control cats did not differ significantly and was 3.03 0.34 kg and 2.92 & 0.15 kg, respectively (mean 5 SE). The maximum weight lifted by the four trained animals ranged from 1,140 to 1,760 gm (mean 1,375 gm). The mean weight of the right FCR muscle from trained animals was 1.362 f 0.156 gm. This represented a 23.6% increase in muscle weight when compared to the mean weight of the left FCR muscle from the same animals (1.096 5 0.118 gm, P = 0.06). However, when the muscle mass of the right FCR from the trained cats was compared to the muscle mass of the right FCR from control cats (0.915 f 0.062 gm), there was a significant increase in muscle weight of 48.6% in the trained animals (P < 0.05). The average daily work and power outputs for the trained cats are shown in Figures 1 and 2, which represent the mean of the four trained animals. Each individual cat showed a similar trend. There was a gradual increase in power output from 100 gm to about 1,000 gm of weight lifted. The power output then showed a greater rise when heavier weights were lifted. The work performed by the cats increased until the animals were lifting about 1,000 gm a t which time the work appeared to level off. Ultrastructural examination of the muscle samples demonstrated degenerative morphological changes such as disrupted sarcolemma, disorganization of myofilaments, vacuolation, and pyknotic nuclei. Such changes were more frequently observed in trained muscle than in control muscle. Fibers from trained and control animals were followed at 1 mm intervals. Areas of focal necrosis occurred in about 5% of the muscle fibers examined in trained animals compared to less than 1%of the control muscle fibers. This preliminary frequency was based on a sample of 1,729 fibers from exercised animals and 746 fibers from control cats. Central nuclei were observed in trained and control muscle; however, their frequency was not determined. Hypertrophic fibers were observed only in trained muscle (Fig. 3). Ultrastructural examination of these fibers revealed disorganization of contractile elements (Fig. 4). The sarcolemma of some fibers was fragmented although the basal lamina remained intact (Fig. 5). Some fibers had extensive regions of disrupted sarcolemma (Fig. 6). Membrane-bound vacuoles were observed in many necrotic fibers (Fig. 7). Pyknotic nuclei were observed in fibers undergoing degeneration. Occasionally, invasive cells were found within a degenerating fiber (Fig. 8). By examining spaced serial sections, the degree of degeneration was determined to vary along the length of the fibers. Figure 9 shows a group of fibers followed a 1-mm intervals through the middle region of the fibers. A spectrum of degenerative changes can be seen in one fiber of this group. This fiber has normal structural features over a portion of its length (Fig. 9A). At this level its sarcolemma is intact and myofilaments appear 135 MUSCLE FIBER TURNOVER i WORK OUTPUT POWER OUTPUT 0+ 3 - c - 3 0 - I L '. w 0 N 0 l * l I I I I 500 I . . I . . . . I . . I . I . . . . 1000 WEIGHT LIFTED IGRANS 1 Fig. 1. Average daily work output (joules) performed by four exercis- ing cats. Fig. 3. Light micrographs of hypertrophic fibers observed in trained . Cross-sectional area of fiber = 5,642 pm2. B) Crossmuscle. ~ 9 5A) 500 1000 YEIGHT LIFTED C GRAHS ) Fig. 2. Average power output (joules per second) performed by four exercising cats. sectional area of fiber = 6,192 pm2. C) Cross-sectional area of fiber = 7,270 pm2. D) Cross-sectional area of fiber = 4,904pm2. 136 C.J.GIDDINGS, W.B. NEAVES, AND W.J. GONYEA Fig. 4. Electron micrograph of a hypertropic fiber in cross section from the middle region of the fiber. The fiber has lost the organization of myofilaments. x 15,200. Fig. 5. Electron micrograph of a necrotic (N) fiber in cross section. Sarcolemma is disrupted (arrowhead) although basal lamina is intact. Mitochondria appear swollen. A healthy fiber (M) is seen next to the necrotic fiber. ~25,225. Fig. 6. Electron micrograph of a necrotic fiber in cross section. Exten- sive areas of disrupted sarcolemma can be seen (arrowhead). Basal lamina is intact. Mitochondria are swollen. x 14,800. Fig. 7. Electron micrograph of a necrotic fiber. Membrane-bound vacuoles (V)and a pkynotic nucleus can be seen. X 14,100. Fig. 8. Electron micrograph of a necrotic fiber (N) in longitudinal section. An invasive cell (I) is observed within the fiber. A normal muscle fiber (M) is adjacent to the necrotic fiber. X3,500. MUSCLE FIBER TURNOVER Fig. 9. Light micrographs of a group of fibers in cross section followed at 1-mm intervals through the midregion of their length. One fiber . the (arrowheads) shows a spectrum of degenerative changes. ~ 9 6A) fiber appears normal. B) The fiber shows vacuolation. C) The fiber is enlarged. D) Only a basal lamina tube remains. Fig. 10. Electron micrograph of the necrotic fiber seen in Figure 9C. 137 The fiber contains many swollen mitochondria (M). Sarcolemma is fragmented. ~5,000. Fig. 11. Electron micrograph of the necrotic fiber seen in Figure 9D. Only a basal lamina tube remains containing a few swollen mitochondra (M).BL, basal lamina. ~ 2 , 2 5 0 . 138 C.J. GIDDINGS, W.B. NEAVES, AND W.J. GONYEA . 0 - FIBER AREA - - TRAINED FCR 00: 0 A-- x * - 0 C n r L - IIIIII 0 - 9-0 S t " I 0 ' ' ~ ~ " " " " " " ' ! " " " " " " ~ ' ' ' ' 4000 2000 6000 8( DO FIBER AREA In FIBER AREA -1 CONTROL FCR g2 X . - 0 + z - - u 0 ( - r - Em: 9-r ;,,j 0 - 0 90 1 , , , , , , , , , , , I , , , , , , , , , , , , * , , , 10 Fig. 12. Histograms of muscle fiber cross-sectional area of trained and control samples. organized. In the next segment, 1 mm away, the fiber has a vacuolated appearance (Fig. 9B). Farther along its length, the same fiber has a much greater cross-sectional area when compared to the previous sections (Fig. 9C). At this level the sarcolemma is fragmented and there is a loss of organization of myofilaments. The mitochondria also appear swollen and disrupted (Fig. 10).The most extreme degenerative changes are seen in Figure 9D where the fiber consists of only a basal lamina tube containing a few swollen mitochondria (Fig. 11). The histograms of fiber areas indicated that trained muscle contained both larger and smaller fibers than were observed in matched control samples (Fig. 12). Many of the largest fibers exhibited degenerative changes over a part of their length. The small fibers observed in the trained muscle were considered to be regenerating or "new" fibers since they were observed not to contain degenerative morphological changes (Fig. 13). Figure 16 shows the ultrastructural characteristics of a small fiber from trained muscle. The fiber has a peripherally located euchromatic nucleus, the contractile elements appear organized and densely packed, and there are no indications of atrophy or degeneration. Satellik cells were present under the basal lamina of control and trained muscle fibers (Fig. 14).In trained muscle "satellite-like" cells were observed containing myofilaments and may represent the development of new fibers (Fig. 15).These cells were observed infrequently. At this point we have not systematically sampled for satellite MUSCLE FIBER TURNOVER Fig. 13.Light micrograph from trained muscle showing very small regenerating or “new” muscle fibers. Compare with figure 16, which shows the ultrastructural features of one fiber (asterisk) from this group. x97. Fig. 14. Electron micrograph of satellite cells beneath the basal lamina (BL)of A) trained fibers ( x 8,430) and B) control fibers ( ~ 8 , 2 6 0 ) . 139 Fig. 15. A) Electron micrograph of a “satellite-like” cell (S)observed in trained muscle. A myonucleus (MN) is seen next to it. ~ 8 , 3 6 0B) . A higher magnification of a portion of the satellite-like cell showing developing myofilaments. ~21,200. 140 C.J. GIDDINGS, W.B. NEAVES, AND W.J. GONYEA Fig. 16. Electron micrograph of a small fiber seen in Figure 13. The fiber shows no degenerative changes and therefore, represents a regenerating or “new” fiber. X3,971. cells to determine the frequency of those containing myofilaments. Such cells were not observed in control muscle. DISCUSSION Muscle fiber necrosis has been reported following acute eccentric exercise (forceful stretching of the muscle) in rats trained to run downhill on a treadmill (Armstrong et al., 1983).It appeared that the necrosis was associated with the acute inflammatory response observed during the initial stage of training. Fiber necrosis was also observed as a result of marathon running in humans (Hikida et al., 1983). In contrast to these studies in which a n acute response to exhaustive exercise was reported, our study demonstrates for the first time that muscle fibers undergo degeneration in response to longterm (chronic) high resistance exercise such as weightlifting in the cat. While downhill running produced eccentric muscle contractions, weight-lifting in the cat has been shown to produce predominantly concentric (shortening of the muscle) in the FCR (Gonyea and BondePetersen, 1978). This study describes morphological changes associated with chronic high resistance exercise. Weight-lifting in cats induces muscle hypertrophy (Gonyea and Ericson, 1976) and may also induce increases in muscle fiber number (Gonyea et al., 1983). The right FCR mean weight from trained cats showed a 23.6% increase when compared to the left FCR from the same animals (P = 0.06). However, since the cats use their left limb a s a brace during weight lifting exercise, the left FCR undergoes a n isometric training effect (Gonyea and Ericson, 1976; Gonyea, 1980). This could reduce differences observed in muscle mass when comparing the left FCR to the right FCR following training. When the muscle mass of the right FCR from trained cats was compared to sex- and weight-matched untrained control cats, there was a significant 48.6% increase in the trained muscle (P < 0.05). Muscle hypertrophy has generally been attributed to increases in muscle fiber size. However, in this study fiber area measurements showed that the FCR muscles from trained cats have both larger and smaller fibers than do control samples (Fig. 12). We found that many of the largest fibers had necrotic foci, although fiber necrosis was not limited to a specific fiber size. This would indicate that perhaps there is a limit to increases in muscle fiber cross-sectional area (hypertrophy) at which time fibers may undergo a degenerative process. The largest fibers observed in this study showed degenerative changes such as disrupted sarcolemma, disorganization of myofilaments, pyknotic nuclei, and vacuolation. It has been postulated that fiber necrosis may be selective for a particular fiber type (Hikida et al., 1983). Although fiber types were not determined in this study, Gonyea and Bonde-Petersen (1978) found no change in fiber type proportions in the FCR following weight-lifting exercise. This would indicate that in the cat FCR, muscle fiber necrosis is not fiber-type specific. Although muscle fiber degeneration was observed a t a higher frequency in trained muscles, neither fiber number nor strength decreases with exercise. In fact, our preliminary results indicate that total fiber number in the FCR increases in response to prolonged weighttraining exercise (Gonyea et al., 1983). Therefore, a mechanism must exist to maintain or increase fiber number. It is known that muscle regeneration occurs in response to injury (Carlson and Faulkner, 1983). Previous studies on biopsy samples from elite bodybuilders described small fibers as atrophic although no other degenerative changes were noted (MacDougall et al., 1982).Small fibers observed in this study did not appear necrotic (Fig. 16) and, therefore, most likely represent regenerating or “new” fibers. New fibers could originate from muscle fiber splitting or through proliferation and fusion of satellite cells (Allbrook, 1981; James, 1973; Van Linge, 1962). Branched fibers have been observed in serial frozen sections following weight-lifting exercise in cats (Gonyea et al., 1977). In addition, direct counts of muscle fibers following nitric acid digestion of the muscle have shown that branched fibers can be found in both weighttrained and control muscles in the cat and also in weightoverloaded wing muscles and control muscles in chickens (Gonyea, manuscript in preparation; Gollnick et al., 1983). However, the significance of branched fibers remains unclear. It has been postulated that branched fibers might arise through longitudinal splitting of mature fibers (Gonyea et al., 1977; Ho et al., 1980; James, 1973; Van Linge, 1962). Fiber branches may also be the result of defective myotube formation or from the fusion of satellite fibers to mature fibers during regeneration (James, 1973; Schmalbruch, 197613). Fiber branching (splitting) was observed in this study and in rats after weight-training (Ho et al., 1980). The mechanism by which branching occurs and the role it might play - - in hyperplasia hasyet to be determined. 141 MUSCLE FIBER TURNOVER New fibers might also originate through profileration and fusion of satellite cells (Allbrook, 1981). Satellite cells have been shown to have myogenic potential in tissue culture and in damaged muscle (Bischoff, 1979; Schmalbruch, 1976a). Changes in satellite cell frequency in response to exercise remains to be determined. In this study, small cells which resembled satellite cells and contained developing myofilaments were observed in trained muscle. Fusion and growth of such cells could provide a mechanism for hyperplasia. The importance of satellite cells in the development of “new” fibers in response to weight-lifting exercise must await further analysis. Since muscle fiber degeneration and regeneration are observed at very low frequencies in control muscle, we postulate that high resistance exercise accelerates muscle fiber turnover. The mean power output (Fig. 1)of trained animals indicated that there might be a threshold for changes within the muscle. The power output showed much steeper rise after the cats had exceeded the 1-kg increment of weight lifted. At this time, however, it is not known whether this is correlated with an increase in the degeneration-regeneration process. The work output appears to level off a t higher weight lifted. This is consistent with previously reported work output in weighttraining cats by Gonyea (1980). Although the load the cats lifted increased, the work output did not continue to increase since the cats performed fewer events at the higher weight. Muscle consists of organized units called motor units which are composed of a n alpha motoneuron and all the fibers innervated by it (reviewed by Burke, 1981). It could not be determined in this study if the degeneration observed involved entire motor units or simply individual fibers of a muscle unit. Muscle fiber degeneration could be due to denervation resulting from damage to the nerve fibers or due to disruption of the sarcolemma a t the neuromuscular junction. Although the structural integrity of the neuromuscular junction has yet to be examined in response to exercise, Ontell and Haller (1980) have shown that the motor end plate remains intact even with severe fiber necrosis in dystrophic mice. Innervation of new fibers or reinnervation of regenerating fibers in response to weight-lifting exercise has not been elucidated. In summary, muscle fiber necrosis was observed in the FCR muscle from cats trained to perform concentric high resistance exercise. The trained muscles showed a higher frequency of degenerative changes than did control muscles. The trained muscles also showed a wider distribution of fiber cross-sectional area than did control muscles. Small regenerating or “new” fibers were observed in the trained muscle; these may provide a mechanism for hyperplasia and account for a portion of the increased strength in the trained cats. LITERATURE CITED Allbrook, D. (1981)Skeletal muscle regeneration. Muscle Nerve, 4t234245. Armstrong, R.B., R.W. Ogilvie, and J.A. Schwane (1983)Eccentric exercise-induced injury to rat skeletal muscle. J. Appl. Physiol., 54t80-93. Bischoff, R. (1979)Tissue culture studies on t h e origin of myogenic cells during muscle regeneration in the rat. In: Muscle Regeneration. A. Mauro, ed. Raven Press, New York, pp. 13-29. Burke, R.E. (1981)Motor units: Anatomy, physiology, and functional organization. In: Handbook of Physiology. The Nervous System Motor Control. V. Brooks, ed. American Physiological Society, Bethesda, MD, Section 1, Vol. 2,pt. 1, pp. 345-422. Carlson, B.M. (1973)The regeneration of skeletal muscle-A review. Am. J. Anat., 137t119-150. Carlson, B.M., and J.A. Faulkner (1983)The regeneration of skeletal muscle fibers following injury: A review. Med. Sci. Sports Exerc., 15t187-198. Gollnick, P.D., D. Parsons, M. Reidy, and R.L. Moore (1983)Fiber number and size in overloaded chicken anterior latissimus dorsi muscle J Appl Physiol ,541292-1297 Gollnick, P D , B F Timson, R L Moore, and M Reidy (1981)Muscular enlargement and number of fibers in skeletal muscles of rats. J. Appl. Physiol., 50t936-943. Gonyea, W.J. (1980)Role of exercise in inducing increases in skeletal muscle fiber number. J. Appl. Physiol., 48r421-426. Gonyea, W.J., and F. Bonde-Petersen (1977)Contraction properties and fiber types of some forelimb and hindlimb muscles in the cat. Exp. Neurol., 57.637-644. Gonyea, W.J., and F. Bonde-Petersen (1978)Electromyographic analysis of two wrist flexor muscles studied during weight-lifting exercise in the cat. In: Biomechanics VI-A. E. Asmussen and K. Jorgensen, eds. University Park Press, Baltimore, Vol 2,pp. 207212. Gonyea, W.J., and G.C. Ericson (1976)An experimental model for the study of exercise-induced skeletal muscle hypertrophy. J. Appl. Physiol., 40t630-633. Gonyea, W.J., G.C. Ericson, and F. Bonde-Petersen (1977)Skeletal muscle fiber splitting induced by weight-lifting exercise in cats. Acta Physiol. Scand., 99t105-109. Gonyea, W.J., C.J. Giddings, and W.B. Neaves (1982)Ultrastructural characteristics of muscle after prolonged weight-training in the cat. Med. Sci. Sports Exerc., 14rlll. Gonyea, W.J., D. Sale, and J.A. Dixon (1983)Increases in muscle fiber number i n response to weight-lifting exercise. Med. Sci. Sports Exer., 15t135. Hikida, R.S., R.S. Staron, F.C. Hagerman, W.M. Sherman, and D.L. Costill (1983)Muscle fiber necrosis associated with human marathon runners. J. Neurol. Sci., 59:185-203. Ho, K.W., R.R. Roy, C.D. Tweedle, W.W. Heusner, W.D. Van Huss, and R.E. Carrow (1980)Skeletal muscle fiber splitting with weightlifting exercise in rats. Am. J. Anat., 157t433-440. James, N.T. (1973)Compensatory hypertrophy in the extensor digitorum longus muscle of the rat. J. Anat., 116t57-65. Korneliussen, H. (1972)Identification of muscle fiber types in “semithin” sections stained with p-phenylene-diamine. Histochemie, 32t95-98. MacDougall, J.D., D.G. Sale, G.C.B. Elder, and J.R. Sutton (1982) Muscle ultrastructural characteristics of elite powerlifters and bodybuilders. Eur. J. Appl. Physiol., 48:117-126. Ontell, M. (1977)Neonatal muscle: An electron microscopic study. Anat. Rec., 189t669-690. Ontell, M. and E. Haller (1980)Necrotic extrafusal muscle fibers of the dystrophic mutant mouse: The ultrastructure of the myoneural junction. Anat. Rec., 197t397-411. Schmalbruch, H. (1976a)The morphology of regeneration of skeletal muscles i n the rat. Tissue Cell, 8r673-692. Schmalbruch, H. (1976b)Muscle fiber splitting and regeneration in diseased human muscle. Neuropathol. Appl. Neurobiol., 2t3-16. ACKNOWLEDGMENTS Tesch, P.A., and L. Larsson (1982)Muscle hypertrophy in bodybuilders. We extend our appreciation to Ms. Barbara Barnes for Eur. J. Appl. Physiol., 49t301-306. her assistance in cat training and to Ms. Alicia Benitez Van Linge, B. (1962)The response of muscle to strenuous exercise. J. Joint Surg., 44Br711-721. for her assistance in preparing the manuscript. This Zar,Bone J.H. (1974)Biostatistical Analysis. Prentice-Hall, Englewood Cliffs, work was supported by grant AM17615 from the NaNJ. tional Institute of Arthritis, Metabolism, and Digestive Diseases.