THE ANATOMICAL RECORD 234:500-512 (1992)

Architectural Design, Fiber-Type Composition, and Innervation of the Rat Rectus Abdominis Muscle TAKA0 HIJIKATA, HITOSHI WAKISAKA,



Department of Anatomy, Faculty of Medicine, University of Tokyo, Tokyo (T.H., T.Y.),and First Department of Oral Anatomy, Hiroshima University School of Dentistry, Hiroshima (H.W.),Japan


The rectus abdominis muscle is architecturally compartmentalized by tendinous intersections and is supplied by multiple thoracic nerves. In this study, the rectus abdominis of the rat has been qualitatively and quantitatively examined with regard to muscle dimensions, fiber organization, fiber-type composition, and innervation. The muscle exhibits architectural heterogeneity and different patterns of innervation among its thoracic, epigastric, and hypogastric parts. The epigastric part, adherent to the rectus sheath via tendinous intersections, represents relatively simple design. It is formed by serially arranged compartments with shorter fibers, compared with the other parts. These compartments are segmentally supplied by thoracic nerves. The hypogastric part is more complex, forms an interdigitation of muscular slips, and has segmental distribution of thoracic nerves in mediolateral direction. The thoracic part much differs from the other parts. It has smaller cross-sectional areas, compartments composed of abundant nonspanning fibers with intrafascicular termination, and non-segmental distribution of thoracic nerves. In addition to these craniocaudal specializations among the three parts, the muscle exhibits mediolateral differences in fiber-type composition. Slow-twitch oxidative fibers are more densely distributed in the medial half region than the lateral, whereas fast-twitch glycolytic fibers follow an inverse pattern. The mediolateral differences in fiber-type composition as well as the craniocaudal specializations in architectural design and innervation imply regionally differentiated recruitments of the muscle in various behaviors. 0 1992 Wiley-Liss, Inc.

Key words: Rat, Rectus abdominis, Architectural design, Innervation, Histochemistry The rectus abdominis muscle (RA) is well known to 1991a),slow/fast fiber ratios (Dum and Kennedy, 1980; be segmented by tendinous intersections (TIs) into a English and Letbetter, 1982; Brichta et al., 1987), and certain number of serially arranged compartments, the content of sensory receptors (Maier, 1979; Rowlerwhich are supplied by different thoracic nerves (Cullen son et al., 1988). Electromyographic studies (English and Brodal, 1937; Tobias and Arnold, 1963). Its re- and Weeks, 1987; Lev-Tov and Tal, 1987; Chanaud et markable segmentation and separate nerve supply to al., 1991b) have revealed that such specializations are compartments reminded some authors of such simple well correlated with differential activation of the musrepetitive structures as trunk muscles of fishes or my- cle during various natural movements of the animal. omeres of embryonic somites, so that this architectural Thus, the muscles can be recognized as organized units pattern was regarded as a remnant reflecting its phy- and detailed knowledge of their structure will dictate logeny andfor ontogeny (Ruge, 1892; Braus, 1929; functional implications for their motor control and usHeinze, 1970). However, it is now well accepted that age. However, the compartmentalization of RAs has the detailed architecture does not simply represent not been analyzed enough from a functional perspecphylogeny and/or ontogeny, but more or less reflects tive. functions. Each compartment in the RA is not always In the present study, we have examined the archisupplied by a single thoracic nerve and variations in tectural design, fiber-type composition, and innervathe number of TIs among mammals do not exactly follow phylogenetic relationships (Eisler, 1912; Konno and Wajima, 1955; Heinze, 1971d). Received January 16, 1992; accepted April 28, 1992. Many other muscles also have compartments that Address reprint requests to T. Hijikata, Department of Anatomy, differ in fiber lengths and attachments (Bodine et al., Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 1982; Richmond and Armstrong, 1988; Chanaud et al., Tokyo 113, Japan. 0 1992 WILEY-LISS, INC.


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1 Fig. 1. A line drawing of rectus abdominis muscles (RAs) of the rat. In the right RA, the thoracic part is found cranially to the rectus sheath, lying directly beneath the pectoral muscles (not drawn), whereas the other parts are enclosed by the rectus sheath. In the left RA, the rectus sheath is removed to expose the epigastric and hypogastric parts.

Fig. 2. Line drawings of the origin of the RA, showing a n interdig itation. a: Superficial slips on both sides cross the mid-line. b Superficial slips are cut to show deep slips.

tion of individual compartments of the rat RA by use of several anatomical methods. We report the architectural details and patterns of innervation of the RA and their regional differences within the muscle. The functional relevance of the regional differences within the RA will be discussed.

tized with a n overdose of sodium pentobarbital and then fixed by immersion or perfusion via the left ventricle with 4% paraformaldehyde.


A total of 27 male Wistar rats (9-10 W, 250-300 g) were used in this study. These animals were bred on a standard chow diet. The animals except ones used for enzyme histochemistry were killed or deeply anesthe-

Morphological Descriptions

Morphological observations were made on the muscles of eight rats with a stereo microscope. Particular attention was paid to tendinous intersections (TIs). The estimation of the amount of fiber fascicles uninterrupted by TIs was made in a single rectus abdominis muscle. The muscle was cut into segments containing 1 TI. These muscle segments were embedded in paraffin,







Thoracic Part

TI 143%

T I 2 3%

c--> Epigastric Part

TI 3 5 % EP


c 3 -

T I 5 2% T I 6 6%

TI778% Hypogastric Part






3 Fig. 3. Compartmentalized structure of the RA. a: A line drawing of the superficial surface of a right RA. Seven light lines show tendinous intersections (TIs). b Schematic drawings of assumed muscle cross sections on which fiber fascicles ending onto TIs (light area) and those uninterrupted by TIs (dark area) were projected. Relative proportions

of areas of uninterrupted fascicles are shown on the right sides. Note that uninterrupted fascicles are abundant in the thoracic and hypogastric parts. c: A schematic drawing of the RA, showing many compartments divided by TIs (space between columns). See Results for compartment abbreviations.

frontally cut into 10 pm thick serial sections, and stained with hematoxylin and eosin. Each fifth serial section was printed, using a video copy processor SCTP75 (Mitsubishi). In each hard copy, transverse lengths of uninterrupted fascicles and TIs were measured on the line perpendicular to the axis of the fibers. From these measurements, relative proportions of cross-sectional areas of uninterrupted fascicles were calculated.

placed on microscope slides for the measurement of sarcomere length. The average sarcomere lenght was determined by measuring the length of a 5-sacromere span at 10 different sites along the fiber by use of a system for digital image analysis. The system consisted of light microscope Optiphoto-2 with a x 100 oil immersion objective (Nikon), video camera C2400 (Hamamatsu), video color board 364 (Raster Ops), Macintosh IIci (Apple), and NIH Image 1.41 software for analysis.

Fiber Microdissection

The muscles of 6 rats were carefully removed from the rectus sheath and stained en bloc using the gold chloride method (Swash and Fox, 1972). These muscles were softened by prolonged storage in glycerin. Two well-stained muscles were selected for systematic fiber analysis. The two muscles were subdivided into longitudinal strips of fascicles from known locations in the muscle, according to Richmond et al. (1985). The strips were microdissected to obtain finer fascicles that ran the entire distance between 2 TIs. From these fascicles, individual fibers were teased and their lengths were measured. Several fibers were

Acetylcholinesterase Staining

Muscles of 4 rats were stained for acetylcholinesterase activity according to Lewis (1961). The Estimation of Cross-SectionalAreas

Cross-sectional areas a t various levels of the RA were estimated by use of the system for digital image analysis (see above). Muscle blocks were cut from the midbelly between TIs and embedded in paraffin. These blocks were cut perpendicularly to the axis of fibers into 10 pm thick sections, which were stained with



TABLE 1. Lengths of fascicles and relative incidence of nonspanning fibers in different compartments

Compartment TL TCrI TM TCdI EP 1 EP 2 EpML EP 3 EP 4 SS-MCr SS-MCd

ss-L DS

Rats 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13

Sarcomere length (pm) 2.54 t 0.04l 2.52 t 0.07 2.72 t 0.10 2.74 f 0.18 2.60 0.09 2.63 0.11 2.64 0.12 2.64 t 0.10 2.48 0.07 2.61 0.14 2.58 0.12 2.62 k 0.12 2.57 f 0.14 2.65 0.11 2.55 f 0.05 2.72 f 0.11 2.62 t 0.15 2.56 2 0.09 2.35 0.07 2.55 f 0.15 2.47 t 0.11 2.61 t 0.10 2.50 0.09 2.54 t 0.09 2.34 0.15 2.60 0.18

* * * * * *


* * *

Length of fascicles (mm) 39-42 (35-38)' 34-39 (31-36) 21-24 (18-20) 20-23 (17-19) 33-37 (29-33) 32-35 (28-31) 16-18 (14-16) 15-16 (13-14) 11-12 (10-11) 10-12 (9-11) 9-11 (8-10) 8-10 (7-9) 21 (19) 18-21 (16-18) 10-15 (9-14) 12-15 (10-13) 13-17 (11-15) 11-18 (10-16) 12-13 (12-13) 14-15 (13-14) 19-20 (18-19) 22-23 (19-20) 28-31 (26-29) 33-35 (30-32) 22-31 (22-30) 24-31 (21-27)

Number of fibers samples 91 58 62 51 83 55 57 53 58 55 62 54 99 58 67 54 65 60 71 51 72 51 133 107 101 55

Percent of nonspanning fibers (%) 100 97 0 0 93 75 0 0 0 0 0 0 76 24 0 0 0 0 0 0 0 0 5 28 1 0

'Mean length f SD (n = 6). 'Normalized to sarcomere length of 2.3 pm.

hematoxylin and eosin. In sections cut well, cross-sectional areas were measured by the system. Fiber Histochemistry

The muscles were removed from 6 deeply anesthetized rats. Under a stereo microscope, the dorsal wall of rectus sheath was carefully stripped to confirm the localization of tendinous intersections and then 3-5 mm thick cross sections were cut from the midbelly between TIs. After the orientation of each cross section was recorded, cross sections were frozen by immersing them either into a mixture of dry ice and acetone or into liquid isopentane cooled over a bath of liquid nitrogen. The frozen samples were cut into 12 Frn thick sections in series on a cryostat. Some sections were incubated for demonstration of myosin ATPase reaction after preincubation a t pH 4.3,4.5,and 10.4 (Brooke and Kaiser, 1970; Suzuki and Cassens, 1980). Other sections were stained with NADH tetrazolium reductase (NADH-TR)(Lojda et al., 1979). Slow-twitch oxidative (SO), fast-twitch oxidative-glycolytic (FOG) and fasttwitch glycolytic (FG) fibers were identified by differences in reactivity for myosin ATPase after acid and alkaline preincubation and activity for NADH-TR (Peter et al., 1972). The relative proportions of the 3 types of fibers were estimated in medial, central, and lateral regions of each section. The 3 regions were determined as strips running from the deep to the superficial sur-

face of the muscle on quarter dividing lines of its transverse axis. In each strip composed of 110-578 fibers, the number of SO, FOG, and FG fibers were counted. Microdissection of Innervation

Nerve supply to the RA was observed on all rats used in morphological descriptions and acetylcholinesterase staining. Intramuscular distribution of nerves was examined in one specimen stained for acetylcholinesterase activity. Under a stereo microscope with approximately x 60 magnification, each primary branch from thoracic nerves was traced into its terminations. RESULTS Muscle Morphology

The rectus abdominis (RA) of the rat was a strap of muscle extending along the whole length of the ventral surface of the trunk (Fig. 1). Its caudal two thirds were enclosed by the rectus sheath composed of aponeuroses of the oblique and transversus abdominis muscles. The muscle on each side arose from the descending ramus of the pubis by 2 or occasionally 3 slips, which crossed the mid-line, alternated with corresponding slips of the opposite side and giving an interdigitation (Fig. 2). The insertion was onto the ventral surface of the manubrium sterni and the medial third of the clavicle. Based on its structural relationships to the rectus sheath, the RA was divided into 3 parts, a thoracic, an epigastric, and a hypogastric part (Figs. 1,3).The three



Fig. 4. Termination modes of muscle fibers. a: A photograph of muscle fibers terminating onto a TI

76). Fibers end with rounded tips in the TI, which appears as a transparent band. b A photograph of a nonspanning fiber with intrafascicular termination ( x 71). A nonspanning fiber with taper profile (light fiber) is apposed to adjacent fiber (dark fiber). Its apposed region is shown by arrowheads.


Fig. 5. Myomyonal junctions and their distribution in compartment TL. The specimen used for this figure is immersed in glycerin to highlight AchE-positive sites. a: A photograph of a left RA stained for AchE activity ( x 1.6). The marked zone is shown at higher magnification ( x 10) in b. Many AchE-positive spots (arrowheads) are scattered between thin positive band of the first TI (top) and broader band

of endplates (bottom). Note that much less AchE-positive spots are found in the left side of the photograph, where the TI intersects the muscle completely from its deep to superficial surface. Compare the density of band of the TI between the left and the right sides. c, d: Photographs of myomyonal junctions ( x 6 7 ) .Muscle fibers are interconnected by myomyonal junctions end to side.

parts were bordered by the second tendinous intersection and the sixth (see below; Fig. 3). The thoracic part cranial to the rectus sheath lay directly beneath the

pectoral muscles. The epigastric part, from the second to the sixth tendinous intersections, was enclosed by the rectus sheath and was intimately adherent to its



TABLE 2. Mean values and SD of nonspannin5 fiber length (mm) from different compartments

Compartment TM TL



Length of fascicles

sampled (mm)



cranially From TI2 to Ins. 20.2 t 1.9 (n=48)** 20.6 f 3.7 (n=28)* 22.8 f 1.9 (n=30)** 21.3 f 2.4 (n=32)**

From Ins. to TI2 25.8 t 1.8 (n=29) 24.0 t 4.6 (n= 12) 28.4 * 2.5 (n=51) 25.8 f 2.7 (n=24)

10 13 10 13

33-37 32-35 39-42 34-39

10 13

21 (21) 18-21 (18-21)

From TI5 to TI3 13.9 t 1.4 (n=34)** 13.3 f 5.7 (n=3)

10 13

28-31 (28-31) 33-35 (33-34)

20.8 t 2.6 (n = 5)* 26.2 1.6 (n= lo)**


(33-35) (-1 (36)

From orig. to TI6




From TI3 to TI5 15.7 * 1.8 (n=31) 16.5 t 1.2 (n= 11) From TI6 t o orig. 16.9 t 2.3 (n= 11) 14.7 ? 1.4 (n= 19)

'In all compartments except the EpML of number 13 rat, mean values differed significantly (*P

Architectural design, fiber-type composition, and innervation of the rat rectus abdominis muscle.

The rectus abdominis muscle is architecturally compartmentalized by tendinous intersections and is supplied by multiple thoracic nerves. In this study...
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