There bee tenne muscles which couer the nether Belly, on either side fiue, called the muscles of the Abdomen. CROOKE Body of Man, 796, 1615
Development of the abdominal wall muscles
The extraembryonic mesoderm divides longitudinally into a paraxial part, from which the dorsal muscles will develop, and a lateral plate, the precursor of the muscles of the abdominal wall.
Somites
The paraxial mesoderm becomes segmented transversely into somites, each of which appears as a mass of mesodermal cells arranged around a central somite cavity, in continuity with the intermediate mesoderm ( Fig. 7-1 A).
Except for the cervical and cranial ones, the somites differentiate into three portions: (1) a dermatome from the outer wall, to form the skin; (2) a myotome from the dorsal part of the inner wall, to form the muscles of the body wall and limbs; and (3) a sclerotome from the ventral part of the inner wall, which forms the skeleton ( Fig. 7-1 B).
Myotomes
Around 5 weeks, the myotomes divide into a ventral division and a smaller dorsal division, each of which will be supplied by an anterior or posterior branch of the corresponding spinal nerve ( Fig. 7-2 ). The individual myotomes formed by the dorsal division remain arranged segmentally, but those formed by the anterior division (on the lateral plate) lose their segmentation before the age of 3 weeks.
Trunk muscles
From the anterior myotomes, precursor cells separate in the thoracic area as discrete buds and emigrate to staging areas in the flank to form large premuscle masses. Primitive myotubes from the myoblasts in these masses assume the orientation that the muscle fibers will later take. As differentiation progresses, these premuscle masses split longitudinally or tangentially into the primordia of individual muscles and fuse with mesodermal material from adjacent myotomes.
As the ribs develop, the ventral extension of the myotomes in the thoracic area moves anteriorly to form the muscles of the anterior abdominal wall. Those in the lumbar area form the psoas and quadratus lumborum, which are involved in flexing the vertebral column, and those in the sacral area form the musculature of the pelvic diaphragm. The dorsal myotomes develop into the extensor muscles of the back. The lumbodorsal fascia forms over them and separates them from the latissimus dorsi and parts of the serratus, which are migratory muscles of the anterior division (see Fig. 8-2 ).
Development proceeds, through final shifting and growth, to reach the fully differentiated state ( Fig. 7-3 ). The rectus abdominis is formed by longitudinal splitting of the ventral end of the fused myotomes. The external oblique and the serratus posterior superior and inferior arise through a tangential split of the lateral sheet, and the internal oblique and transversus arise from the medial sheet; the remaining part of the myotomal processes form the internal and external intercostals. At 6 weeks, the muscles are differentiated, although in a more lateral position than in the adult. In fact, the recti are still widely separated at 10 weeks, a condition that, if persistent, would result in diastasis recti. Some of the myotomal material degenerates and disappears entirely or remains as vestigial fibrous structures to form the aponeuroses of the anterior trunk muscles, or as the nonmuscular sacrotuberous ligament. In contrast, the tendons do not originate from the muscles but develop from the local connective tissue to become secondarily attached to the muscles.
The number of muscle fibers is established in the neonatal period, but the fibers may grow by the addition of sarcomeres at either end or by an increase in diameter. Satellite cells are added to the muscle fiber syncytium as the fibers grow. It is from these cells that muscle fibers may regenerate after surgical or other injury.
The mesenchyme underlying the rectus abdominis and transversus abdominis is continuous with that covering the levator ani. The transversalis fascia will develop from this portion of the retroperitoneal tissue, a layer that is separate from the epimysium of the muscles of the body wall.
Anomalies
Prune belly syndrome
Although several theories have been championed, the embryogenesis of this anomaly (absence or hypoplasia of the abdominal muscles, distention of the bladder, ureters, and renal pelves, and cryptorchidism) is not understood. Muscular change secondary to distention of the urinary tract, with or without ascites, is a doubtful cause; an obstructive lesion is not found and known obstructive lesions such as urethral valves do not result in the syndrome. A primary mesodermal defect may be at fault, because both of the involved systems—the urinary tract and the abdominal wall—arise from the mesoderm of the paraxial intermediate and lateral plates.
The defect starts before the seventh week, when the several muscles differentiate from the somatic mesoderm of the anterior division of the myotomes (see Fig. 7-3 ). The first lumbar segment has been implicated in the dysgenesis because normally much of the oblique and transverse muscles develop from this location; the hypoplasia is maximum here and is less pronounced above and below. However, defects of the lower limbs indicate that the dysgenesis may extend to the lower lumbar and sacral segments and absence of the upper portion of the rectus suggest involvement of the lower thoracic region.
The effects of the anomaly vary from minimal hypoplasia to complete absence of muscle fiber, but the medial and lower portions of the abdomen are uniformly involved. A sheet of fibrous tissue, which is firmly attached to the peritoneum, takes the place of the muscles ( Fig. 7-4 ). Occasionally, congenital megalourethra is found. The bladder is large and thick walled, often with a pseudodiverticulum on the dome, and is attached to the umbilicus ( Fig. 7-5 ). The trigone is large, and reflux is common. The bladder neck is widely dilated far down into the prostatic urethra. The prostate itself is poorly developed, usually consisting of only a shell. The upper tracts are dilated, and renal dysplasia and hydronephrosis are not uncommon findings. Cryptorchidism, accompanied by short spermatic vessels, is the rule.
Anterolateral and lower abdominal body wall: structure and function
Surgical access to the contents of the abdomen and pelvis is through the abdominal wall and requires incisions that gain the greatest exposure with the least disturbance of muscular and fascial layers and the nerve and blood supply to them.
The body wall has three major layers—skin, musculofascial, and peritoneal—to be traversed during surgery ( Table 7-1 ).
| SKIN AND SUPERFICIAL FASCIA |
| Dermis: Supports the epidermis |
| Subcutaneous fascia: superficial (dartos); deep (Scarpa’s, Buck’s, Colles’) |
| Deep muscular fascia: external investment of body wall muscles (external oblique, internal oblique, transversus abdominis) |
| MUSCLES OF BODY WALL RETROPERITONEAL TISSUE |
| Outer stratum: investment of body wall muscles (transversalis fascia and its pelvic extensions) |
| Intermediate stratum: investment of urinary tract organs (Gerota’s fascia, prostatic sheath, lateral vesical pedicle, broad ligament) |
| Inner stratum: investment of intestinal tract (supporting connective tissue of the peritoneum) |
| PERITONEUM |
The external oblique is the most powerful muscle of the body wall and the internal oblique, only a little less so. In contrast, the transversus abdominis is the thinnest and contributes least structurally but is important surgically because the principal vessels and nerves lie on the surface of its investing fascia as they run in the direction of its fibers. Thus, when one is opening the transversus, injury to a nerve is possible and incorporation in a suture can occur when closing.
Although the intrinsic fascial layer that intimately covers the external surface of each of these muscles is not well developed, it is strong enough to hold sutures spaced 1 cm apart and 1 cm deep even if little or no muscle is included in them.
The muscles and their relationships can be most clearly comprehended by first topographically viewing the surface of the body wall, then removing successive muscle layers until the deepest structures of the body wall, the transversalis fascia and the muscles of the posterior body wall, are reached.
Topography
From the surface, the more superficial muscular layers of the anterolateral body wall can be seen ( Fig. 7-6 ). The oblique course of the external oblique is clear, especially in its upper portion. Note the insertion of slips from the external oblique between similar slips from the serratus anterior inferior and the passage of slips of the external oblique beneath the latissimus dorsi . The pectoralis major inserts above the serratus muscles. The belly of the external oblique joins its aponeurosis, which, in turn, is incorporated into the anterior rectus sheath beginning at the semilunar line. The tendinous intersections of the rectus abdominis have a role as fixation points for contraction of the muscle in its upper portion.
