Development of the Rectus Abdominis DISCUSSION The present study indentified a candidate primitive rectus abdominis as a plate-like mesenchymal condensation at the anterior midline area in the smallest embryos studied (CRL 7 mm) (Fig. 1). We believed the midline condensation to be analogous to the so-called “abdominal band” previously re- ported by Munger and Munger2 in their study of fetal mice and rats. However, the anlage remained restricted above the umbilicus, reaching the level near the brachial plexus. More- over, in the next stage, the rectus abdominis was not seen near the midline but was observed in the relatively lateral part of the body wall. Accordingly, we wondered how it is possible that the midline condensation or abdominal band develops into the “laterally” located rectus muscle. The ab- dominal band shown in Fig. 1 might be a mesenchymal condensation not for the rectus abdominis but for the ven- tral epidermis. If so, that band may have a different role in humans compared to mice and rats. Rozen, et al.13 intro- duced a theory of rectus abdominis muscle development: 1) the ventral longitudinal column containing myogenic cells or the primitive rectus abdominis divides into the “two heads of origin” comprising an attachment at the pubic symphysis medially and that from the upper border of the pubic crest laterally; 2) the two muscle slips develop cranially to reach the costal cartilage. However, according to the present re- sults, rather than the inferior part near the pubic bone anla- ge, the muscle is most likely to start development above the umbilicus, especially in front of the liver at 7 weeks. Thus, the developing rectus abdominis seemed to require mechan- ical support from the posterior side, by the liver and ribs. The rectus muscle below the umbilicus increased its width and thickness at 9 weeks. Nevertheless, we have no evi- dence of the two-heads near the pubic bone among the spec- imens examined. According to Niikura, et al.,4 the most in- ferior part of the rectus abdominis is not composed of two heads at even 25 weeks. Thus, the two definite heads or ori- gins, if present in adults, may develop after birth. Apart from two specimens of CRL 36 mm, all of the pres- ent specimens were at stages of development prior to clo- sure of the anterior abdominal wall or before resolution of physiological umbilical hernia. Conversely, the rectus ab- dominis and its sheath commenced developing in specimens with a herniated gut. It may be that below the umbilicus, even after resolution of hernia, the urachus and umbilical ar- teries exert a negative mechanical effect on the development of the posterior rectus sheath. However, without closure of the abdominal and chest walls, the anterior (or posterior) sheath could develop as an insertion of the external oblique (or the transversus abdominis). Otherwise, closure of the anterior chest wall and resolution of the hernia are likely to cause a topographical change in the rectus abdominis from the early lateral site to the final midline area. Growth of the ribs and of the lower limb may also contribute to the an- teromedial migration of the rectus abdominis. We found that after the physiological umbilical hernia had resolved, the posterior sheath seemed to secondarily disappear below the umbilicus along the inferior epigastric artery. The umbilical artery was separated from the missing sheath by the peritoneal cavity. We first considered that de- veloping fatty tissue was committed to the secondary disap- pearance of the posterior sheath, but the fatty tissue ap- peared at a much later stage, such as at 15 weeks.4 Changes in mechanical stress after hernia resolution seemed to in- duce this disappearance. These observations together with those of Niikura, et al.4 suggest that the posterior sheath be- low the umbilicus completely disappears by 25 weeks of gestation. At the stage just after the herniated gut returned, the linea alba began to develop as a transversely directed fi- brous tissue. Thus, resolution of the hernia could be an im- portant accelerating factor in the differentiation of connec- tive tissues in the anterior midline area, albeit only one of a few factors. Rizk’s bilaminar sheaths, as well as his pro- posed digastric configuration of the abdominal muscles,5,6 may result from functional adaptation to mechanical de- mands during trunk muscle function after birth. The large umbilicus appears to stop myotomal cell mi- gration in order to make a pair of myogenic cell pools or the abdominal band.2 The band seems to be mechanically strong enough to receive the anterior sheath from the exter- nal oblique, although the liver also supports the inner as- pect. We interpreted the figures of Manley, et al.15 to indi- cate that even in specimens with unsuccessful fusion of the bilateral anlagen of the sternum in hoxb4 mutant mice, the rectus muscle or the sternalis (a variant of the pectoralis: see below) seems to develop. However, in AP-2alpha tran- scription factor knockout mice, the abdominal band as well as the covering abdominal epidermis failed to develop.16 If gastroschisis (i.e., unsuccessful fusion of the bilateral upper abdominal walls) is due to such a loss of transcription fac- tor function, the rectus abdominis may also not develop. However, if gastroschisis is the result of anomalous yolk sac development,17,18 we may be able to identify the rectus Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012 1033