Development of the Rectus Abdominis Moreover, at 7 weeks, the supracostal part of the rectus ab- or margin, the rectus abdominis was adjacent to the pecto- dominis was consistently seen at the superficial side of the ralis major, which was developing from the superolaterally internal intercostal (Fig. 3). In all four specimens at 7 weeks, located shoulder. The transverses abdominis was continu- above the level of the umbilical vein exit from the liver, the ous with the transverses thoracis and, at the ribs and ster- connective tissue laminae extended medially from each of num, the diaphragm insertions were interdigitated with the the EO, IO and transverse muscles. Thus, development of two transversus muscles. The inferior epigastric artery was the sheath started from the superior part. The development identified at 7 weeks (Fig. 3G). However, ramification of of the anterior rectus sheath was followed by that of the this artery was not seen at this stage. In addition, the rectus posterior sheath. At the early stage, the anterior sheath did abdominis was very narrow along the mediolateral axis at not reach the medial margin of the rectus abdominis, while the level below the umbilicus, and the attachment to the de- the posterior sheath was difficult to discriminate from a veloping pubis appeared to be single (not two in number; mesenchymal condensation along the primitive peritoneum Fig. 3G insert). (Fig. 3B-E). The connective tissue lamina from the EO At 8-9 weeks, fusion of the bilateral anlagen of the ster- consistently covered the supracostal rectus muscle. The num was completed, and the umbilical hernia had resolved. skin differentiated from the surface ectoderm. At the superi- At that time, the rectus abdominis increased in thickness and the anterior and posterior sheaths reached the medial margin of the rectus abdominis (Fig. 4) and completely enclosed it (Figs. 5 and 6). Simultaneously, the superior and inferior Fig. 3. Anterior rectal sheath appears in horizontal sections of a 7-week embryo (CRL 21 mm). HE staining. Panel A (or Panel G) is the most superior (or inferior) level of the figure. The rectus abdominis extends superiorly in the superficial side of the rib cage (panels A-D). However, the anterior chest wall is not closed by the sternum but by the surface ectoderm and underlining mesenchymal tissues (stars in panel B). Panels C and D are higher magnification views of the bilateral rectus muscles shown in panel B. The PC is filled with loose mesenchymal tissue (panels B and C). Panel E, the level of the umbilical vein exiting the liver, with the ribs also present. The anterior sheath (arrowheads), originating from the external oblique (EO), covers the rectus abdominis at levels above the umbilicus (panels B-E). In panel F (the level of the umbilical hernia) and panel G (a level below the umbilicus), the rectus sheath as well as the transversus (T) is difficult to identify. In panel G, the rectus abdominis is artifactually divided into two parts during the histological procedure. The rectus abdominis is reduced in size at the level below the umbilicus (panel G). An insert of panel G demon- strates the most inferior part of the muscle comprising of a single small band adjacent to the pubic bone anlage (pubis). Panels A and C-G were prepared at the same magnification (scale bar in panel A). D, diaphragm; IEA, inferior epigastric artery; IO, internal oblique; PC, primitive pleural cav- ity; T, transverses abdominis/thoracis; UA, umbilical artery; UR, urachus. Fig. 4. Posterior rectus sheath appears in horizontal sections of an 8-week embryo (CRL 29 mm). HE staining. Panel A (or Panel F) is the most superior (or inferior) level of the figure. In panels A-C, the rectus abdominis (R) at- taches to the anterior aspect of the ribs, but it does not extend medially to reach the sternum (S). The rectus abdominis is located at the superficial side of the internal intercostal (II). The asterisk in panel A indicates bleed- ing. The anterior sheath (arrowheads), originating from the external oblique (EO), covers the rectus abdominis at all levels (panels A-F). In panel D, the level of the umbilical vein exiting the liver, note the absence of ribs. In panel E (a level of the umbilical hernia) and panel F (a level below the umbilicus), the posterior sheath (arrows) originates from the transversus abdominis (T). In panel F, the midline area is composed of loose mesenchymal tissue (asterisk) and the anterior sheath (arrowheads) does not reach the midline area. All panels were prepared at the same magnification (scale bar in panel A). D, diaphragm; IEA, inferior epigastric artery; IO, internal oblique; T, transverses abdominis/thoracis; UA, umbilical artery; UR, urachus. Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012 1031