Jae Do Yang, et al. mesenchymal condensations in the abdominal wall. Those present. A thick, loose mesenchymal tissue layer existed be- two condensations appeared to correspond to the undiffer- tween the EO and the thin surface ectoderm. The diaphragm entiated rectus abdominis and lateral abdominal muscles, was not yet present at the posterior side of the adrenal, respectively (Fig. 1). We regarded the midline condensation while the pleural cavity extended inferiorly as a loose mes- as the so-called “abdominal band” reported by Munger and enchymal space between the peritoneum and the posterior Munger2 in fetal mice and rats. The candidate rectus muscle body wall. anlage had already extended longitudinally from the level The abdominal muscles and their associated connective of the inferior margin of the brachial plexus to the level be- tissues appeared to be differentiated between 6 and 7 weeks. low, but immediately above the umbilicus. At the earliest At that time, the bilateral anlagen of the sternum had begun stage, neither the muscle fibers nor the rectus sheath was to unite with the superior part (i.e., the manubrium), but it seen. The developing ribs were still restricted in the postero- had not reached the inferior part. However, in three of four lateral part of the body wall. In two of three specimens at 6 specimens at 7 weeks, the rectus abdominis had already ex- weeks (Fig. 2), the rectus abdominis, EO and IO muscles tended between the laterally located sternum and the pubis were identified as independent muscle sheets at the level without any interruption alongside the umbilical hernia. above the umbilicus (strictly, above the umbilical vein exit from the liver). However, the transversus abdominis and thoracis were difficult to identify. The rectus sheath was not Fig. 1. The abdominal wall is occupied by the umbilical veins in tilted hori- zontal sections of a 5-week embryo (CRL 7 mm). HE staining. Panel A (or Panel D) is the most superior (or inferior) in the figure. Panels E-H are high- er magnification views of the primitive rectus abdominis shown in panels A-D, respectively. Panel C displays the level of the umbilical hernia. Arrows in panels B-D (or E and F) indicate developing ribs (or candidate anlagen of the abdominal wall muscles). The plate-like, midline condensation (arrows in panel F) appears to correspond to the abdominal band in mice and rats (see text for explanation). Panels A-D or panels E-H were prepared at the same magnification (scale bar in panels D and H). Fig. 2. Abdominal wall muscles appear in horizontal sections of a 6-week embryo (CRL 16 mm). HE staining (panels A-C) and silver impregnation (panels D-F). Panel A (or Panel F) is the most superior (or inferior) level of the figure. The rectus abdominis (R) appears to be differentiated at levels above the umbilicus (panels A and B), but it is identified as a mesenchymal cell cluster alongside the umbilicus (panels C and D) and is difficult to iden- tify below the umbilicus (star in panels E and F). Likewise, the oblique mus- cles (EO, IO) are not identified below the umbilicus (panels E and F). The rectus abdominis is located in the lateral part of the body wall rather than the anterior site. The rectus sheath (arrowheads) was not seen at any lev- el. The diaphragm has not yet developed. The future inferior part of the pleural cavity is filled with loose tissue (PC in panels A-C). All panels were prepared at the same magnification (scale bar in panel B). In all figures in- cluding Fig. 1, the upper side of each panel corresponds to the anterior or ventral side of the body. AO, aorta; SA, serratus anterior; SMA, superior mesenteric artery; UA, umbilical artery; UR, urachus; EO, external oblique; IO, internal oblique; PC, primitive pleural cavity. 1030 Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012