Jae Do Yang, et al. epigastric arteries became clear along the posterior aspect of tissues at 8-9 weeks (Figs. 4F and 6G, H). the rectus abdominis. In four fetuses at 9 weeks, the inferior Notably, in the inferior part of the two largest specimens epigastric artery divided into two major braches below the (CRL 36 mm) (Fig. 6G), the posterior sheath was in the umbilicus in a single specimen (Fig. 5F) and above the um- process of disappearing along the course of the inferior epi- bilicus in three specimens (Fig. 6D-F). In regards to the gastric artery and vein. At this site, the umbilical artery did muscle, the medial end facing the herniated gut was espe- not directly attach to the anterior body wall, but there was a cially thick (Fig. 5E). The posterior sheath was clearly seen mesentery-like structure connecting with a limited, midline originating from the transverses abdominis and IO (Figs. 4E area (Figs. 4F and 6F). We did not find any tendinous inter- and 5F). However, the posterior sheath was absent along the sections of the rectus abdominis; however, the horizontal supracostal part of the rectus abdominis, as the muscle belly sections used in the study were not ideal for making such of the transverses abdominis/thoracis reached the midline an observation. Likewise, we did not find two heads of the area to insert into the sternum with or without a short apo- muscle near or at the attachment to the pubis (Fig. 6H). At neurosis. Although the internal intercostal occupied the in- 8 weeks (Fig. 4), abundant vessels appeared in the develop- tercostal space, we were unable to discriminate the inner- ing subcutaneous tissue between the EO and skin. Overall, most intercostal from the internal intercostal in the present rectus abdominis differentiation was found to commence sections. The 11th rib received the internal oblique at levels above the umbilicus at 6 weeks, and extended inferiorly. of the umbilical hernia (Fig. 5E). Sometimes (3 of 8 speci- Thus, the muscle was absent or thin in the levels below the mens at 8-9 weeks), the rib cage did not reach an inferior umbilicus until 8 weeks: the inferior part increased its width level, including the umbilical vein exit from the liver. The and thickness at 9 weeks. Conversely, it did not develop linea alba was composed of transversely directing fibrous from a two-headed origin1 at or near the pubis. Fig. 5. Posterior rectus sheath becomes clear in horizontal sections of a 9-week fetus (CRL 35 mm). HE staining. Panel A (or Panel F) is the most su- perior (or inferior) level of the figure. The bilateral sternal anlagen have al- ready fused (panels A and B). The supracostal part of the rectus abdominis (R) is located on the superficial side of (panel A), but in panels B and C, at the same layer as the internal intercostal (II). The anterior sheath (arrow- heads), originating from the external oblique (EO), covers the rectus ab- dominis at all levels (panels A-F). In panel D, the level of the umbilical vein exiting the liver, the ribs are seen. A midline raphe-like structure can be seen (asterisk in panel D). Panel E, a level of the umbilical hernia, which in- cludes the 11th rib that receives the internal oblique (IO). In panel F, a level below the umbilicus, three lateral muscles are separated by relatively wide loose mesenchymal tissues. Arrows in panels E and F indicate the posteri- or sheath originating from the transversus abdominis (T). All panels were prepared at the same magnification (scale bar in panel A). SMA, superior mesenteric artery. D, diaphragm; IEA, inferior epigastric artery; S, sternum. Fig. 6. Secondary change of the posterior rectus sheath after resolution of the umbilical hernia: horizontal sections of a 9-week fetus (CRL 36 mm). HE staining. Panel A (or Panel H) is the most superior (or inferior) level of the figure. In panels A-C, the rectus abdominis (R) is well developed in the an- terior aspect of the internal intercostal (II). In panel D, the level of the um- bilical vein exit, note the absence of ribs. The anterior sheath (arrowheads) is clearly identified at the medial side of the rectus abdominis (panels E-H). At a level below the umbilicus, part of the posterior sheath is absent (arrow with star in panel G) and the midline area (asterisk in panels G and H) is composed of dense connective tissue. The inferior epigastric artery (IEA) runs upward from panel H to panel F as a single trunk, and divides into two major branches at a level between panels E and F. All panels were pre- pared at the same magnification (scale bar in panel A). D, diaphragm; IO, internal oblique; T, transverses abdominis/thoracis; UA, umbilical artery; UR, urachus; EO, external oblique; S, sternum. 1032 Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012