Development of the Rectus Abdominis Thus, the rectus abdominis carries a distinct supracostal part. However, a limited study using mice and rats2 indicat- ed development commenced earlier than both chest wall closure and resolution of the physiological umbilical her- nia. The muscle fibers of the rectus abdominis are interrupt- ed by three fibrous bands or tendinous intersections at spe- cific sites above and at the level of the umbilicus.1 Gościcka and Murawski3 reported that the rectus muscle fibers ap- peared at 10 weeks of gestation, the epigastric arteries at 13-14 weeks, and the tendinous intersections at around 18 weeks. Despite the above-mentioned studies, fetal develop- ment of the rectus abdominis remains poorly understood. The rectus sheath of the anterior abdominal wall is com- posed of three aponeurotic laminae or leaves with decussat- ing fibers originating from the external oblique (EO), inter- nal oblique (IO) and transverses abdominis (TA): the lamina from the EO joins the anterior rectus sheath, that from the TA connects with the posterior rectus sheath, and the lami- nae from the IO merge with both sheaths above the umbili- cus.1 The posterior sheath is absent below the umbilicus be- cause all laminae join mutually to form the anterior sheath. Although Niikura, et al.4 demonstrated that the posterior sheath was absent below the umbilicus in human fetuses at 20-25 weeks, this was not the focus of their investigation. For the sheath above the umbilicus, Rizk5,6 postulated the so-called “bilaminar theory”: a classical lamina from each of the EO and IO is divided into two laminae according to the collagen fiber direction. That theory provided a better biomechanical model for the adult rectus sheath as an inter- mediate tendon of the bilateral abdominal wall muscles. However, our interpretation of the rat fetus findings of Rizk and Adieb7 is that they failed to demonstrate fetal develop- ment of the rectus sheath. Differentiation in the bilaminar and decussating collagen fibers in the rectus sheath5,6 may be established by mechanical demands after birth. Instead of the sternalis, a well-known variant of the su- pracostal rectus muscle, sometimes innervated by the inter- costal nerve,8-12 we speculated that in fetuses the rectus ab- dominis might extend more superiorly along the rib cage compared to adults. Consequently, the aim of the present study was to improve understanding of the fetal develop- ment of the rectus abdominis muscle and its associated sheath in humans. We followed the development of those structures in stained sections derived from human embryos and fetuses aged between 5 and 9 weeks of gestation. Of interest to surgeons, we also addressed fetal development of the inferior epigastric artery, especially that of the medial and lateral rows of the musculocutaneous perforators.13 MATERIALS AND METHODS The study was performed in accordance with the provisions of the Declaration of Helsinki 1995 (as revised in Edinburgh 2000). We examined paraffin-embedded histological sec- tions derived from 18 embryos and fetuses at 5-9 weeks of estimated gestational age: three embryos at each of 5 weeks [Crown-Rump Length (CRL) 7-10 mm] and 6 weeks (CRL 15-17 mm), and four embryos or fetuses at each of 7 weeks (21-25 mm), 8 weeks (27-29 mm) and 9 weeks (31-36 mm). All sections were horizontal, 5 microns thick, prepared at in- tervals of 20 microns, and stained with hematoxylin and eo- sin or silver impregnation. This work was a qualitative study with no specific measures and we did not use any specific instrument other than a light microscope. All specimens were part of the collection of the Embryol- ogy Institute of Universidad Complutense, Madrid. Speci- mens were from women who had had miscarriages or ecto- pic pregnancies at the Department of Obstetrics of the University. We were unable to rule out the presence of pa- thologies due to the nature of the specimens. However, the study sought to identify the morphology most commonly evident in fetuses at each stage. Approval for the study was granted by the ethics committee of the University (No. B08/374). RESULTS In all figures, the upper side of each panel corresponds to the anterior or ventral side of the body. Apart from the two largest specimens (CRL 36 mm), all specimens were at stages of development prior to resolution of the physiologi- cal umbilical hernia. Moreover, anterior chest wall closure was completed through fusion of the lower ends of the bi- lateral sternal anlagen (the so-called sternal bands14) at 8-9 weeks. Thus, other than 4 specimens at 9 weeks, the mid- line area of the lower chest wall, in front of the liver or heart, was filled with loose mesenchymal tissue covered by the surface ectoderm. We found that the rectus abdominis and its sheath commenced development at early stages in which both the herniated gut and the loose tissue covering of the liver and heart were seen. In three specimens at 5 weeks, there were two plate-like Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012 1029