Original Article http://dx.doi.org/10.3349/ymj.2012.53.5.1028 pISSN: 0513-5796, eISSN: 1976-2437 Yonsei Med J 53(5):1028-1035, 2012 Development of the Rectus Abdominis and Its Sheath in the Human Fetus Jae Do Yang, 1 * Hong Pil Hwang, 1 * Ji Hyun Kim,2 Jose Francisco Rodríguez-Vázquez,3 Shin-ichi Abe,4 Gen Murakami,5 and Baik Hwan Cho1,6 Departments of 1Surgery and 2Anatomy, Chonbuk National University Medical School, Jeonju, Korea; 3Department of Anatomy and Embryology II, Faculty of Medicine, Complutense University, Madrid, Spain; 4Oral Health Science Center hrc-8 and Department of Anatomy, Tokyo Dental College, Chiba, Japan; 5Division of Internal Medicine, Iwamizawa Kojin-kai Hospital, Iwamizawa, Japan; 6Research Institute of Clinical Medicine, Chonbuk National University Hospital, Jeonju, Korea. Received: October 21, 2011 Revised: December 6, 2011 Accepted: December 6, 2011 Corresponding author: Dr. Baik Hwan Cho, Department of Surgery, Chonbuk National University Medical School, 20 Geonji-ro, Deokjin-gu, Jeonju 561-712, Korea. Tel: 82-63-250-1570, Fax: 82-63-250-6197 E-mail: chobh@jbnu.ac.kr *Jae Do Yang and Hong Pil Hwang contributed equally to this work. ∙ The authors have no financial conflicts of interest. Purpose: Although the rectus abdominis and its sheath are well known structures, their development in the human fetus is poorly understood. Materials and Meth- ods: We examined rectus abdominis and sheath development in semiserial horizon- tal sections of 18 fetuses at 5-9 weeks of gestation. Results: Rectus muscle differ- entiation was found to commence above the umbilicus at 6 weeks and extend inferiorly. Until closure of the anterior chest wall via fusion of the bilateral sternal anlagen (at 7 weeks), the anterior rectal sheath originated from the external oblique and developed towards the medial margin of the rectus abdominis at all levels, in- cluding the supracostal part. After formation of the anterior sheath, fascial laminae from the internal oblique and transversus abdominis contributed to formation of the posterior rectus sheath. However, the posterior sheath was absent along the supra- costal part of the rectus abdominis, as the transversus muscle fibers reached the ster- num or the midline area. Therefore, it appeared that resolution of the physiological umbilical hernia (8-9 weeks) as well as chest wall closure was not required for de- velopment of the rectus abdominis and its sheath. Conversely, in the inferior part of the two largest fetal specimens, after resolution of the hernia, the posterior sheath underwent secondary disappearance, possibly due to changes in mechanical stress. Conclusion: Upward extension of the rectus abdominis suddenly stopped at the margin of the inferiorly developing pectoralis major without facing the external in- tercostalis. The rectus thoracis, if present, might correspond to the pectoralis. Key Words: Rectus abdominis, rectus sheath, pectoralis, sternalis, sternum, rib © Copyright: Yonsei University College of Medicine 2012 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non- Commercial License (http://creativecommons.org/ licenses/by-nc/3.0) which permits unrestricted non- commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. INTRODUCTION The rectus abdominis extends between the rib cage and pubis, and is supplied by the lower intercostal and corresponding segmental abdominal nerves (Th7-L1). According to Gray’s Anatomy,1 at the superior end, the rectus abdominis is at- tached by three slips of muscle to the fifth, sixth and seventh costal cartilages. 1028 Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012