The Human Rectus Sheath: Formation And Microscopic Organisation

Abstract

Background: The pattern of formation of the rectus sheath from the aponeuroses of external oblique, internal oblique and transversus abdominis muscles shows regional variations. The prevalence and location of the arcuate line which marks the inferior extent of the posterior wall of this sheath exhibits inter-population differences. Data on the Kenyan population however remains scarce and yet the location of this line may be important when harvesting rectus abdominis muscle for musculocutaneous flaps. Further, it is not known whether the variations on how the rectus sheath is formed and in the position of the arcuate line influence the histomorphology of this sheath. This knowledge may help explain the contribution of the rectus sheath in the biomechanics of the ventral abdominal wall. Objective: To describe the pattern of formation and microscopic organization of the rectus sheath as well as location of the arcuate line. Study design: A descriptive cross-sectional study. Materials: Specimens were collected from eighty subjects of both gender aged 18 to 70 years. Of these, 31(16 male, 15 female) were collected from Chiromo and Nairobi City Mortuaries during autopsies. The remaining 49 (21 male, 18 female) were acquired from cadavers used for routine dissection by first year medical students at the Department of Human Anatomy, University of Nairobi. Methods: The rectus sheath was exposed through a midline incision followed by dissection and clearance of the superficial fascia covering the anterior wall of this sheath. The pattern of formation of the sheath and position of the arcuate line were studied and documented. In the autopsy materials, five millimeter thick sections were harvested and processed for light microscopy by paraffin embedding. From these, seven micrometer thick sections were cut then stained with Masson's Trichrome and Weigert's Resorcin Fuchsin to demonstrate collagen and elastic fibres respectively. Photographs were taken to illustrate the pattern of formation and the microscopic organisation of the rectus sheath. Data analysis: Analogue photomicrographs taken were scanned using Hewlett Parkard® Scanjet seamier then entered into the Scion Image Beta 4.0.3.2 software (Scion Corporation, Fredrick, Maryland) for measurement of the thickness of the anterior and posterior walls of the rectus sheath. Data collected were coded then entered into SPSS computer software (version 15.0 Chicago, Illinois) for statistical analysis. The student t test was used in determining gender variations. A p-value < 0.05 was considered significant. Results: The anterior wall of the rectus sheath was aponeurotic in all cases while the posterior wall of the rectus sheath was aponeurotic in 71 (88.5%) cases, the rest were musculoaponeurotic. The internal oblique abdominis aponeurosis above the arcuate line split in all cases to enclose the rectus abdominis muscle. The arcuate line was present in 64 (80.4%) cases bilaterally as a single structure. This line was located closer to the umbilicus than the pubic symphysis, consistently at the last intersection of rectus abdominis muscle. Microscopically, both walls of the rectus sheath were made of three distinct zones: superficial, intermediate and deep. The superficial and deep zones contained loosely arranged collagen and elastic fibres while the intermediate zones were made of compact bundles of collagen fibres. These bundles in the anterior wall of the rectus sheath were obliquely oriented above the arcuate line and transverselv disposed below this line. This corresponded with the alignment of the fascicles of internal oblique abdominis muscle. The intermediate zone in the posterior wall of the rectus sheath contained transversely oriented collagen fibre bundles in line with the fascicles of transversus abdominis muscle. Conclusion: The microscopic organisation of the rectus sheath is determined by its pattern of formation and not by the position of the arcuate line. This sheath is mainly formed by the aponeuroses of internal oblique and transversus abdominis. Further, the arcuate line occurs more cranially and can accurately be located using tire most distal intersection of rectus abdominis muscle. This information may be important to surgeons when harvesting rectus abdominis muscle for musculocutaneous flaps