Identification of a sphincter at the sigmoidorectal canal in humans: histomorphologic and morphometric studies

In a recent study we demonstrated that the rectosigmoid junction is a large bowel segment 2.8 +/- 0.9 cm in length, which we have termed the 'sigmoidorectal canal' (SRC). In that study we showed that the SRC exerted increased intraluminal pressure, contained a thick, circular, smooth muscle layer, and reacted to sigmoid colon (SC) and rectal contraction by relaxation and contraction, respectively. These findings suggest the existence of an anatomical sphincter at the SRC. To investigate this hypothesis, in the present study we carried out a morphometric (quantitative) analysis of the musculature of the SC, SRC, and rectum. Twenty-two cadavers (13 male, 9 female; mean age = 32.3 +/- 17.6 years) were investigated. Histomorphologic studies comprised staining transverse (15 cadavers) and longitudinal (7 cadavers) sections from the SC, SRC, and rectum. Specimens were stained with H&E, Masson's trichrome, and silver stains. Morphometric studies, using image analyzer computer equipment, measured the muscle thickness of the muscularis externa (ME) of the SC, SRC, and rectum. The ME of the SRC was markedly thickened compared to the ME of the SC or rectum. In 16 specimens, the increased thickness involved mainly the circular muscle layer and in six specimens both the circular and longitudinal layers. Connective tissue septa connected the adventitia with the submucosa: these 'adventitial septa' contained nerve ganglia. 'Localized bundles' of smooth muscle fibers were detected between the circular muscle fibers. The morphometric findings showed that the mean thickness of the circular muscle layer measured 440.3 +/- 80.1 microm in the SC, 1166.1 +/- 255.4 microm in the SRC, and 394.9 +/- 123.9 microm in the rectum. This study strongly suggests the presence of an anatomic sphincter at the SRC. This sphincter may explain the higher pressure at the SRC and the response of the SRC to rectal and SC contractions. The localized smooth muscle bundles in the ME may represent a rectosigmoid pacemaker.