Morphometry and connective tissue composition of the sigmoid colon in adult Kenyans

Abstract

Background The sigmoid colon in Africans is commonly affected by volvulus formation. Anatomical characteristics of this part of the colon are thought to provide some of the contributory explanation for this predisposition. Published study results for Indians, South Americans, Middle East and some African populations indicate a higher prevalence of sigmoid volvulus in males. Epidemiological evidence shows a low incidence of diverticulosis, another sigmoid colon pathology, in Africans as compared to Caucasians. The exact pathogenesis of diverticular disease of the colon remains debatable. Suggestions have been made that differences in the histomorphology of the sigmoid colon wall may explain the higher and lower incidences in Caucasian and African populations respectively. In Caucasians, the arrangement and amount of collagen and elastic fibres show regional differences. Colonic submucosal collagen fibres crosslink and become tightly packed in small bundles whereas the elastic fibre content increases proximo-distally especially in the taenia coli. These regional changes are associated with loss of wall tensile strength and predisposition to diverticular disease, common in distal sigmoid colon. Physiologically, this distal segment of the sigmoid colon is also known to present a high intraluminal pressure zone. Recent studies have demonstrated an anatomical basis for this phenomenon- an increase in the thickness of the circular muscle forming the "rectosigmoid sphincter". There is limited information on the histomorphology of the African sigmoid colon to investigate structural factors that may protect against diverticulosis. Objective To study the morphometric features of the sigmoid colon and mesocolon and to describe the histomorphology of the sigmoid colon wall in adult Kenyans. Study design A descriptive cross sectional study. Materials and methods Ninety six sigmoid colons (fifty eight male subjects) were harvested at autopsy. Measurements of length of the sigmoid colons and root length and height of mesocolon were taken. From these measurements the sigmoid length: mesocolic root length ratio and sigmoid length: mesocolic height ratio were calculated, and compared for gender. Sections from the proximal, middle and distal segments were processed for light microscopy and stained with Weigert's resorcin fuchsin and Masson's trichrome stains to demonstrate collagen and elastic fibres. Point counting was done to quantify the relative thickness of the submucosa and the inner circular muscle layer. Data management and presentation Means and frequencies of the lengths of the sigmoid colon and the height and root lengths of the sigmoid mesocolon were generated using SPSS program. The sigmoid length: mesocolic root length ratio and sigmoid length: mesocolic height ratios were compared for gender using the Student t-test. P < 0.005 was considered statistically significant. The ratio of the circular muscle to the submucosa was compared for the proximal, mid and distal sigmoid colon segments using analysis of variance (ANOVA). Data on sigmoid colon and mesocolon morphometry was represented in form of tables and graphs while that on histomorphology of the sigmoid colonic wall was presented as photomicrographs. Results The mean sigmoid colon length was higher in males (36.9cm) than in females (32.6cm) (p = 0.007). Most (41.7%) of the sigmoid colons measured 30-34cm long. Males had shorter mesocolon roots and longer mesocolon heights. Elastic fiber bundles were demonstrated in the interphase between mucosa and the submucosa, the submucosa and the circular muscle layer and that between the circular and longitudinal muscular layers of the sigmoid colon wall. The density of elastic fibers increased proximodistally in the external muscular layers. The circular muscle layer showed a relative increase in thickness proximodistally. Conclusion The greater mean lengths of the sigmoid colon and smaller mesocolic root lengths in males may be part of the anatomical basis for the higher incidence of sigmoid volvulus in males. The elastic bundles between layers of the colon wall may be helpful in transmission of stress and improve the capacity of the wall to withstand intracolic pressure - which may partly be a probable protective mechanism against diverticulosis in most Africans. The thickness of the circular muscle layer in the distal sigmoid colonic wall is consistent with the existence of an anatomical rectosigmoid sphincter.