Three-dimensional Heterogeneity of Smooth Muscle Fiber Density Anterior to the Rectum in Males: Quantitative Analysis with Implications for Transanal Total Mesorectal Excision

Purpose Transanal total mesorectal excision for rectal cancer in men poses a risk of urethral injury. The morphology of smooth muscle tissues around the rectum is reportedly characterized by differences in fiber density; however, quantitative analysis of these tissues for surgical applications is lacking. This study aimed to quantitatively analyze the histological properties of fiber density and the spatial extent of the smooth muscle anterior to the male rectum. Method This descriptive cadaveric study involving six adult cadavers was conducted at Tokyo Medical and Dental University. Serial histological sections were prepared from tissues in the region anterior to the rectum, and immunostaining and three-dimensional reconstruction were performed to evaluate the spatial distribution of the smooth muscle. Smooth muscle fiber densities were measured in different regions of the smooth muscle anterior to the rectum and statistically analyzed. Results The three-dimensional heatmap revealed a gradual change in fiber density within the smooth muscle anterior to the rectum, with a low density in the superior part and a high density in the inferior part. In mid-sagittal immunostained sections, the smooth muscle anterior to the rectum exhibited a significant difference in fiber density, averaging 23.22% ± 5.50% in the superior area and being significantly higher at 46.99% ± 12.92% in the inferior area. Conclusion Heterogeneity in fiber density between the superior and inferior smooth muscle anterior to the rectum suggests that these differences could serve as landmarks, providing crucial positional information to avoid urethral injury during transanal total mesorectal excision.