Anatomy & Embryology of the Colon, Rectum & Anus

We begin with the anal canal and pelvic floor. In colorectal surgery, it’s important to distinguish between the anatomic anal canal and the surgical anal canal. Continence depends on a coordinated set of muscles: the internal and external anal sphincters, and the puborectalis. The canal is lined by different epithelial types, with the dentate line as the critical landmark. That line separates endoderm from ectoderm, and it also marks a division in nerve supply, blood supply, and lymphatic drainage.Moving proximally, we examine the rectum. Its divisions, its blood supply from the superior, middle, and inferior rectal arteries, and the surrounding fascial planes are central to cancer surgery. Structures like the mesorectum and Denonvilliers’ fascia guide oncologic dissection. We also need to know the anorectal spaces — the ischioanal and supralevator spaces, among others — because they can harbor or spread infection.The colon itself extends from the cecum to the sigmoid. Its hallmark features are the taeniae coli, the haustra, and the appendices epiploicae. The colon is supplied by both the superior and inferior mesenteric arteries, with the watershed zone at the splenic flexure carrying clinical significance. Venous drainage, lymphatic drainage, and autonomic innervation form a complex network with surgical and oncologic implications.Finally, embryology. The rectum and anus derive from the hindgut, while the colon comes from both midgut and hindgut. Midgut rotation during fetal life occurs in three stages; errors in this process lead to anomalies such as malrotation, non-rotation, or omphalocele. Other congenital disorders to recognize include Hirschsprung’s disease, colonic atresia, and the spectrum of anorectal malformations.