taenia coli

**Anatomy** - Three bands: (1) taenia mesocolica (attachment of the transverse and sigmoid mesocolon), (2) taenia omentalis (attachment of the omentum/epiploic appendages on the transverse colon), and (3) taenia libera (free, no attachments). - They are thickened strips of the outer longitudinal layer of the muscularis externa. Between the taeniae the longitudinal layer is thin. - Because the taeniae are shorter than the colon's length, the colon is gathered into **haustra** (sacculations). Haustra are separated by plicae semilunares inside the lumen. - Other features of the colon that distinguish it from the small bowel: **taeniae coli, haustra, and epiploic appendages** (fat-filled peritoneal pouches). - The three taeniae **converge at the base of the vermiform appendix**. Following the anterior taenia (taenia libera) down the cecum is a classic way to find the appendix during appendectomy. - Near the rectosigmoid junction, the taeniae broaden and fuse into a continuous longitudinal muscle coat, so the **rectum has no taeniae or haustra**.

**Embryology and histology** - Muscularis externa is derived from splanchnic mesoderm. The inner layer is circular and the outer layer is longitudinal (myenteric/Auerbach plexus lies between them).

**Clinical significance** - **Appendectomy:** the taeniae guide the surgeon to the appendix base. - **Sigmoid volvulus / colonic diverticulosis:** diverticula are false (pseudo) diverticula, with mucosa and submucosa herniating through the muscularis at weak points where the vasa recta penetrate the circular muscle, typically between the taenia mesocolica and the taeniae antimesenteric. This is most common in the sigmoid colon. Low-fiber diet and high intraluminal pressure are risk factors. - **Radiology:** loss of haustra ("lead pipe colon") in chronic ulcerative colitis reflects loss of normal haustral pattern. Haustra are visible on plain film and barium enema; the small bowel instead shows valvulae conniventes (plicae circulares) that cross the whole lumen. - **Toxic megacolon / Hirschsprung:** colonic dilation with effacement of haustral markings can be seen. - **Surgery:** taeniae are used as sites for placing seromuscular sutures in some colonic procedures and are the basis for the taenia-based colonic orientation (preventing twisting during anastomosis).

**High-yield pearls** - Three taeniae, three names: mesocolica, omentalis, libera. - Taeniae + haustra + epiploic appendages = colon (not rectum, not appendix). - Haustra are formed because the taeniae are shorter than the colon. - Follow the taeniae to the appendix.