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Anal canal above vs below the pectinate line
The pectinate line is the junction of hindgut and ectoderm, so almost everything changes across it — epithelium, blood supply, lymph, nerves and the diseases that arise.
| Feature | Above the pectinate line | Below the pectinate line |
|---|---|---|
| Origin | Hindgut endoderm | Ectodermal proctodeum |
| Epithelium | Columnar | Stratified squamous (non-keratinized in the pecten, keratinized in the perianal skin) |
| Arteries | Superior rectal artery (from the inferior mesenteric artery) | Inferior rectal arteries (from the internal pudendal) |
| Veins | Superior rectal vein → inferior mesenteric vein → portal system | Inferior rectal veins → internal pudendal → internal iliac (systemic) |
| Lymph | Internal iliac (and inferior mesenteric) nodes | Superficial inguinal nodes |
| Nerves | Visceral (autonomic), sensitive only to stretch — lesions are painless | Somatic (inferior rectal nerves from the pudendal nerve) — lesions are painful |
| Hemorrhoids | Internal: painless bright-red bleeding, prolapse | External: perianal swelling; a thrombosed one is an acutely painful blue lump |
| Cancer | Adenocarcinoma | Squamous cell carcinoma (HPV-associated) |
The middle rectal arteries anastomose with both supplies, and the rectal venous plexus is a portosystemic anastomosis.
An anal fissure lies below the line (usually posterior midline), so it is very painful. See anal canal and pelvic and inguinal lymph drainage.