pectinate line
Summary
The pectinate (dentate) line marks the transition from hindgut-derived columnar epithelium to ectoderm-derived squamous epithelium in the anal canal, roughly 2 cm inside the anus. It is a key anatomical landmark that determines differences in blood supply, innervation, lymphatic drainage, and venous drainage above versus below it, which explains distinct clinical presentations of pathology occurring on either side.
Detail
The pectinate line is formed at the junction of the hindgut and the proctodeum during embryonic development, marking the boundary between endoderm (above) and ectoderm (below). Above the line: columnar epithelium, arterial supply from superior rectal artery (branch of IMA), venous drainage via superior rectal vein into the portal system, lymphatic drainage to internal iliac nodes, and visceral innervation (insensitive to pain, temperature, touch) via the inferior hypogastric plexus—hence internal hemorrhoids arising above the line are painless. Below the line: stratified squamous epithelium, arterial supply from inferior rectal artery (branch of internal pudendal artery), venous drainage via inferior rectal vein into the internal pudendal vein and then systemic (caval) circulation, lymphatic drainage to superficial inguinal nodes, and somatic innervation via the inferior rectal nerve (branch of pudendal nerve, S2-S4)—making external hemorrhoids and anal fissures below the line painful. This line is also clinically significant for anal cancer: squamous cell carcinoma arises below the pectinate line and spreads to inguinal nodes, while adenocarcinoma arises above and spreads to iliac/hypogastric nodes. Portal-systemic anastomosis at this site (superior rectal vein vs inferior/middle rectal veins) is important in portal hypertension, leading to anorectal varices distinguished from hemorrhoids. Anal crypts and glands located at the pectinate line can become obstructed, leading to anorectal abscesses and fistulas.
Sources
- First Aid for the USMLE Step 1
- Moore's Clinically Oriented Anatomy
- Kaplan USMLE Step 1 Anatomy Lecture Notes
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