Anal canal above versus below the pectinate line
The pectinate line (also called the dentate line) is an irregular circle in the anal canal that divides it into an upper and a lower part with different origins.
Remember
Below the pectinate line the lining is squamous (flat, layered cells, like skin), the nerves are somatic (the type that supplies skin) and sense pain, and drainage goes to the superficial inguinal lymph nodes in the groin; above it the pattern is the reverse.
The pectinate line is formed by the anal valves, which are small folds of lining. It divides the anal canal into an upper and a lower part with different origins. The upper part comes from the embryonic hindgut (the back end of the developing gut), and the lower part comes from the proctodeum (a pit of surface tissue formed as the embryo folds).
Below the line, the lining is stratified squamous epithelium (flat, layered cells, like skin). The nerve supply is somatic, the type that supplies skin and muscle, and it runs in the inferior rectal nerves, which are branches of the pudendal nerve. The area therefore senses pain, touch, pressure and temperature, so a small tear in the lower canal, an anal fissure, causes sharp pain on passing stool. Lymph (the fluid that drains from tissues and is filtered by lymph nodes) goes to the superficial inguinal nodes in the groin. Veins drain through the inferior rectal and internal pudendal veins into the systemic circulation.
Above the line, the lining is columnar (glandular) epithelium, like the rectum above it. The nerves are visceral, the automatic system that serves internal organs, and they sense stretch only. Their supply comes from the inferior hypogastric plexus, a nerve network in the pelvis. This gives a vague, poorly localised discomfort instead of sharp pain. Lymph goes to the internal iliac nodes deep in the pelvis.
A simple picture helps: below the line the lining behaves like skin, so it hurts like skin. Inflammatory or malignant disease at the anal margin, which lies below the pectinate line, often causes tender, enlarged nodes in the groin.
Key facts
- Lining
- below the line it is stratified squamous epithelium, like skin; above the line it is columnar.
- Nerves
- below the line they are somatic (S2 to S4, through the inferior rectal nerves) and sense pain, touch, pressure and temperature; above the line they are visceral and sense stretch only.
- Lymph nodes
- below the line lymph goes to the superficial inguinal nodes in the groin; above the line it goes to the internal iliac nodes.
- Arteries
- below the line the supply is the inferior rectal artery, a branch of the internal pudendal artery; above the line it is the superior rectal artery, the end of the inferior mesenteric artery.
- Veins
- below the line they drain through the inferior rectal and internal pudendal veins into the systemic circulation; above the line they drain through the superior rectal vein into the portal system.
- Clinical clue
- pain and spread to the groin nodes both point to disease below the pectinate line.
Exam traps
- Disease below the pectinate line does not drain to the internal iliac nodes, because those nodes drain the canal above the line; below the line, lymph goes to the superficial inguinal nodes.
- Pain below the pectinate line is not carried by somatic nerves from L1 to L2, because the lower anal canal is supplied by S2 to S4.
- The deep inguinal nodes are not the first station for lymph from the anal margin, because they collect lymph from deeper structures of the lower limb, with the superficial nodes feeding in.
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