CorpusHuman Anatomy
DigestiveRegion

Pectinate line

Linea pectinata

Also known as: dentate line

The pectinate line is the scalloped line formed by the anal valves about halfway down the anal canal. It roughly marks the junction of endodermal hindgut and ectodermal proctodeum, so epithelium, blood supply, lymph drainage and nerve supply all change across it.

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Overview

The pectinate line is one of the most examined landmarks in gastrointestinal anatomy because almost every attribute of the canal changes across it. Above the line the canal is derived from the endodermal cloaca, lined by columnar epithelium, supplied by the superior rectal artery, drained to the portal system and the internal iliac nodes, and innervated by autonomic fibres. Below it the canal is derived from the ectodermal anal pit (proctodeum), lined by stratified squamous epithelium, supplied by the inferior rectal artery, drained to the systemic veins and the superficial inguinal nodes, and innervated by somatic nerves.

The practical consequences follow directly. Internal haemorrhoids and low rectal adenocarcinomas are painless, while external haemorrhoids, fissures and anal squamous cancers are painful. Cancers above the line spread to pelvic and mesenteric nodes, while those below spread to the groin nodes. Portal hypertension can open the rectal portosystemic anastomosis here.

Location

Anal canal, about 2 cm above the anal verge, along the lower ends of the anal columns.

Parts & subdivisions

  1. Embryology
    Above: endoderm (hindgut, cloaca). Below: ectoderm (proctodeum/anal pit).
  2. Epithelium
    Above: simple columnar. Below: stratified squamous, becoming keratinised beyond the white line.
  3. Arteries
    Above: superior rectal artery (IMA). Below: inferior rectal artery (internal pudendal).
  4. Veins
    Above: superior rectal vein → IMV → portal. Below: inferior rectal vein → internal pudendal → internal iliac → IVC.
  5. Lymph
    Above: internal iliac and inferior mesenteric nodes. Below: superficial inguinal nodes.
  6. Nerves
    Above: visceral autonomic fibres (stretch only, painless). Below: somatic inferior rectal nerve (painful).
  7. Tumours
    Above: adenocarcinoma. Below: squamous cell carcinoma (HPV-16).

Function

  • A landmark that predicts the behaviour of anorectal disease.

Clinical significance

Internal vs external haemorrhoids

Internal haemorrhoids (above the line, visceral innervation) bleed painlessly. External haemorrhoids (below, somatic innervation) are acutely painful when thrombosed. Banding is therefore done above the line, where it is painless.

Mnemonics

Above vs below

Above the line it is all "Internal": Internal haemorrhoids, Insensate (visceral), drained to the Internal iliac and Inferior mesenteric nodes. Below the line the rules are reversed: External, Excruciating, and drained to the groin (superficial inguinal nodes).

Test yourself

Internal haemorrhoids are typically painless because the mucosa above the pectinate line is:
  1. Supplied by the inferior rectal nerve
  2. Covered by keratinised skin
  3. Innervated by visceral (autonomic) afferents sensitive only to stretch
  4. Drained by the superficial inguinal lymph nodes

Answer: Innervated by visceral (autonomic) afferents sensitive only to stretch

Above the pectinate line the canal is hindgut, with visceral innervation: painless bleeding, superior rectal (portal) venous drainage and internal iliac nodes. Below the line it is somatic (inferior rectal nerve, painful), with systemic veins and superficial inguinal nodes.

Lymph from the anal canal below the pectinate line drains first to the:
  1. Internal iliac nodes
  2. Inferior mesenteric nodes
  3. Para-aortic nodes
  4. Superficial inguinal nodes

Answer: Superficial inguinal nodes

The ectodermal part of the anal canal drains to the superficial inguinal nodes, like the perineal skin. Anal squamous carcinoma therefore spreads to the groin.

More digestive system questions

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.