The anal canal: haemorrhoids, condyloma and anal carcinoma
- Lesion
Swollen anal veins cause bright red bleeding; HPV can cause warts and, with high-risk types, cancer of the anal canal.
What it is
The anal canal has three zones: an upper zone lined by columnar rectal epithelium, a middle transitional zone and a lower zone of stratified squamous epithelium. Tumours mirror these linings: adenocarcinoma, squamous cell carcinoma and basaloid squamous carcinoma (the old cloacogenic carcinoma). Haemorrhoids are dilated collateral veins of the anal and perianal venous plexuses, caused by persistently raised venous pressure.
Structure
- Haemorrhoids: thin-walled dilated submucosal vessels, often with thrombosis, haemorrhage and recanalization.
- Condyloma: papillary squamous proliferation with koilocytes.
- Squamous cell carcinoma: nests of malignant squamous cells, often from anal intraepithelial neoplasia.
- Basaloid carcinoma: small dark immature cells with peripheral palisading.
Step by step
- Venous pressure in the anal cushions rises (straining, pregnancy, portal hypertension).
- Vessels of the haemorrhoidal plexus dilate and supporting tissue weakens.
- Cushions slide down and bleed; thrombosis causes pain.
- Separately, HPV infects the transitional zone; high-risk types integrate and inactivate p53 and RB.
- Intraepithelial neoplasia progresses to invasive squamous carcinoma.
Clinical notes
- Haemorrhoids are rare under thirty except in pregnancy.
- Anal varices of portal hypertension are more common and much less dangerous than oesophageal varices.
- Distal anal cancers drain to inguinal nodes and systemic veins, bypassing the liver.
Test yourself
Think of the answer, then tap the question to see it.
What is the main pathogenic factor in haemorrhoids?
Persistently raised venous pressure in the haemorrhoidal plexus, mostly from straining at stool and pregnancy.
Which virus is linked to anal squamous cell carcinoma?
High-risk human papillomavirus, especially type 16.
From which zone does basaloid (cloacogenic) carcinoma arise?
From immature basal cells of the transitional zone of the anal canal.
Elsewhere in DurgiPedia
- Large intestine, appendix and anal canal
- Hindgut and cloaca: the urorectal septum and the pectinate line
Connections
Microorganisms1
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Internal Hemorrhoid (StatPearls (NCBI Bookshelf), 2025); Anatomy, Abdomen and Pelvis: Anal Canal (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.