Large intestine, appendix and anal canal !
- Organ
The large intestine has no villi; its lining is straight glands full of mucus cells that absorb water and make stool slide.
Updated 2026-11-01
See it in 3D: Cecum · Appendix · Ascending colon · Transverse colon · Sigmoid colon · Rectum · Anal canal
What it is
The colon and rectum have a flat surface with straight, tightly packed crypts rich in goblet cells, designed to absorb water and lubricate stool. The outer longitudinal muscle gathers into three teniae coli. The appendix is a narrow tube packed with lymphoid follicles. The anal canal changes from columnar to squamous epithelium at the pectinate line, a landmark for hemorrhoids and anal cancers.
Structure
- Colon mucosa: no villi, straight tubular crypts, many goblet cells, absorptive colonocytes.
- Muscularis externa: inner circular layer and three teniae coli; haustra are the pouches between them.
- Appendix: crypts that are irregular, lymphoid follicles with germinal centres that may form a ring in the mucosa and submucosa, and a complete longitudinal layer.
- Rectum: deeper crypts, many goblet cells, a continuous longitudinal muscle layer (the teniae spread out).
- Anal canal: simple columnar epithelium above, a transitional zone, non-keratinized stratified squamous below the pectinate line, then keratinized skin with hairs at the anal verge.
Above vs below the pectinate line
| Feature | Above the pectinate line | Below the pectinate line |
|---|---|---|
| Epithelium | Columnar (rectal type) | Stratified squamous |
| Origin | Endoderm (hindgut) | Ectoderm (proctodeum) |
| Pain sensation | Visceral, largely painless | Somatic, painful |
| Hemorrhoids | Internal, usually painless bleeding | External, painful when thrombosed |
| Typical cancer | Adenocarcinoma | Squamous cell carcinoma (often HPV related) |
Clinical notes
- Ulcerative colitis: continuous inflammation from the rectum upward, limited to mucosa and submucosa, with crypt abscesses and distorted crypts.
- Colorectal cancer usually develops from adenomatous polyps through dysplasia (the adenoma-carcinoma sequence).
- Acute appendicitis: obstruction of the lumen, then neutrophils infiltrating the muscularis propria, the key histological criterion.
- Diverticula are herniations of mucosa and submucosa through weak points where vessels cross the muscle wall.
- Entamoeba histolytica causes flask-shaped ulcers of the colon, still seen in the region.
Test yourself
Think of the answer, then tap the question to see it.
What features distinguish colonic mucosa from small-intestinal mucosa?
The colon has no villi and no circular folds; it shows straight crypts packed with goblet cells.
What histological finding confirms acute appendicitis?
Neutrophils infiltrating the muscularis propria of the appendiceal wall.
Which epithelium lines the anal canal below the pectinate line?
Non-keratinized stratified squamous epithelium, which becomes keratinized skin at the anal verge.
Which layers of the wall does ulcerative colitis usually affect?
The mucosa and submucosa; transmural inflammation suggests Crohn disease instead.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Small intestine: villi, crypts, enterocytes, Paneth cells; duodenum vs jejunum vs ileum
- The GI tract wall: mucosa, submucosa, muscularis, serosa; enteric plexuses
- Enteroendocrine cells and GALT
Related conditions
- Ulcerative colitis
- Colorectal cancer
- Acute appendicitis
- Diverticular disease and diverticulitis
- Haemorrhoids
- Crohn disease
Microorganisms
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 23.5 The Small and Large Intestines - Anatomy and Physiology 2e | OpenStax (OpenStax (Rice University), 2022); Small & Large Intestine | SEER Training (National Cancer Institute (SEER Training), 2026); 23.1 Overview of the Digestive System - Anatomy and Physiology 2e | OpenStax (OpenStax (Rice University), 2022).
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Representative model, for educational purposes only.