Appendix
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Here is a comprehensive overview of the appendix (vermiform appendix) drawn from authoritative medical textbooks:
Vermiform Appendix
Anatomy
The vermiform appendix is a narrow, blind-ended tube that projects from the posteromedial wall of the cecum, just below the ileocecal valve. It develops embryologically as part of the midgut, forming as a bud from the midgut loop between the 8th and 12th weeks of gestation.
Key measurements:
- Average length: 9-10 cm (range: 5-35 cm)
- Average diameter: ~6 mm
Position: The appendix assumes a highly variable position. Unlike the rest of the colon - where the longitudinal muscle forms teniae coli - the appendix has a continuous circumferential longitudinal muscle layer and no teniae. Surgically, this is useful: following the teniae of the cecum leads directly to the appendiceal base.
Five positions of the appendix. Retrocecal is most common (~65%), followed by pelvic/subcecal (~31%). - Sleisenger and Fordtran's GI and Liver Disease, Fig. 120.1
| Position | Frequency |
|---|---|
| Retrocecal (ascending) | ~65% |
| Subcecal / pelvic (descending) | ~31% |
| Transverse retrocecal | ~2% |
| Pre-ileal (paracecal) | ~1% |
| Post-ileal (retroileal) | ~0.5% |
McBurney's point: The classic surface landmark is located at the junction of the lateral and middle thirds of a line from the right anterior superior iliac spine to the umbilicus. However, the appendix is within 5 cm of this point in less than 50% of cases, explaining why tenderness at McBurney's point is not universal in appendicitis.
Blood Supply, Venous Drainage & Lymphatics
- Arterial supply: The appendicular artery - a branch of the ileocolic artery (the last branch of the superior mesenteric artery) - runs within the mesoappendix to supply the appendix.
- Venous drainage: Via the appendicular vein → ileocolic vein → superior mesenteric vein → portal vein.
- Lymphatics: Drain to the ileocolic lymph nodes, shared with the terminal ileum and ascending colon.
- Peritoneum: The appendix is an intraperitoneal structure with its own mesentery (mesoappendix).
Histology
Microscopically, the appendix resembles the large intestine but with several distinguishing features:
- Crypts (intestinal glands) are present but shallow and irregular (unlike the deep, packed crypts of the cecum).
- The most distinctive feature is a massive collection of lymphoid follicles (aggregated lymphoid nodules) extending from the lamina propria through the muscularis mucosae into the submucosa.
- This dense lymphoid tissue makes the appendix an important component of the gut-associated immune system - sometimes described as a "safe house" for symbiotic gut microbiota.
Function
Once considered a vestigial organ, current understanding recognizes two roles:
- Immune function: The appendix is rich in lymphoid tissue (GALT - gut-associated lymphoid tissue) and likely plays a role in mucosal immunity, particularly in early life.
- Microbiome reservoir: It has been proposed to serve as a reservoir for beneficial gut bacteria, potentially aiding recolonization after gastrointestinal illness.
Acute Appendicitis
Appendicitis is the most common abdominal surgical emergency. Inflammation is classified as:
| Stage | Gross / Microscopic Features |
|---|---|
| Acute (suppurative) | Serosal injection and edema; neutrophilic infiltrate involving the muscularis propria circumferentially; mucosal ulceration and microabscesses |
| Gangrenous | Transmural inflammation; focal mural necrosis; greenish/black discoloration; prominent vascular thrombosis |
| Perforated | Breakdown of the necrotic wall; spillage of contents; adjacent abscess formation or generalized peritonitis |
Note: Mucosal inflammation alone ("catarrhal" inflammation) is not sufficient for microscopic diagnosis of appendicitis - inflammation must extend to the muscularis propria.
Pathogenesis (two main hypotheses):
-
Obstruction hypothesis: A fecalith or lymphoid hyperplasia obstructs the lumen → raised intraluminal pressure → venous hypertension → ischemia → bacterial invasion and necrosis. However, fecaliths are found in only 3.6-27% of cases, and elevated intraluminal pressure is present in only a minority.
-
Mucosal ulceration hypothesis: Bacterial or viral enteric infection (CMV has been implicated) causes primary mucosal ulceration → bacterial invasion by normal colonic flora. Mucosal ulceration is found in up to 75% of appendicitis cases and is a more consistent finding than fecaliths.
Appendiceal Tumors
- Neuroendocrine tumors (NETs) are the most common appendiceal neoplasm (60-80% of appendiceal tumors), and are found incidentally in ~1-1.5% of appendectomy specimens.
- Lymph node metastasis rates correlate with tumor size: 15% for <1 cm, 47% for 1-2 cm, 86% for >2 cm.
- Management is guided by size: small NETs (<1 cm) can be treated with simple appendectomy; larger or high-risk tumors require right hemicolectomy with lymphadenectomy.
Sources:
- Color Atlas of Human Anatomy Vol. 2, Internal Organs, pp. 338-342
- Sleisenger and Fordtran's Gastrointestinal and Liver Disease, pp. 2436-2438
- Sabiston Textbook of Surgery, p. 1613
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