Microvilli, stereocilia and cilia, with different structure and function
Expansion
| Structure | Core | Motile | Site and function |
|---|---|---|---|
| Microvilli | Actin | No | Brush border of intestine and renal proximal tubule; increase surface area |
| Stereocilia | Actin (long microvilli) | No | Epididymis (absorption), inner ear hair cells (mechanotransduction) |
| Cilia | Microtubules, 9+2 | Yes | Respiratory tract, fallopian tube, ependyma |
The naming is misleading: stereocilia are not cilia at all but long microvilli, which is why they are non-motile and actin-based.
Microvilli increase the intestinal surface area about twentyfold, and combined with plicae circulares and villi give the total amplification of about 600-fold.
Cilia beat 12 to 15 times per second with a fast forward stroke and slow recovery, propelling mucus. Their 9 plus 2 arrangement with dynein arms is disrupted in primary ciliary dyskinesia, giving bronchiectasis, sinusitis, infertility and situs inversus in half of cases (Kartagener syndrome).
Loss of microvilli in coeliac disease and other causes of villous atrophy reduces absorptive surface, which is the direct cause of malabsorption.