Continuous, fenestrated and sinusoidal, in increasing permeability
Mnemonic
Three types, in order of increasing leakiness:
- Continuous - unbroken endothelium with tight junctions. Muscle, skin, lung, and the brain, where the tight junctions form the blood brain barrier
- Fenestrated - pores in the endothelium, basement membrane intact. Gut, kidney glomerulus, endocrine glands, choroid plexus. Built for rapid exchange
- Sinusoidal (discontinuous) - large gaps and an incomplete basement membrane. Liver, spleen, bone marrow, adrenal. Allow cells and proteins through
“Continuous keeps everything in, fenestrated lets fluid through, sinusoidal lets cells through.”
The pattern follows function precisely: the brain must exclude, the gut and kidney must exchange fluid quickly, and the marrow must release whole cells into the circulation.
Circumventricular organs are the exception in the brain, being fenestrated so they can sample the blood: the area postrema (the vomiting centre, and why chemotherapy causes nausea), the posterior pituitary and the hypothalamic nuclei.
Expansion
| Type | Structure | Location |
|---|---|---|
| Continuous | Unbroken endothelium, complete basement membrane, tight junctions | Muscle, skin, lung, brain (with especially tight junctions forming the blood-brain barrier) |
| Fenestrated | Pores of 60 to 80 nm, often with a diaphragm; complete basement membrane | Gut mucosa, endocrine glands, renal glomerulus (here without diaphragms), choroid plexus |
| Sinusoidal (discontinuous) | Large gaps, incomplete or absent basement membrane | Liver, spleen, bone marrow, adrenal cortex |
The pattern is entirely functional:
- Continuous where exclusion matters, most extremely in the brain
- Fenestrated where rapid filtration or absorption of small molecules is required, which is why the glomerulus and the gut mucosa are fenestrated
- Sinusoidal where whole cells or large proteins must cross, which is why newly formed blood cells leave the marrow and why hepatocytes have direct access to plasma through the space of Disse
Pericytes surround continuous and fenestrated capillaries, providing contractile support and, in the brain, contributing to the blood-brain barrier. Their loss is an early event in diabetic retinopathy, producing microaneurysms.