Blood-brain barrier

The blood-brain barrier (BBB) forms a physical resistance to the passage of lipophobic substances from cerebral capillaries into the brain and is a key reason why there is no CSF enhancement following intravenous contrast media in CT and MR imaging.

Gross anatomy

The BBB is formed by a combination of endothelial cells, pericytes, and astroglial and perivascular macrophages along the cerebral capillary walls.

In general, capillary walls in the human body can consist of three different types:

• continuous: present in areas which have a BBB
  • continuous interendothelial tight junctions
  • no pinocytosis
  • no fenestrations
• fenestrated: present in areas which lack the BBB
• sinusoidal: not found in the brain

In the brain, the majority of capillary walls are of the continuous type, with tight junctions and a continuous basement membrane

Areas which contain fenestrated capillaries, and thus lack the blood brain-brain barrier, are:

•  circumventricular organs (CVO)
  • area postrema
  • median eminence
  • subforniceal region
  • posterior pituitary gland
  • pineal gland
  • organum vasoculosum
  • lamina terminalis
• choroid plexus
• dura mater

Generally, lipophilic solutes can cross the BBB, including:

• caffeine
• ethanol
• CO₂
• O₂
• nicotine
• radiopharmaceuticals used in PET and SPECT imaging

Hydrophilic solutes, in general, are unable to cross it, e.g. water soluble CT/MR contrast media. 

Related pathology

There are a multitude of conditions associated with disruption of the BBB:

• temporal lobe epilepsy (TLE) ¹
• posterior reversible encephalopathy syndrome (PRES)
• hypoxia, ischaemia and infarction
• tumours
• inflammatory conditions, e.g. meningitis
• trauma
• intracranial irradiation
• multiple sclerosis
• progressive multifocal leukoencephalopathy: the JC virus can cross the BBB