Figure 2.

Possible routes of M. tuberculosis entry into CNS. M. tuberculosis may enter the brain parenchyma directly via capillary blood flow through the blood-brain barrier (BBB, Figure 2.1). This may be via paracytosis (A), transcytosis (B) or within an infected cell (C). M. tuberculosis may also pass through the blood-CSF barrier lining the ventricles, which contains an epithelial choroid plexus layer along with an endothelial layer through the same mechanisms (Figure 2.2). In the meninges, the blood-arachnoid barrier separates the unfiltered interstitial fluid and blood from the dura from the sub-arachnoid space within the CNS via an epithelial layer of arachnoid barrier cells. The arachnoid barrier cells are poorly vascular, and exchange is likely to be minimal. In addition, blood vessels (BV) passing through the subarachnoid space represent another blood-CSF barrier which does not contain a choroid plexus layer. M. tuberculosis in the subarachnoid space may be able to pass across the brain-CSF barrier formed by an ependymal layer into the brain parenchyma (Figure 2.3). Created with BioRender.com.