Fig. 4 |. Routes of leukaemia central nervous system invasion.

Leukaemia cell invasion of the central nervous system (CNS) mostly relates to acute leukaemias. Leukaemia cells have been shown to invade the CNS by breaching the blood–brain barrier of parenchymal or leptomeningeal vessels⁶⁸ (part a), crossing the blood–cerebrospinal fluid (CSF) barrier of the choroid plexus¹⁷⁹ (part b) or travelling along the abluminal surface of emissary blood vessels that exit the bone marrow through fenestrations in the bone and transition into leptomeningeal vessels (part c)⁵⁶. The choroid plexus is a secretory tissue in the brain that is responsible for producing CSF. It contains fenestrated vessels and a monolayer of ependymal cells with tight junctions, ion pumps and transporters that filter out many cells, ions and proteins to produce CSF. The meninges comprise three membranous layers–the dura, the arachnoid and the pia mater–that form a continuous physical barrier surrounding the brain parenchyma and spinal cord. The dura mater is the outermost layer that is adjacent to the calvarium (skull) or vertebral bone. The subsequent arachnoid and pia mater are connected and form the leptomeninges. Between these two layers that form the leptomeninges is the subarachnoid space, which is filled with an acellular CSF that physically cushions the brain and spinal cord.