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The pia mater is the innermost layer of the meninges and together with the arachnoid mater is referred to as the leptomeninges. It is closely related to the surface of the brain and unlike the arachnoid mater extends into the sulci 1.
The pia mater is separated from the arachnoid mater by the CSF-filled subarachnoid space and only connected to this outer membrane by the arachnoid trabeculae 2. It is impermeable to CSF 1.
It is a highly vascular layer containing blood vessels that supply the underlying surface of the brain particularly over the cerebrum 3. At the point where blood vessels enter the underlying neural tissue, extensions of the pia mater are carried with them creating a perivascular space 1. Because the brain does not have a true lymphatic system this perivascular space is important in allowing the movement of substances back into the CSF.
At certain points in the third, fourth and lateral ventricles the pia mater forms a double layer and is covered by ependymal cells to create the tela choroidea. In the third ventricle a double-layered tela choroidea forms the velum interpositum 4.
At certain points the tela choroidea bulges into the ventricle and is closely associated with numerous villous processes and an underlying vascular bundle. This is the choroid plexus and involved in the production of CSF 1.
History and etymology
"Pia mater" derives from medieval Latin for "soft mother". This term was created in Stephen of Antioch's translation of Hali Abbas in the 12th century 5. Arabic medicine at that time conjectured that the meninges gave rise to all the membranes of the body and expressed relationships between different tissue types in terms of familial relationships (mother, son, daughter, etc.).
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