In short, it is the innermost layer of the meninges, a delicate fibrous tissue entirely covering the central nervous system. The other two layers are the dura mater (outer layer) and the arachnoid mater (inner layer). When it becomes infected or irritated, the consequence is meningitis. Cerebrospinal fluid fills the subarachnoid space overlying the pia mater, which is impermeable. The pia mater, along with the arachnoid mater, arises during embryogenesis from the neural crest and the dura mater from the mesoderm.
See Cerebrospinal fluid (CSF), Embryogenesis, Meningises, Meningitis, Mesoderm, Molecular layer, Neural crest, Proliferative ventricular zone