(/ˈpaɪ.ə ˈmeɪtər/ or /ˈpiː.ə ˈmɑːtər/ often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord.
Pia mater is medieval Latin meaning “tender mother.” The other two meningeal membranes are the dura mater and the arachnoid mater.
Pia mater is a thin fibrous tissue that is impermeable to fluid. This allows the pia mater to enclose cerebrospinal fluid. By containing this fluid the pia mater works with the other meningeal layers to protect and cushion the brain. The pia mater allows blood vessels to pass through and nourish the brain. The perivascular space created between blood vessels and pia mater functions as a lymphatic system for the brain. When the pia mater becomes irritated and inflamed the result is meningitis.
Pial invasion is an important predictor of tumor recurrence in benign and atypical meningiomas.