Pia

Pia mater ( or), 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.

Both the pia and arachnoid mater are derivatives of the neural crest while the dura is derived from embryonic mesoderm.

The pia mater is a thin fibrous tissue that is permeable to water and small solutes.

The pia mater allows blood vessels to pass through and nourish the brain.

The perivascular space between blood vessels and pia mater is proposed to be part of a pseudolymphatic system for the brain (glymphatic system).

When the pia mater becomes irritated and inflamed the result is meningitis.