Pia Mater

The pia mater is the innermost layer of the meninges that lies below and adjacent to the arachnoid. It follows the surface of the brain and spinal cord, covering them snugly and extending into every sulcus (groove), fissure, and depression of the cerebrum and cerebellum surface. It has the aspect of a thin mesh, with a network of tiny blood vessels interlacing it. Since it lies contiguous to one another, both the pia mater and the arachnoid are collectively called the leptomeninges, and the space between them is the subarachnoid space.

The pia mater is composed of vascular loose connective tissue which contains delicate collagen fibers, flat fibroblasts, and some macrophages. It is a very thin membrane, with the word ''pia'' meaning tender, as it adheres closely to the brain surface. Thin blood vessels run from the pia mater into the substance of the brain to supply the external layer of the cerebral and cerebellar cortex (however, the deeper parts of the brain are supplied by branches from the middle cerebral and basilar artery).

Function

Like the other two layers of the meninges, the pia matter function is to protect the brain surface from trauma damage and infections. Because of its high degree of permeability, water and small molecules in the cerebrospinal fluid can seep into the cerebral cortex interstitial fluid. This permeability is regulated by the astrocytes, which is a support cell of the brain nerve cells.

Above, a schematic picture showing the pia mater below the subarachnoid space and over cerebral cortex.