The thin, innermost layer of the meninges, the pia mater adheres closely to the spinal cord, passing into the anterior median fissure, conus medullaris, filum terminale, and nerve roots, and brain, all the way into the sulci and fissures of the cortex. It fuses with ependyma (membranous linings of the ventricles) to form the choroid plexus, which produces cerebrospinal fluid. The pia mater consists of mostly collagen and reticular fibers.