What Are the Meninges?
The meninges are a three-layer protective membrane system that covers the brain and spinal cord, consisting of the dura mater (outermost layer), the arachnoid mater (middle layer), and the pia mater (innermost layer). 1
Anatomical Structure
Three Distinct Layers
Dura mater: The outermost, thickest layer that forms both an outer endosteal layer related to the skull and spine bones, and an inner layer closely applied to the arachnoid mater 2
Arachnoid mater: The middle layer with an outer parietal component that is impermeable to cerebrospinal fluid (CSF) due to tight intercellular junctions 2
Pia mater: The innermost delicate layer that directly adheres to the surface of the brain and spinal cord, separated from neural tissue by the glial limitans 3, 2
Leptomeninges vs. Pachymeninges
The leptomeninges consist of the arachnoid and pia mater together, and these two layers are separated by the subarachnoid space which contains circulating CSF 1
The pachymeninges refer to the dura mater alone 4
The leptomeninges derive embryologically from neural crest cells (neuroectodermal origin), while the pachymeninges have a different developmental origin 4
Functional Significance
Cerebrospinal Fluid Compartment
The subarachnoid space between the arachnoid and pia mater normally contains approximately 140 mL of CSF in adults, a volume that does not vary significantly with height or weight 1, 5
Approximately 800 mL of CSF is produced over 24 hours by the choroid plexus, meaning the entire CSF volume is replaced more than 5 times each day 1, 5
CSF flows from the lateral ventricles through the foramina of Monro into the third ventricle, through the aqueduct of Sylvius to the fourth ventricle, then through the foramina of Magendie and Luschka to the base of the brain, traveling caudally to the lumbar sac and rostrally over the cerebral convexities 1, 5
Barrier Functions
The pachymeninges (dura) are located outside the blood-brain barrier, making them extra-axial structures 4
The leptomeninges form part of the blood-CSF barrier system and regulate the movement of pathogens, molecules, and cells into and out of the CSF and brain parenchyma 4, 3
The arachnoid barrier cells are connected by tight and adherens junctions that control permeability 3
Developmental and Homeostatic Roles
The meninges are essential for normal brain development, with alterations in leptomeningeal formation or loss of normal signaling functions contributing to cortical malformations 4, 6
Meninges contribute to neural tissue homeostasis by secreting trophic factors including FGF2 and SDF-1 6
The pia mater is reflected from the brain surface onto arteries and veins, creating perivascular spaces (Virchow-Robin spaces) that play roles in interstitial fluid drainage and inflammatory responses 2
Clinical Relevance
Anatomical Compartmentalization
Irregular trabeculae partition the subarachnoid space throughout its length, and delicate pial blood vessels traverse it 1
The meninges penetrate deeply into CNS structures, projecting between anatomical regions, forming the stroma of the choroid plexus, and creating the perivascular space around every parenchymal vessel 6
Disease Implications
Pachymeningeal metastases arise most commonly from breast and prostate cancers, while leptomeningeal metastases have completely different biology, prognosis, and management 4
Leptomeningeal metastases occur when tumor cells disseminate into and grow within the leptomeninges, with cancer cells accessing this space through hematogenous spread via the choroid plexus, direct seeding from brain metastases, perineural invasion, or retrograde venous extension 1
The most common sites of leptomeningeal tumor involvement are the base of the brain (basilar cisterns or posterior fossa) and the base of the spine (cauda equina), likely due to slow CSF flow in these locations 1