What are the meninges?

The Meninges: Structure and Function

The meninges are a three-layered protective membrane system that covers and protects the brain and spinal cord, consisting of the dura mater (outermost), arachnoid mater (middle), and pia mater (innermost layer). 1

Anatomical Structure

The meninges comprise three distinct layers:

1. Dura Mater (outermost layer):

   • Tough, fibrous layer closest to the skull
   • Composed predominantly of fibroblasts
   • Contains fenestrated blood vessels and an elaborate lymphatic system
   • Hosts a diverse immune cell population distinct from the CNS 2, 3
   • Forms two layers: an outer endosteal layer related to the skull bones and an inner layer applied to the arachnoid 4

2. Arachnoid Mater (middle layer):

   • Contains epithelial-like barrier cells connected by tight junctions
   • Forms the blood-CSF barrier that regulates movement of pathogens, molecules, and cells
   • The outer parietal layer is impermeable to CSF due to tight intercellular junctions
   • Connected to the pia mater by arachnoid trabeculae that span the subarachnoid space 2, 4

3. Pia Mater (innermost layer):

   • Delicate layer directly adherent to the brain and spinal cord surface
   • Collagen and basement membrane-rich
   • Reflects onto blood vessels entering the brain
   • Shows regional heterogeneity across different brain regions 2, 4

Between the arachnoid and pia mater lies the subarachnoid space, which contains cerebrospinal fluid (CSF). In adults, this space normally contains approximately 140 mL of CSF, which is completely replaced more than 5 times daily (approximately 800 mL produced over 24 hours). 1

Recent research suggests a possible fourth meningeal membrane called the Subarachnoid Lymphatic-like Membrane (SLYM), which may subdivide the subarachnoid space and facilitate CSF transport. 5

Functional Roles

The meninges serve multiple critical functions:

1. Physical protection of the brain and spinal cord from trauma

2. CSF circulation and homeostasis:

   • CSF is produced by the choroid plexus
   • Flows through the ventricular system and circulates in the subarachnoid space
   • Flows from lateral ventricles through foramina of Monro → third ventricle → aqueduct of Sylvius → fourth ventricle → foramina of Magendie and Luschka → subarachnoid space 1

3. Immune surveillance and protection:

   • The dura mater hosts a rich immune landscape that protects against invading pathogens
   • Leptomeningeal cells (arachnoid and pia) can secrete cytokines during infection 4, 3

4. Interface between CNS and periphery:

   • Essential for brain development and homeostasis
   • Provides pathways for drainage of interstitial fluid 2, 4

5. CSF drainage:

   • Specialized leptomeningeal cells form channels in arachnoid granulations for CSF drainage into venous sinuses 4

Clinical Significance

The meninges are involved in various pathological conditions:

1. Leptomeningeal metastases:

   • Cancer cells can gain access to the CSF and leptomeninges through:
     • Hematogenous spread through arachnoid vessels
     • Direct invasion through choroid plexus
     • Extension from other metastases
     • Tracking along peripheral nerves
     • Surgical spillage during tumor resection 1
   • Most common in breast cancer, lung cancer, melanoma, and gastrointestinal cancers 6
   • Poor prognosis with median survival of 2-4 months without treatment 6

2. Meningiomas:

   • Arise from arachnoid cells in the meninges
   • Can develop in any location where meninges exist 1
   • Most pediatric meningiomas (90%) are supratentorial or intraventricular 1

3. Meningitis:

   • Inflammation of the meninges, typically due to infection
   • Leptomeningeal cells secrete cytokines during bacterial meningitis 4

4. Neurodegenerative diseases:

   • Meningeal pathways for interstitial fluid drainage appear to be blocked by amyloid-beta in Alzheimer's disease 4

Recent Advances in Meningeal Biology

Recent research has revealed that each meningeal layer contains molecularly distinct types of fibroblasts, and the traditional three-layer model may be more complex than previously thought. The emerging view of the meninges is as an essential interface between the CNS and periphery, critical to brain development and homeostasis. 7, 2

Understanding meningeal structure and function is crucial for comprehending various neurological conditions and developing targeted therapeutic approaches for diseases affecting this important protective system.