What Are Leptomeninges?
The leptomeninges are the two innermost layers of the meninges—specifically the arachnoid mater and pia mater—that directly surround the brain and spinal cord and contain the cerebrospinal fluid (CSF) in the subarachnoid space between them. 1
Anatomical Structure
The meninges consist of three layers covering the central nervous system (CNS): 1
- Dura mater (pachymeninx): The outermost fibrous layer that also serves as the inner periosteum of the skull 2
- Arachnoid mater: The middle layer, part of the leptomeninges 1, 3
- Pia mater: The innermost layer, part of the leptomeninges, directly adherent to the brain and spinal cord surface 1, 3
The Subarachnoid Space
The CSF is contained in the subarachnoid space, which separates the arachnoid and pia mater. 1 This space normally contains approximately 140 mL of CSF in adults, a volume that remains relatively constant regardless of height or weight. 1, 4
The subarachnoid space is compartmentalized by irregular trabeculae throughout its length, and delicate pial blood vessels traverse it. 1
Embryological Origin
The leptomeninges derive from the neural crest, specifically from mesenchymal cells of neuroectodermal origin, developing as structures intimately related to the CNS from early developmental stages. 5 Their formation is closely linked with neuronal migration and cortical development. 5
Functional Characteristics
The leptomeninges are anatomically and functionally distinct from the pachymeninges (dura mater) in critical ways: 1
- Blood-CSF barrier location: The leptomeninges form part of the subarachnoid space within the blood-CSF barrier system, while the pachymeninges are outside the blood-brain barriers and therefore extra-axial 1, 5
- Cellular structure: Leptomeningeal cells line CSF spaces and are connected by tight junctions (in the arachnoid barrier) and desmosomes/gap junctions elsewhere 2, 6
- Barrier function: The arachnoid barrier cells are epithelial-like with tight and adherens junctions that regulate movement of pathogens, molecules, and cells into and out of the CSF and brain parenchyma 7
Clinical Relevance
The distinction between leptomeningeal and pachymeningeal disease is critical because they have completely different biology, prognosis, blood supply, and management approaches. 1, 5
Leptomeningeal metastases characteristics:
- Tumor cells disseminate into and grow within the leptomeninges, with cancer cells freely circulating throughout the neuraxis driven by pulsatile CSF flow 1
- Cells exist as adherent plaques on brain/spinal cord surfaces or as free-floating cells/clusters 1
- Common primary cancers include breast (especially infiltrating lobular subtype), lung (particularly small cell), melanoma, and gastrointestinal malignancies 1
Pachymeningeal metastases characteristics:
- Most commonly arise from breast and prostate cancers 1, 5
- Are extra-axial and have different therapeutic implications due to location outside blood-brain barriers 1, 5
CSF Dynamics Within the Leptomeningeal Space
CSF is produced by the choroid plexus at approximately 500-800 mL per day, meaning the entire CSF volume is replaced 4-5 times daily. 1, 4 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