General
- Development of the meninges starts early in gestation and reaches the basic adult forms by the end of the first trimester.
Meningeal precursors
- Derived from both neural crest and mesodermal cells.
22 to 24 days of gestation
- Neural tube fuses
- A single layer of cells, with some attachments to the neural crest, surrounds the developing neural axis.
24 to 28 days of gestation
- A thicker, looser collection of mesenchymal cells further covers the neural tube
33 to 41 days of gestation
- Meninges completely envelops the developing spinal cord and brain
- This mesodermal-derived cellular network, along with the neural crest–derived monocellular layer will differentiate into the meninx primitiva (primary meninx).
34 to 48 days of gestation
- As the pluripotent meninx primitiva develops, it subdivides into two distinct layers
- Outer portion: Ectomeninx
- Is dense and compact
- Is the precursor to the dura and the bones of the neurocranium, thus the close apposition of dura and skull stems from their shared embryological ancestry.
- Inner layer: Endomeninx
- Is more loosely arranged.
45 to 55 days of gestation
- The inner portion of the endomeninx, containing the neural crest–derived cells covering the neural tube, begins to form the pia during the gestation interval
55 days of gestation
- Cerebrospinal fluid invades the endomeninx, cavitations (future cisterns) begin to appear in the outer portion of the endomeninx and become obvious
- Although the dura and pia are distinguishably formed structures by this point of development, a distinct arachnoid layer is not obvious and may not appear until much later during fetal development.
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