Meninges

The meninges appear during early development and consist of the pachymeninges or dura mater, and the leptomeninges, which is made up of the arachnoid mater and pia mater; all components show considerable structural and functional heterogeneity. In the trunk and caudal regions of the head, the meninges originate from paraxial mesenchyme, while the meninges in the skull anterior to the mid-brain are derived from cranial neural crest cells (arising from the posterior diencephalic neural crest cells). The pia mater on the surface of the spinal cord and brain is seen already on day 24 of embryonic development, while the dura mater can be seen in the basal areas by day 41. The arachnoid mater becomes separated from the dura mater by day 57. The meninges develop from two condensations which appear in the head region: the outer condensation gives rise to the skeletogenous layer, while the inner condensation (dural- limiting layer) gives rise to the pia and arachnoid. These two mesenchymal condensations subsequently differentiate into two different tissue components: the internally located connective tissue (dense in the dura mater and loose in the leptomeninges) and an epithelial component, on the surface of the meninges. The dura mater is a tough, thick sheet of dense connective tissue, containing venous sinuses, while the arachnoid is a delicate connective tissue sheath attached to the inner surface of the dura mater. The subarachnoid space is located between the arachnoid and the pia mater and contains delicate trabeculae and cerebrospinal fluid (CSF). Meningeal cells are involved in the control of development of the cerebellum, by influencing both neuronal migration and radial glia cell differentiation via different cellular interactions. The innermost layer (pia), produces the basement membrane (BM) covering the cortex and serves as the source for blood vessels in the superficial region of the cerebral cortex. The middle layer (arachnoid) plays an important role in the resorption of CSF, whereas the outermost layer (dura) is a collagenous structure that is tightly attached to the calvarium (skull) and has an important role in its development.