Walsh & Hoyt: Optic Nerve Sheaths

The leptomeninges that surround the optic nerve the dura mater, arachnoid, and pia mater are continuous with the leptomeninges of the brain. Embryologically, the meninges develop from different sources: the dura from mesenchyme, and the arachnoid and pia largely from ectoderm. The dura mater near the globe splays out slightly and fuses with the sclera surrounding the optic nerve. At the apex of the orbit, the dura fuses with the periosteum and annulus of Zinn, then passes through the foramen, where it merges with the intracranial dura that lines the base of the skull. Thus, the intracranial portion of the optic nerve is surrounded only by pia mater. Normal human optic nerves have a sparse population of mast cells in their dura mater (much less so in the arachnoid). These cells become more numerous and can be found in the optic nerve parenchyma in injured eyes, although it is not known whether they can incite injury to the optic nerve or are merely epiphenomenal to the primary disease.