Optic Nerve

The optic stalk is the initially hollow structure connecting the optic vesicle with the forebrain. At approximately 6 weeks gestation, axons from developing ganglion cells pass through vacuolated cells from the inner wall of the optic stalk. Emanating from the center of the primitive nerve is the hyaloid artery. A glial sheath forms around the hyaloid artery. As the hyaloid artery regresses, a space between the hyaloid artery and the glial sheath enlarges. Bergmeister's papilla represents a remnant of these glial cells around the hyaloid artery. The extent of the Bergmeister's papilla is dependent on the amount of hyaloid and glial cell regression. Glial cells in this area migrate into the optic nerve and form the primitive optic disc. The glial cells around the optic nerve and the glial part of the lamina cribrosa come from the inner layer of the optic stalk, which is of neural ectoderm origin. Later, there is development of a mesenchymal (neural crest cell) portion of the lamina cribrosa. By the third month, the optic nerve shifts nasally as the temporal aspect of the posterior pole enlarges. The tissue of Kuhnt, which circum-ferentially surrounds the intraocular part of the optic nerve and acts as a barrier between the optic nerve and retina, comes from glial tissue in the region of the disc and mesenchyme from nearby developing retinal vasculature. Myelinization of the optic nerve starts at the chiasm at about 7 months gestation and progresses toward the eye. Normally, myelinization stops at the lamina cribrosa at about 1 month after birth. At birth, the myelin is thin, with the layers of myelin increasing into late childhood.

Myelinated nerve fibers occur if the myelinization continues past the lamina cribrosa (Fig. 1-20). The best explanation as to why myelinization passes the lamina cribrosa is the presence of heterotopic oligodendrocytes or glial cells within the retinal nerve fiber layer. This concept contrasts with the theory that there is a congenital defect in the lamina cribrosa that allows myelinization to progress into the retina. Autopsy studies of myelinated nerve fibers have failed to show a defect in the lamina cribrosa; therefore, myelinated nerve fibers most probably represent ectopic myelinization.100,101 Myelinization of the nerve fibers is often associated with high myopia and ambly-opia. Patients with this disorder should be aggressively treated by correcting the refractive error and initiating occlusion

FIGURE 1-20. Photograph of myelinated nerve fibers emanating from the disc. Myelinated nerve fibers will cause a local scotoma; however, in the macular and foveal region, their presence does not usually preclude good central vision.

therapy of the sound eye to treat the amblyopia, as good visual outcome can be achieved even when there is macular involvement.103

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