Cyclopia: histogenesis of the single optic nerve

We report a case of cyclopia with alobar holoprosencephaly in a fetus examined at 18 weeks of gestation. There was a single eyelid opening and two small eyeballs showing partial fusion at the posterior portion within a single orbit. A single optic nerve emerged from the midline of the posterior portion of the eyeballs. Histologically, within the nerve, a highly cellular central zone and a less cellular marginal zone were distinguishable. The former was composed mainly of glial cells, and only the latter contained argyrophilic axons. Immunohistochemical expression of tau, a microtubule-associated protein expressed in outgrowing axons, was demonstrated in the marginal zone. The central zone showed staining for glial fibrillary acidic protein. These findings indicate that the axon fibers from each eyeball coursed, respectively, along the unilateral marginal zone of the single nerve, and that most neuroepithelial cells in the central zone developed into astrocytes in the absence of axons.