Ocular malformations of Xenopus laevis exposed to nickel during embryogenesis

The pathogenesis of eye anomalies induced by exposure to Ni2+ (as nickel chloride) during embryogenesis was studied in the frog, Xenopus laevis. Eyes of control and Ni(2+)-exposed tadpoles were examined without staining using a dissecting microscope, by light microscopy of histological sections, and by electron microscopy. The ocular abnormalities of Ni(2+)-exposed tadpoles included (a) microphthalmia, (b) hypopigmentation, (c) hernias and cysts of the choroid and retina, and (d) iris coloboma; cataracts were uncommon. The pathogenesis of the ocular lesions appears to involve diffuse or focal dysplasia and loss of the retinal pigment epithelium, with dystrophy of photoreceptor outer segments and protrusion of neuroretina through gaps in the pigment epithelium. This study confirms that Ni2+ is a potent ocular teratogen for Xenopus embryos and points to the retinal pigment epithelium as a primary cellular target for Ni(2+)-induced embryotoxicity.