Histological examination of chick embryos with ocular hypoplasia.

From so-called "dead-in-pipped-eggs" provided by the Poultry Breeding Center of Gifu Prefecture, we obtained two chick embryos with hypoplastic eyes on external examination, and studied the visual system and cranial nerve nuclei innervating the extraocular muscles. The upper bills of both embryos deviated toward one side when observed with the naked eye. In histological sections from the most severely affected eyeball of each case primordial retinal tissue in the poorly developed orbit seemed to be at the stage of optic vesicle formation; it had not differentiated into individual layers. The optic nerve was absent from these eyeballs and the neural retina had multiple folds. The ciliary ganglion was smaller on the more severely affected side than on the contralateral side which had only mild hypoplasia. The optic nerve, which was present on the contralateral side, bifurcated at the level of the chiasma close to the diencephalic base; one branch ran toward the ipsilateral optic tectum and the other toward the contralateral brain region. The oculomotor and trochlear nerve nuclei could be identified bilaterally in one of the embryonic brains but not in the other. These findings suggest that hypoplasia of the neural retina and optic nerve in an orbit leads to bifurcation of the contralateral optic nerve which, under normal conditions, should completely cross at the chiasma. This may also suggest that retinal afferents from the brain are also affected in the hypoplastic eyeball.