Aberrant axon growth of the chick embryo retina during normal development

Axon growth behavior in the optic nerve was examined using a carbocyanine dye, DiI, as a tracer, DiI facilitated clear visualization of the whole growth pattern of the optic nerve, i.e. the initial association of axons, fasciculated growth within the optic fiber layer and flattened growth cones in both living and fixed chick embryo retinae. Retrograde labelling with DiI in fixed retinae revealed that a considerable number of ganglion cells were apparently misdirected, extending their axons toward the periphery of the retina during normal development. The maximum proportion of aberrant ganglion cells reached about 15% of the total upon staining with a single DiI crystal. Misdirection was predominantly observed in retinae prepared from 6- to 8-day-old chick embryos. In embryos more than 9 days old, however, distinction of aberrant ganglion cells from normal ones became difficult, so that any degeneration of misdirected ganglion cells could not be clarified. Almost all of the misdirected ganglion cells were oriented centrifugally to the retinal periphery. These results indicate that misdirection occurs spontaneously during normal development even within the retina.