RETINAL AXON DIVERGENCE IN THE OPTIC CHIASM - MIDLINE CELLS ARE UNAFFECTED BY THE ALBINO MUTATION

The visual pathway in albino animals is abnormal in that there is a sm aller number of ipsilaterally projecting retinal ganglion cells, There are two possible sites of gene action that could result in such a def ect, The first site is the retina where the amount of pigmentation in the retinal pigment epithelium is correlated with the degree of ipsila teral innervation (LaVail et al, (1978) J, Comp, Neurol. 182, 399-422) , The second site is the optic chiasm, the site of retinal axon diverg ence. We investigated these two possibilities through a combination of in vivo and in vitro techniques, Our results demonstrate that the gro wth patterns of retinal axons and the cellular composition of the opti c chiasm in albino mice are similar to those of normally pigmented mic e, consistent with the albino mutation exerting its effects in the ret ina, and not on the cells from the chiasmatic midline, We directly tes ted whether the albino mutation affects the chiasm by studying 'chimer ic' cultures of retinal explants and chiasm cells isolated from pigmen ted and albino mice, Crossed and uncrossed axons from pigmented or alb ino retinal explants display the same amount of differential growth wh en grown on either pigmented or albino chiasm cells, demonstrating tha t the albino mutation does not disrupt the signals for retinal axon di vergence associated with the albino optic chiasm, Furthermore, in vitr o, a greater proportion of albino retinal ganglion cells from ventrote mporal retina, origin of uncrossed axons, behave like crossed cells, s uggesting that the albino mutation acts by respecifying the numbers of retinal ganglion cells that cross the chiasmatic midline.