Abnormal axons in the albino optic tract.

Purpose: There have been suggestions that the misrouting of axons at the optic chiasm leading to the abnormal ratio of crossed to uncrossed axons in the optic tract of albinos is a consequence of abnormal timing of ganglion cell axon outgrowth. The sequence of genesis of ganglion cell classes and their axon outgrowth is correlated with the deep-to-superficial distribution of their axons by size in the mammalian optic tract. Optic tract axon order in albino and normally pigmented ferrets was, therefore, examined to determine whether an abnormal pattern of retinal ganglion cell genesis and axon outgrowth is evident in albinos.

Methods: Light and electron microscopy were used to study axon diameters and myelin thickness of axons in the optic tracts of adult albino and pigmented ferrets.

Results: In the optic tract, large-diameter axons are confined superficially in the normally pigmented ferret but are present throughout its depth in albinos. The abnormally located large axons and neighboring small-diameter axons in the albino have an abnormal axon diameter/myelin thickness ratio; large-diameter axons are poorly myelinated, and small-diameter axons exhibit an abnormally thick myelin sheath. These deep abnormal axons originate from the contralateral retina.

Conclusions: In addition to the known disruptions of normal organization in the visual system, albinos have an abnormal axon diameter distribution and a population of morphologically abnormal axons in the optic tract. These abnormal axons may represent the population of aberrantly crossed axons found in all albino mammals.