In the mammalian visual system, retinal ganglion cells send their axons to several distinct structures in the brain. One of these, the lateral geniculate nucleus (LGN), receives a substantial projection from both eyes in higher mammals such as the cat1-3. How does this pattern of input arise during development? One possibility is that retinal ganglion cell axons project from the very beginning to the appropriate side of the brain. Another is that the initial projection may contain many inappropriate connections which are eliminated after the axons reach their target, the LGN. We thought the Siamese cat might provide an opportunity to study these alternatives. Guillery and others4-7 have shown that in adult Siamese cats, the LGN receives a disproportionately large input from the contralateral eye and a correspondingly diminished input from the ipsilateral eye due to a genetic mutation at the albino locus. We report here that this abnormal pattern of input is present from the earliest stages of development of the retinogeniculate projection and arises as a consequence of the misrouting of fibres at the optic chiasm. These findings suggest that during normal development, growing optic fibres can be directed to the appropriate side of the brain.
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