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Retinal projections to the thalamus in albino frogs
Author(s) -
Grant S.,
Waller W.,
Bhalla A.,
Kennard C.
Publication year - 2002
Publication title -
ophthalmic and physiological optics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.147
H-Index - 66
eISSN - 1475-1313
pISSN - 0275-5408
DOI - 10.1046/j.1475-1313.2002.00086_34.x
Subject(s) - biology , retina , anatomy , neuroscience , retinal , decussation , diencephalon , geniculate , neuropil , optic nerve , optic chiasm , thalamus , retinal pigment epithelium , visual system , central nervous system , biochemistry , nucleus
Melanin in the embryonic retinal pigment epithelium (RPE) is essential for normal development of the mammalian visual system: in albino and other mutant mammals lacking this pigment, many temporal retinal ganglion cells (RGCs) that usually project ipsilaterally misroute their axons across the optic chiasm to the opposite side of the brain. In human albinos, this misrouting results in higher visual centres receiving predominantly monocular input from the opposite eye and in defective binocular stereovision. The RPE–RGC signalling mechanisms that govern the chiasmatic pathway choices of ganglion cell axons are unknown, but may involve gap junctions, which are a conserved feature of the embryonic retina in all vertebrate classes. To determine whether chiasmatic abnormalities also generalize to non‐mammalian pigment mutants, we compared the distribution of crossed and uncrossed retinal projections to the thalamus in normal and albino Xenopus frogs. In both phenotypes, retinal terminations from the opposite eye covered ∼80% of the neuropil of Bellonci and corpus geniculatum thalamicum (analogous to the mammalian lateral geniculate nuclei), with uncrossed inputs occupying ∼75 and 25%, respectively, of the two terminal fields. Ipsilateral projections originated from ∼3000 RGCs in the wild‐type and albino frogs, and in both phenotypes they were restricted to a ventrotemporal crescent comprising ∼40% of the total retinal area. In sum, we found no evidence of chiasmatic misrouting in albino Xenopus , despite the fact that this species normally possesses retinal projections with a substantial ipsilateral component. Our findings support other recent evidence that a lack of RPE melanin leads to defective development of the retina and its central pathways only in mammals.