Eur J Anat, 1 (1): 3-11 (1997)

Lamination and visual topography in the lateral geniculate nucleus of normal and achiasmatic dogs

Dale Hogan¹, Preston E. Garraghty² and Robert W. Williams¹

1.-Department of Anatomy and Neurobiology, University of Tennessee School of Medicine, 855 Monroe Avenue, Memphis, Tennessee 38163, USA.2.- Program in Neural Science, Department of Psychology, Indiana University, Bloomington, Indiana 47405, USA

ABSTRACT We have identified an autosomal recessive muta­tion in Belgian sheepdogs that markedly decrea­ ses the number of retinal axons that cross at the optic chiasm. In many mutants, the chiasm is completely eliminated and all retinal axons pro­ject ipsilaterally. We have examined laminar anatomy and visuotopic maps in the lateral genicu­ late nucleus (LGN) of these mutant dogs and compared these features with those of normal dogs. The pattern of lamination in normal dogs is similar to that in cats. In mutant dogs, the size and shape of the LGN is essentially the same as in normal dogs. However, lamination in the A layers is severely perturbed. In the rostromedial part of the LGN, as observed in parasagittal sec­tions, layers A and Al appear to be fused. The A­ Al fusion occurs at the topographic representation of the vertical meridiano The overall visuoto­ pic organization of the LGN is the same in both normal and mutant dogs. Lower fields are loca­ ted rostrolaterally, upper fields caudomedially, and monocular fields are located laterally. In mutants, layer A receives retinotopic input from all of the nasal hemiretina, resulting in an abe­rrant visuotopic map covering the whole ipsilate­ral visual field. As a consequence, the LGN con­ tains mirror-image maps of visual space in adjacent layers. This result is similar in principie, but opposite in polarity, to the mapping errors seen in Siamese cats (a mutation in which the size of the ipsilateral projection is decreased). Thisvisual system mutation demonstrates a robust affinity between nasal and temporal retinal axons and specific LGN layers that drives the establishment of visuotopic maps, even when these axons originate from a single eye. The fusion of layers A and Al indica tes that the complete separation of the A layers requires asynchronous activity from the two eyes.

Keywords: optic chiasm, retinal ganglion cells, retinogeniculate, axon guidance, ON/OFF orga­ nization.