The lateral geniculate nucleus in normal and naturally-strabismic monkeys: neuronal density and laminar area

Purpose. Our goal was to determine if the retinogeniculate pathways in monkeys with natural infantile esotropia differ from those in normal monkeys. It is important to rule out abnormalities of the optic chiasm and lateral geniculate nucleus (LGN) as part of the search for mechanisms that might explain infantile esotropia. [Methods. LGNs were removed from 4 normal and 3 naturally-strabismic monkeys and sectioned in the coronal plane. Serial 60 \Lm thick slices were Nissl and CytOx stained and analyzed at high magnification. Neuronal soma were counted using stereological methods in midline, medial and lateral LGN regions within the magnocellular and parvocellular layers. The overall area and configuration of each layer was also analyzed. Mean densities were compared using the t-test with significance defined as p < .05. Results. A nasotemporal asymmetry of neuronal density was present in each animal, with densities in layers corresponding to the contralateral eye (i.e. nasal retina, layers 1, 4 and 6) higher than those corresponding to the ipsilateral eye (i.e. temporal retina, layers 2, 3 and 5). Strabismic monkeys did not have evidence of an albino abnormality, i.e. the nasotemporal asymmetry was of comparable magnitude in strabismic and normal monkey and there was no evidence of laminar fusion. Parvocellular layers had higher neuronal densities than magnocellular layers in normal and strabismic animals. As has been reported in normal humans, densities in LGN regions corresponding to the inferior visual field exceeded those in regions corresponding to the superior visual field. Conclusions. Macaque monkeys who have infantile esotropia have nasotemporal asymmetries in LGN neuron density and laminar area comparable to those found in normal animals. The esotropia and defects of binocular vision in these animals are not due to an albino-like misroutine in which too many fibers project from each eye to the contralateral LGN.