We analyzed the modifications of the retinal neurons in a heterozygous mutant small eye mouse, the Sey(Dey). This mouse presents a mutation in chromosome 2 which affects the gene Pax6 and other nearby genes, such as the Wt1 gene and the gene of the Reticulocalbin. The eyes of these animals do not have lenses and their retinas present important morphological alterations: in the anterior portion they are joined to the cornea, they are found detached from the pigment epithelium, they present folds that form rosettes in some zones and alteration of the lamination can be observed. The partial loss of the genes affected does not prevent the formation of the different layers of the retina, but does affect its thickness, principally of the plexiform layers; moreover, the internal limiting membrane is found disorganized. All the neuronal populations are present in the retina of these animals and express the same neurochemical markers as the control animals, but the number of Pax6(+) cells is notably reduced. In these retinas a marked disorganization of the distribution of the dendrites and axons is observed and a notable reduction in the axons of ganglion cells. These results suggest that, although it does not appear determinant in the differentiation of the distinct neuronal types of the retina, the partial lack of genes of the heterozygotes +/Sey(Dey) provokes important morphological and neurochemical modifications in the cytoarchitecture of the retina.