Ocular pathology in mice with a transgenic insertion at the microphthalmia locus

A DNA insertional mutation at the microphthalmia locus (mi) in a transgenic mouse has been developed and given the allele symbol of mitg (Krakowsky et al., 1993). Mice homozygous for this transgene have eyes markedly reduced in size and relatively unpigmented. In this study, we examined the morphology of these eyes using light and electron microscopy. Transgenic homozygous (mitg/mitg) animals have a structurally normal choroid which lacks melanocytes but contains occasional leukocytes. The elastica of Bruch's membrane is absent except in an occasional site. The retinal pigmented epithelium (RPE) appears dramatically abnormal. It displays cellular heterogeneity, residual basal infoldings and apical microvilli, rare and immature melanosomes, and numerous cilia-like structures. Occasionally, cells of the RPE appear to have extruded into or from the choroid. The photoreceptor cells are devoid completely of outer segments and partially of inner segments. Numerous active macrophages are present between the amelanotic RPE and neuro-retina and also within the vitreous body. The anterior uveal tract is underdeveloped and hypomelanotic. This new microphthalmia model exhibits ocular pathology with similarities and differences to other mutations and the mi (microphthalmia) locus.