Complicated colobomatous microphthalmos in the BW rat: a new form of inherited retinal degeneration

A new model of inherited retinal degeneration has been found in the rat. It is inherited in association with a number of other ocular defects, including microphthalmos, coloboma, retinal dysplasia, optic nerve hypoplasia and/or aplasia, as well as medullation of the nerve fiber layer of the retina. Together, these abnormalities constitute a condition referred to as complicated colobomatous microphthalmos. This condition was originally discovered in the Bmn strain of rats but subsequently transferred to a new genetic background in the Bmn-wys strain of rats (BW). This facilitated the histological evaluation of both the developmental and degenerative ocular defects in the adult animals. A well defined pattern emerged relating eye size, optic nerve size and retinal histology. Normal-sized eyes had normal-sized optic nerves and normal retinal histology while intermediate-sized eyes with no optic nerves had uniformly thin retinas. In contrast, intermediate-sized eyes with small optic nerves had areas of both normal thickness and thin retina. All of these eyes developed retinal degeneration characterized by a late onset and slow progression associated with normal phagocytic activity in the pigment epithelium and a tendency for the rod outer segments to fragment into very thin structures rather than accumulate as lamellar debris. This indicates that the retinal degeneration in the BW model differs in many respects from the well studied RCS model.