Retinal degeneration occurs spontaneously at high incidence in both an inbred and a noninbred colony of Osborne-Mendel rats. On the basis of histopathologic examination, the degeneration appears to have its onset relatively late in life, at about the age of 9 months, and increases in severity, subsequently. The process begins with focal degeneration of photoreceptors in the postequatorial retina and results in total loss of receptor cells followed by disruptive involvement of the inner retinal layers.
von Sallmann L, Grimes P. Spontaneous Retinal Degeneration in Mature Osborne-Mendel Rats. Arch Ophthalmol. 1972;88(4):404–411. doi:10.1001/archopht.1972.01000030406011