The distribution of melanin in the developing optic cup and stalk and its relation to cellular degeneration.

The early distribution of melanin in the developing optic cup and stalk and its relationship to cellular degeneration have been studied during intrauterine development in hamsters, mice, ferrets, and humans. The extensive degenerative changes that occur in the region of the optic fissure do not involve any pigment-bearing cells. Melanin is not formed at the site of the fissure until after the processes of fusion and the associated degenerative changes are complete. In contrast to this, there is a small region near the junction of the cup and stalk where melanin is particularly related to degenerative changes. Melanosomes form dense clumps associated with basophilic, apparently degenerate areas of cytoplasm, and these melanosome complexes themselves undergo further degenerative changes. The two types of degenerative changes, pigmented and unpigmented, are limited to the same two regions of the developing eye in all four species. However, the pigment degeneration is more extensive in ferrets and humans than in mice and hamsters. These observations raise the possibility that the lysis of melanosomes plays a significant but undefined local role in ocular development.