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Melanosome abnormalities of ocular pigmented epithelial cells in beagle dogs with hereditary tapetal degeneration.

Eyes of laboratory beagle dogs with an inherited tapetal degeneration were abnormally lightly pigmented. The development of pigmentation was followed morphologically from 7 days postnatal to 9 years of age. At all postnatal ages the iris pigmented epithelia contained no normal melanosomes, only organelles resembling secondary lysosomes or residual bodies. The ciliary body pigmented epithelium contained a variety of melanosome organelles at the earliest stages examined, but in fewer numbers than in normal animals. These included premelanosomes, partially melanized and some fully melanized pigment granules. However, the melanin deposition was usually patchy and irregular. With time, many of these granules appeared to condense into residual bodies. The retinal pigmented epithelium in peripheral and inferior posterior regions of affected animals never contained normal appearing melanin granules at any stage of postnatal development. The iris and choroidal stroma had melanosomes of normal size and shape, but many fewer than in normal animals. These results imply that there is local cellular control over melanosome production and regression, since the melanosome abnormalities do not follow the anterior to posterior development of pigment in ocular epithelia. It is proposed that a defect in synthesis of the matrix component of melanosomes could result in absent or abnormal deposition of melanin and initiate a process of autophagy of these organelles.