Slate (slt) is an autosomal recessive mutation which affects the pigment granule attributes, size, shape and number, in the cortex and medulla of the hair, in the epithelial melanocytes of the eye, and in the neural crest-derived melanocytes of the uveal tract and Harderian gland. Pigment granules are reduced in number in all of the pigmented sites, they are unusually large, and they are quite irregular in shape. In addition, there is a tendency of slt granules to clump together in the medulla of the hair.

Studies of pigment granule genesis in the epithelial, uveal and Harderian gland melanocytes indicate that the reduced number of granules found in the presence of slt is due both to the synthesis of fewer melanin granules, and to the fusion of individual granules into progressively larger pigmented bodies. Granule size thus increases at the expense of granule number. Since granule shape becomes more irregular as granule size increases, it appears that the effects of slt on pigment granule size, numberand shape are inter-related.