The slaty (Dctslt) mutation is known to reduce the activity of dopachrome tautomerase, which converts dopachrome to 5,6-dihydroxyindole-2-carboxylic acid in the pathway of eumelanin synthesis and to inhibit melanosome maturation in melanocytes. However, it is not known whether the inhibition of melanosome maturation in slaty melanocytes is developmentally regulated. To address this point, changes in the morphology and maturation of melanosomes in cultured epidermal melanocytes derived from newborn mice of wild-type (black) and slaty mutant were surveyed under the electron microscope. In black melanocytes (Dct+), almost all melanosomes were elliptical stage IV melanosomes. However, in slaty melanocytes, numerous spherical stage III melanosomes with globular depositions of pigment in addition to elliptical stage III melanosomes with intraluminal fibrils were observed. Mixed-type melanosomes containing both globular deposition and intraluminal fibrils of pigment were also observed. In slaty melanocytes, spherical and mixed-type melanosomes were gradually decreased after birth, whereas elliptical melanosomes were gradually increased. Stage IV melanosomes were very few in slaty melanocytes, and the number did not increase after birth. These results suggest that the slaty mutation blocks the melanosome maturation at stage III and affects the melanosome morphology (elliptical or spherical) in a developmental stage-specific manner. Anat Rec, 2007. © 2007 Wiley-Liss, Inc.