Melanocytes in the modiolus of the guinea pig cochlea.

The cochleae of pigmented and albinotic guinea pigs aged 6-8 weeks were examined by electron microscopy for melanocytes in the modiolus. Two different types of melanocyte were observed: i) The first (Type A) was characterized by spherical, uniformly pigmented stage IV melanosomes. No signs of melanogenesis were seen in this perivascular pigment cell form, which is found mainly in the region of the cochlear plexus. ii) The second, very rarely observed type of melanocyte (Type B) was characterized by fusiform to oval melanosomes in various stages of maturation. Connective tissue cells containing fusiform to oval melanosomes enclosed in membrane-bound vacuoles were frequently found. These two types can be regarded as corresponding to uveal (Type A) and dermal (Type B) melanocytes. The connective tissue of the cochlear plexus seems to be derived from the leptomeninx.