Neural-tube-derived melanocyte subsets undergo commitment to their distinct lineages in culture

Neural-crest-derived melanocytes populate two anatomical sites in the chicken, the epidermis of regenerating feathers and the uveal tract of the eyes. These two anatomical populations of melanocytes differ morphologically and functionally. Morphologically, feather and uveal melanocytes synthesize structurally different pigment granules (melanosomes). Feather melanosomes are rod-shaped, 0.2 x 0.8 micron, whereas uveal melanosomes are larger and more oval, 0.6 x 0.9 micron. Functionally, feather melanocytes continuously synthesize melanosomes during feather regeneration, and transfer these melanosomes to neighboring keratinocytes. Ocular melanocytes, on the other hand, synthesize melanosomes until their cytoplasm becomes congested with melanosomes, at which time the melanocytes become melanogenically dormant and do not transfer granules to neighboring cells. Cultures of melanocytes established from neural tubes of Light Brown Leghorn chick embryos produce two populations of melanocytes containing small (0.45 micron) or larger (0.90 micron) melanosomes which resemble the two types described in situ. Both types of melanocytes emigrate from along the entire length of the neural tube during several embryonic stages. Melanocyte cultures developed from neural tubes of the Recessive White breed of chicken, which has tyrosinase-negative, feather melanocytes and pigmented, functionally normal uveal melanocytes, also develop a mixture of amelanotic and pigmented melanocytes which maintain their respective characteristics even after separation by flow cytometry and reculture. These findings suggest that epidermal and uveal melanocytes are two distinct sub-populations of melanocytes whose commitment to separate lineages can occur in culture in the absence of their respective target tissue environment.