Melanogenesis of avian neural crest cells in vitro is influenced by external cues in the periorbital mesenchyme

Avian melanoblast differentiation was studied by explantation of the neural tube and periorbital mesenchyme. Outgrowths from the mesenchymal explants consisted of a mixed population of melanocytes, melanoblasts and fibroblasts, whilst typical neural crest populations migrated from the neural tube explants. Cells that differentiated within explants of mesenchyme, produced elongate black eumelanosomes of normal ultrastructure which were identical to those found in the ocular connective tissues. However, melanoblasts that differentiated within outgrowths of mesenchyme or neural tube produced round brown melanosomes of highly abnormal ultrastructure. Some of these melanosomes contained a few disorganised melanosomal filaments whilst others had granular melanin with complete absence of filaments. This abnormality of phenotype was invariant over a range of culture conditions that modified cell behaviour, the timing of differentiation and the abundance of the pigmented cells. These experiments suggest that local factors in the mesenchyme are essential for the induction of melanogenesis in the presumptive connective tissue melanocyte.