Melanotic differentiation of the avian neural crest

Abstract

This thesis is concerned with melanocyte differentiation of the neural crest cells in the embryonic Brown Leghorn fowl (Gallus gallusl Melanocyte differentiation in vivo was compared with that occurring in three experimental systems in vitro . Premigratory neural crest cells were isolated by the neural tube explanation technique of Cohen and Konisberg (1975). Differentiation of these isolated cells was compared with that which occurred in the postmigratory crest cells of the periorbital mesenchyme and corneal stroma.Examination of melanocyte shape in vivo demonstrated that cell shape is extrinsically determined by features of the local micro-environment contrary to what has been reported in the mouse tissue. Observations of melanocyte shape change in vitro indicated that the environmental control of shape occurs by modulation of locomotory activity.Melanogenesis within isolated cells was found to be different from that occurring in mesenchymal explants, or connective tissues and ocular epithelia in vivo. The isolated melanoblasts produced an ultrastructurally abnormal brown melanosome with an altered elemental composition, whereas those cells which differentiated within ocular tissue or mesenchymal explants produced black eumelanosomes.The group of cells which colonise the corneal stroma are not totally committed to their fate as fibroblasts at the time of migration from the orbital mesenchyme. Some of the migratory cells retain the ability to become melanocytes, although this is rapidly lost after stromal colonisation.Finally, melanocyte and fibroblast differentiationare presented as alternatives within the pigmented mesenchyme and attention is drawn to the fact that the abnormally pigmented crest cells in vitro resemble human malignant melanoma cells.