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Abstract:
The skin of the white mutant axolotl larva is pigmented differently from that of the normal dark due to a local inability of the extracellular matrix (ECM) to support subepidermal migration of neural crest-derived pigment cell precursors. In the present study, we have compared the ECM of neural crest migratory pathways of normal dark and white mutant embryos ultrastructurally, immunohistochemically and biochemically to disclose differences in their structure/composition that could be responsible for the restriction of subepidermal neural crest cell migration in the white mutant axolotl. When examined by electron microscopy, in conjunction with computerized image analysis, the structural assembly of interstitial and basement membrane ECMs of the two embryos was found to be largely comparable. At stages of initial neural crest cell migration, however, fixation of the subepidermal ECM in situ with either Karnovsky-ruthenium red or with periodate-lysine-paraformaldehyde followed by ruthenium red-containing fixatives, revealed that fibrils of the dark matrix were significantly more abundant in associated electron-dense granules. This ultrastructural discrepancy of the white axolotl ECM was specific for the subepidermal region and suggested an abnormal proteoglycan distribution. Dark and white matrices of the medioventral migratory route of neural crest cells had a comparable appearance but differed from the corresponding subepidermal ECMs. Immunohistochemistry revealed only minor differences in the distribution of fibronectin, laminin, collagen types I, and IV, whereas collagen type III appeared differentially distributed in the two embryos. Chondroitin- and chondroitin-6-sulfate-rich proteoglycans were more prevalent in the white mutant embryo than in the dark, especially in the subepidermal space. Membrane microcarriers were utilized to explant site-specifically native ECM for biochemical analysis. Two-dimensional gel electrophoresis of these regional matrices revealed a number of differences in their protein content, principally in constituents of apparent molecular masses of 30-90,000. Taken together our observations suggest that local divergences in the concentration/assembly of low and high molecular mass proteins and proteoglycans of the ECM encountered by the moving neural crest cells account for their disparate migratory behavior in the white mutant axolotl.
