Hyperpigmentation in the Silkie fowl correlates with abnormal migration offate-restricted melanoblasts and loss of environmental barrier molecules

In most homeothermic vertebrates, pigment cells are confined to the skin. R ecent studies show that the fate-restricted melanoblast (pigment cell precu rsor) is the only neural crest lineage that can exploit the dorsolateral pa th between the ectoderm and somite into the dermis, thereby excluding neuro ns and glial cells from the skin. This does not explain why melanoblasts do not generally migrate ventrally into the region where neurons and glial ce ll derivatives of the neural crest differentiate, or why melanoblasts do no t escape from the dorsolateral path once they have arrived at this destinat ion. To answer these questions we have studied melanogenesis in the Silkie fowl, which is a naturally occurring chicken mutant in which pigment cells occupy most connective tissues, thereby giving them a dramatic blue-black c ast. By using markers for neural crest cells (HNK-1) and melanoblasts (Smyt h line serum), we have documented the development of the Silkie pigment pat tern. The initial dispersal of melanoblasts is the same in the Silkie fowl as in Lightbrown Leghorn (LBL), White Leghorn (WLH), and quail embryos. How ever, by stage 22, when all ventral neural crest cell migration has ceased in the WLH, melanoblasts in the Silkie embryo continue to migrate between t he neural tube and somites to occupy the sclerotome. This late ventral migr ation was confirmed by filling the lumen of the neural tube with DiI at sta ge 19 and observing the embryos at stage 26. No DiI-labeled cells were obse rved in the sclerotome of LBL embryos, whereas in the Silkie embryos DiI-fi lled cells were found as far ventral as the mesentery. In addition to this extensive ventral migration, we also observed considerable migration of mel anoblasts from the distal end of the dorsolateral space into the somatic me soderm (the future parietal peritoneum), and into the more medioventral reg ions where they accumulated around the dorsal aorta and the kidney. The abi lity of melanoblasts in the Silkie embryos to migrate ventrally along the n eural tube and medially from the dorsolateral space is correlated with a la ck of peanut agglutinin (PNA)-binding barrier tissues, which are present in the LBL embryo. The abnormal pattern of melanoblast migration in the Silki e embryo is further exaggerated by the fact that the melanoblasts continue to divide, as evidenced by BrdU incorporation (but the rate of incorporatio n is not greater than seen in the LBL). Results from heterospecific graftin g studies and cell cultures of WLH and Silkie neural crest cells support th e notion that the Silkie phenotype is brought about by an environmental dif ference rather than a neural crest-specific defect. We conclude that melano blasts are normally constrained to migrate only in the dorsolateral path, a nd once in that path they generally do not escape it. We further conclude t hat the barriers that normally restrain melanoblast migration in the chicke n are not present in the Silkie fowl. We are now actively investigating the nature of this barrier molecule to complete our understanding of melanobla st migration and patterning. (C) 2001 Wiley-Liss, Inc.