THE DELAYED ENTRY OF THORACIC NEURAL CREST CELLS INTO THE DORSOLATERAL PATH IS A CONSEQUENCE OF THE LATE EMIGRATION OF MELANOGENIC NEURAL CREST CELLS FROM THE NEURAL-TUBE

Neural crest cells migrate along two pathways in the trunk: the ventra l path, between the neural tube and somite, and the dorsolateral path, between the somite and overlying ectoderm. In avian embryos, ventral migration precedes dorsolateral migration by nearly 24 h, and the onse t of dorsolateral migration coincides with the cessation of ventral mi gration. Neural crest cells in the ventral path differentiate predomin antly as neurons and glial cells of the peripheral nervous system, whe reas those in the dorsolateral path give rise to the melanocytes of th e skin. Thus, early- and late-migrating neural crest cells exhibit uni que morphogenetic behaviors and give rise to different subsets of neur al crest derivatives. Here we present evidence that these differences reflect the appearance of specified melanocyte precursors, or melanobl asts, from late- but not early-migrating neural crest cells. We demons trate that serum from Smyth line (SL) chickens specifically immunolabe ls melanocyte precursors, or melanoblasts. Using SL serum as a marker, we first detect melanoblasts immediately dorsal and lateral to the ne ural tube beginning at stage 18, which is prior to the onset of dorsol ateral migration. At later stages every neural crest cell in the dorso lateral path is SL-positive, demonstrating that only melanoblasts migr ate dorsolaterally. Thus, melanoblast specification precedes dorsolate ral migration, and only melanoblasts migrate dorsolaterally at the tho racic level. Together with previous work (Erickson, C. A., and Goins, T. L., Development 121, 915-924, 1995), these data argue that specific ation as a melanoblast is a prerequisite for dorsolateral migration. T his conclusion suggested that the delay in dorsolateral migration (rel ative to ventral migration) may reflect a delay in the emigration of m elanogenic neural crest cells from the neural tube. Several experiment s support this hypothesis. There are no melanoblasts in the ventral pa th, as revealed by the absence of SL-positive cells in the ventral pat h, and neural crest cells isolated from the ventral path do not give r ise to melanocytes when explanted in culture, suggesting that early, v entrally migrating neural crest cells are limited in their ability to differentiate as melanocytes. Similarly, neural crest cells that emigr ate from the neural tube in vitro during the first 6 h fail to give ri se to any melanocytes or SL-positive melanoblasts, whereas neural cres t cells that emigrate at progressively later times show a dramatic inc rease in melanogenesis under identical culture conditions. Thus, the t iming of dorsolateral migration at the thoracic level is ultimately co ntrolled by the late emigration of melanogenic neural crest cells from the neural tube. (C) 1998 Academic Press.