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 ventral 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 onset of dorsolateral migration coincides with the cessation of ventral migration. Neural crest cells in the ventral path differentiate predominantly as neurons and glial cells of the peripheral nervous system, whereas those in the dorsolateral path give rise to the melanocytes of the skin. Thus, early- and late-migrating neural crest cells exhibit unique morphogenetic behaviors and give rise to different subsets of neural crest derivatives. Here we present evidence that these differences reflect the appearance of specified melanocyte precursors, or melanoblasts, from late- but not early-migrating neural crest cells. We demonstrate that serum from Smyth line (SL) chickens specifically immunolabels melanocyte precursors, or melanoblasts. Using SL serum as a marker, we first detect melanoblasts immediately dorsal and lateral to the neural tube beginning at stage 18, which is prior to the onset of dorsolateral migration. At later stages every neural crest cell in the dorsolateral path is SL-positive, demonstrating that only melanoblasts migrate dorsolaterally. Thus, melanoblast specification precedes dorsolateral migration, and only melanoblasts migrate dorsolaterally at the thoracic level. Together with previous work (Erickson, C. A., and Goins, T. L., Development 121, 915-924, 1995), these data argue that specification as a melanoblast is a prerequisite for dorsolateral migration. This conclusion suggested that the delay in dorsolateral migration (relative to ventral migration) may reflect a delay in the emigration of melanogenic neural crest cells from the neural tube. Several experiments support this hypothesis. There are no melanoblasts in the ventral path, as revealed by the absence of SL-positive cells in the ventral path, and neural crest cells isolated from the ventral path do not give rise to melanocytes when explanted in culture, suggesting that early, ventrally migrating neural crest cells are limited in their ability to differentiate as melanocytes. Similarly, neural crest cells that emigrate from the neural tube in vitro during the first 6 h fail to give rise to any melanocytes or SL-positive melanoblasts, whereas neural crest cells that emigrate at progressively later times show a dramatic increase in melanogenesis under identical culture conditions. Thus, the timing of dorsolateral migration at the thoracic level is ultimately controlled by the late emigration of melanogenic neural crest cells from the neural tube.