Clonal analysis, transplantation and lineage-tracing experiments revealed that in vitro and in vivo pluripotent cells coexist with lineage-restricted precursors in migrating neural crest and in early crest-derived tissues

The multiple cell types of the vertebrate peripheral nervous system (PNS) are generated from a transient population of neural crest stem cells (NCSCs) derived from the dorsal portion of the developing neural tube (Le Douarin, 1982). A central issue in developmental biology is to understand how an initially multipotent stem cell becomes fate restricted giving rise to distinct cell types at precise embryonic locations (Edlund and Jessell, 1999). Cell type-specificity could be achieved by the differentiation of committed precursors, or could reflect the action of local cues imposing particular fates on pluripotent cells (reviewed in Anderson et al., 1997). Multipotency of neural crest cells has been demonstrated both in avian and rodent neural crest cells (Baroffio et al., 1988; Bronner-Fraser and Fraser, 1988; Fraser and Bronner-Fraser, 1991; Ito et al., 1993; Sextier-Sainte-Claire Deville et al., 1992; Sieber-Blum and Cohen, 1980; Stemple and Anderson, 1992). Clonal analysis, transplantation and lineage-tracing experiments revealed that in vitro and in vivo pluripotent cells coexist with lineage-restricted precursors in migrating neural crest and in early crest-derived tissues (reviewed in Anderson et al., 1997; Le Douarin, 1986; Le Douarin and Ziller, 1993). The ratio of fate-restricted precursors to pluripotent cells present in neural crest-derived structures increases with age. Thus, it is thought that fate restriction of neural crest cells is a progressive process occurring during migration and at sites of differentiation. The molecular basis of fate restriction in neural crest cells is believed to involve exposure to environmental signals (reviewed in Anderson et al., 1997; Le Douarin and Ziller, 1993). However, it is often unclear whether the observed restriction of neural crest cells to particular fates also represents reduced developmental potential. Is a neural crestderived cell that in lineage-tracing experiments or in clonal cultures gives rise only to a particular sublineage indeed committed to this lineage? Demonstrating lineage commitment requires a challenge to the precursor cells by changing the environment of the cell, a process that might induce alternative cell fates. Back-transplantation into avian embryos during early phases of neural crest migration suggested that late emigrating neural crest cells display restricted developmental capacities (Artinger and Bronner-Fraser, 1992). Clonal analysis of NCSC cultures identified instructive extracellular signals promoting particular sublineages at the expense of other fates (Shah et al., 1994, 1996). A neuregulin (NRG1) isoform, glial growth factor (Marchionni et al., 1993) induces gliogenesis, while bone morphogenetic protein 2 (BMP2) 3781 Development 126, 3781-3794 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV2442