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The neural crest is a multipotent precursor population which ultimately generates much of the peripheral nervous system, epidermal pigment cells, and a variety of mesectodermal derivatives. Individual multipotent neural crest cells are capable of some self-renewing divisions, and based upon this criteria can be considered stem cells. Considerable progress has been made in recent years toward understanding how this important population of progenitor cells is initially established in the early embryo, and how cell-intrinsic and non-cell-intrinsic factors mediate their subsequent lineage segregation and differentiation.
Figure 1 Patterns of neural crest migration and major derivatives produced by neural crest originating at different axial levels in avian embryos. For a more complete description of derivatives, see Le Douarian (1982). FB-forebrain; MB-midbrain; R-rhombomere; OV-otic vesicle. (Modified from Sechrist et al., 1993.)
Figure 2 Inductive interactions implicated in formation of the neural crest in amphibian and
avian embryos. (A) In Xenopus, the initial patterning of the prospective neural plate commences
at gastrula stages. MBP4 in the ectoderm is antagonized by noggin, chordin, and follistatin
protein secreted by the organizer and the chordomesoderm derived from it. Region of the
ectoderm lying farthest from the organizer are thought to see higher levels of BMP signaling
and give rise to epidermis. Regions of low BMP activity, originating closer to the organizer,
form the neural plate. Intermediate regions form the neural plate border or the neural folds.
Neural crest induction may require a second signal emanating from the prospective epidermis
of underlying mesoderm. This signal can be mimicked by growth factors of the FGF and Wnt
families. (B) By neural plate stages in Xenopus, the dorsoventral pattern of the future neural
tube is largely established. NC-neural crest; NP-neural plate; FP-floorplate; No-notochord. (C)
In avian embryos, specification of the neural crest may not occur until the neural folds are
closing. Signals from the ectoderm (arrowheads) are implicated in this process and can be
mimicked by BMPs and related growth factors.
Figure 3 Expression of the transcription factor Slug in chick and Xenopus embyros. (A) In
situ hybridization of a 10-somite-stage chick embryo using a Slug-specific probe. Migrating
neural crest cells express Slug, as do dorsal regions of the closed and closing neural tube with
the potential to form neural crest. (B) Similarly, in a stage 18 Xenopus embryo, Slug expression
is seen in the massive cranial folds and dorsally along the anterioposterior axis of the closing
neural tube. As in chick, Slug continues to be expressed by neural crest cells as they begin to
migrate. (C) Single cell lineage analysis in chick embryos indicates that the progeny of single
dorsal neural tube cells contribute to multiple ectodermal derivatives even after the onset of
Slug expression. (Modified from Selleck and Bronner-Fraser, 1995.) DiI and LRD injections
were carried out in closed neural tubes and open neural folds from stages 8–10. Colored blocks
represent the tissue contributions made by labeled cells after injection at that position. Similar
results have been obtained in mouse and Xenopus embryos.