Neural tube derived Wnt signals cooperate with FGF signaling in the formation and differentiation of the trigeminal placodes.
BACKGROUND
Neurogenic placodes are focal thickenings of the embryonic ectoderm that form in the vertebrate head. It is within these structures that the precursors of the majority of the sensory neurons of the cranial ganglia are specified. The trigeminal placodes, the ophthalmic and maxillomandibular, form close to the midbrain-hindbrain boundary and many lines of evidence have shown that signals emanating from this level of the neuraxis are important for the development of the ophthalmic placode.


RESULTS
Here, we provide the first evidence that both the ophthalmic and maxillomandibular placodes form under the influence of isthmic Wnt and FGF signals. Activated Wnt signals direct development of the Pax3 expressing ophthalmic placodal field and induce premature differentiation of both the ophthalmic and the maxillomandibular placodes. Similarly, overexpression of Fgf8 directs premature differentiation of the trigeminal placodes. Wnt signals require FGF receptor activity to initiate Pax3 expression and, subsequently, the expression of neural markers, such as Brn3a, within the cranial ectoderm. Furthermore, fibroblast growth factor signaling via the mitogen activated protein kinase pathway is required to maintain early neuronal differentiation within the trigeminal placodes.


CONCLUSIONS
We demonstrate the identity of inductive signals that are necessary for trigeminal ganglion formation. This is the first report that describes how isthmic derived Wnt signals act in concert with fibroblast growth factor signaling. Together, both are necessary and sufficient for the establishment and differentiation of the ophthalmic and maxillomandibular placodes and, consequently, the trigeminal ganglion.
Canning
Claire A
CA
Institute of Medical Biology, A*STAR, 8A Biomedical Grove, #06-06 Immunos, Singapore 138648, Republic of Singapore. claire.canning@imb.a-star.edu.sg
Lee
Lily
L
Luo
Sarah Xinwei
SX
Graham
Anthony
A
Jones
C Michael
CM
D000818:Animals; D015153:Blotting, Western; D002454:Cell Differentiation / Q000502:physiology*; D002642:Chick Embryo; D051524:Fibroblast Growth Factor 8 / Q000235:genetics / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D054259:Neural Tube / Q000166:cytology / Q000196:embryology* / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D017468:Receptors, Fibroblast Growth Factor / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D012668:Trigeminal Ganglion / Q000196:embryology / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D051153:Wnt Proteins / Q000235:genetics / Q000378:metabolism*
D017468:Receptors, Fibroblast Growth Factor; D051153:Wnt Proteins; D051524:Fibroblast Growth Factor 8