Abstract
BackgroundNeurogenic 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.ResultsHere, 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.ConclusionWe 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.
Highlights
Neurogenic placodes are focal thickenings of the embryonic ectoderm that form in the vertebrate head
The isthmic Fgf8 domain had not yet responded to activated Wnt signaling by this time point; we could only detect ectopic expression of Fgf8 9 hours post-electroporation of Wnt1 (CAC and CMJ, unpublished results). These results suggest that the earliest expansion of the trigeminal placode may be responding to Wnt signals alone, we do not rule out the role of related fibroblast growth factor (FGF) and Wnt family members expressed within the midbrainhindbrain region
We provide evidence that FGF signaling is necessary to maintain early neuronal differentiation within placodal tissue
Summary
Neurogenic placodes are focal thickenings of the embryonic ectoderm that form in the vertebrate head. The sensory neurons of the head develop differently from those of the trunk In the trunk, these neurons are derived exclusively from the neural crest, while in the head, sensory neurons are generated from neural crest and focal thickenings of the embryonic ectoderm, the neurogenic placodes [1,2]. Neural Development 2008, 3:35 http://www.neuraldevelopment.com/content/3/1/35 ganglion arise from two distinct placodes, the ophthalmic and the maxillomandibular, which form alongside the midbrain-hindbrain junction. They have a neural crest cell component [1,3,4]. It conveys somatosensory information from the face, but there is relatively little understanding of how its early development is controlled, with respect to the development of the ophthalmic and maxillomandibular placodes
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