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The role of the nasal pit and Fibroblast Growth Factor signaling in avian craniofacial development Szabo Rogers, Heather Lynn

Abstract

We characterized the role of FGFs (Fibroblast growth factors) during upper lip fusion in the chicken embryo model. We first undertook an expression study of members of the Sprouty family, intracellular inhibitors of FGF signaling, in the upper face to assess where active FGF signaling is occurring. We found that Sprouty 1, 2, and 4 are expressed in nested, overlapping domains within the fusing upper lip in the frontonasal mass and maxillary prominence. Furthermore the areas that expressed Sprouty family members are adjacent to areas that also express Fgf8 . Next we tested the requirement for FGF signals in the developing lip by implanting beads soaked in an FGF receptor antagonist, SU5402. We hypothesized that loss of FGF signaling would induce a cleft lip through reduced proliferation and increased cell death. We were surprised to find that mesenchyme in the zone of fusion was FGF-independent. In contrast, clefts were induced when beads were placed distant to the zone of fusion, near the top of the nasal slit. We then showed that cranially implanted SU5402 beads decreased active FGF signaling along the lateral edge of the frontonasal mass. Furthermore we showed that the cranially implanted SU5402 beads resulted in reduced proliferation of the cranial region of the frontonasal mass, and increased cell death along the lateral edge of the frontonasal mass. In contrast, beads in the fusion zone had limited effects on gene expression, cell proliferation and cell death. We therefore identified an FGF-dependent growth centre in the cranial frontonasal mass. We proposed a new theory of facial fusion, whereby the FGF-dependent cranial growth centre displaces the globular process towards the maxillary prominence. This led us to hypothesize that the FGF-rich nasal pit epithelia which is directly adjacent to the frontonasal mass growth center influences morphogenesis of the upper beak. We found that the ectopic nasal pits had the capacity to form ectopic cartilage and bone in competent facial mesenchyme. Although we could not attach specific identities to the induced skeletal elements, these experiments show for the first time that the nasal pit epithelium is capable of providing skeletogenic patterning information.

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