Conditional inactivation of FGF8 signaling in the frontonasal ectoderm results in dysmorphology of the frontonasal prominence and severe craniofacial defects

Abstract

Cranial neural crest cells are a multipotent migratory cell population that gives rise to the majority of the developing head and face, including bones, cartilage, connective tissue, cranial nerves and pigment cells. During embryonic development, a population of cranial neural crest cells migrates toward the developing frontonasal prominence (FNP) and gives rise to specific mid-facial structures such as the nasal, premaxillary and maxillary bones. The ectoderm of the FNP is known to be essential for neural crest cell viability and differentiation. Although many genes are known to be involved in the patterning of this region, FGF8 signaling has been implicated in playing a particularly critical role. To examine the role of FGF8 signaling in the developing mid-facial region, we conditionally inactivated the Fgf8 gene specifically in the frontonasal ectoderm, using the Cre-loxP recombination system and an Ap2a promoter. Tissue specific inactivation was confirmed by the loss of Fgf8 expression in the FNP and not in the apical ectodermal ridge of the limbs. Embryonic day (E)10.5 embryos display transient crania bifida (delayed closure of the forebrain neuropore), ring-shaped FNP and an absence medial and lateral nasal pits. In addition mutant embryos also lack the maxillary component of the first branchial arch and have a reduced manibular prominence. Skeletal preparations of E14.5 embryos demonstrate a tubular shaped hypoplastic nasal cartilage, absence of distal lower and upper jaw components, including the nasal bones which persist through to E17.5. In addition, E17.5 embryos lacked multiple bones of the skull vault. Only the frontal bones were present and these were abnormal in shape and did not project medially. We are currently investigating the effects of the conditional inactivation of Fgf8 on the migration and viabililty of cranial neural crest cells as part of the mechanistic origins of the craniofacial anomalies observed. Collectively, these results demonstrate a tissue specific requirement for FGF8 signaling in the frontonasal region as a critical component of normal craniofacial morphogenesis.