Hedgehog signaling is required for cranial neural crest morphogenesis and chondrogenesis at the midline in the zebrafish skull

Abstract

Neural crest cells that form the vertebrate head skeleton migrate and interact with surrounding tissues to shape the skull, and defects in these processes underlie many human craniofacial syndromes. Signals at the midline play a crucial role in the development of the anterior neurocranium, which forms the ventral braincase and palate, and here we explore the role of Hedgehog (Hh) signaling in this process. Using sox10:egfp transgenics to follow neural crest cell movements in the living embryo, and vital dye labeling to generate a fate map, we show that distinct populations of neural crest form the two main cartilage elements of the larval anterior neurocranium: the paired trabeculae and the midline ethmoid. By analyzing zebrafish mutants that disrupt sonic hedgehog (shh)expression, we demonstrate that shh is required to specify the movements of progenitors of these elements at the midline, and to induce them to form cartilage. Treatments with cyclopamine, to block Hh signaling at different stages, suggest that although requirements in morphogenesis occur during neural crest migration beneath the brain, requirements in chondrogenesis occur later, as cells form separate trabecular and ethmoid condensations. Cell transplantations indicate that these also reflect different sources of Shh, one from the ventral neural tube that controls trabecular morphogenesis and one from the oral ectoderm that promotes chondrogenesis. Our results suggest a novel role for Shh in the movements of neural crest cells at the midline, as well as in their differentiation into cartilage, and help to explain why both skeletal fusions and palatal clefting are associated with the loss of Hh signaling in holoprosencephalic humans.

MicroRNA Mirn140 modulates Pdgf signaling during palatogenesis

“…Crest-derived palatal precursors, fate mapped in previous studies 3,4 , migrate around the eye to reach the oral ectoderm and the subset of these crest cells migrating rostrally to the eye must additionally migrate around the optic stalk (see below). Both rostrally and caudally migrating crest cells express pdgfra throughout migration to the oral ectoderm (Fig.…”

“…7a). Normally in zebrafish, caudally migrating neural crest cells give rise to the lateral palatal skeleton, while rostrally migrating cells fill in the medial palatal skeleton 3 . Therefore, shape change after Mirn140 knockdown could be due to changes in the relative contributions of caudal versus rostral crest cells to the palatal skeleton.…”

“…In amniotes, the palate serves to separate the nasal and oral cavities and is generated through an intricate series of morphogenic events that include early neural crest cell migration and cell-cell signaling during the formation of facial prominences, as well as later generation and fusion of palatal shelves. While later events involving palatal shelves have not been described in zebrafish, palatal precursors migrate both rostral and caudal to the eye to condense upon the oral ectoderm in amniotes 1 as well as zebrafish 2,3 and evidence continues to accumulate that the early signaling environment governing palatogenesis is also largely equivalent [3][4][5][6] . For instance, Hh signaling is crucial for palatogenesis in humans and zebrafish 3,4,7 .…”

“…While later events involving palatal shelves have not been described in zebrafish, palatal precursors migrate both rostral and caudal to the eye to condense upon the oral ectoderm in amniotes 1 as well as zebrafish 2,3 and evidence continues to accumulate that the early signaling environment governing palatogenesis is also largely equivalent [3][4][5][6] . For instance, Hh signaling is crucial for palatogenesis in humans and zebrafish 3,4,7 . Zebrafish and amniotes also share expression patterns of palatogenic genes such as Shh 4,8 , Fgf8 4,9,10 and Pdgf receptor alpha (Pdgfra) [11][12][13] .…”

Why the short face? Developmental disintegration of the neurocranium drives convergent evolution in neotropical electric fishes

“…All the signaling molecules discussed above are known to perform multiple functions during development (Dworkin, Boglev, Owens, & Goldie, 2016; Harada, Sato, & Nakamura, 2016; McCarthy et al., 2016; Sudheer et al., 2016; Wada et al., 2005). It is therefore unlikely that modulation of these signaling molecules is regulated by a single gene.…”