Locally released retinoic acid repatterns the first branchial arch cartilages in vivo

The fates of cranial neural crest cells are unique compared to trunk neural crest. Cranial neural crest cells form bone and cartilage and ultimately t hese cells make up the entire facial skeleton. Previous studies had establi shed that exogenous retinoic acid has effects on neurogenic derivatives of cranial neural crest cells and on segmentation of the hindbrain. In the pre sent study we investigated the role of retinoic acid on the skeletal deriva tives of migrating cranial neural crest cells. We wanted to test whether lo w doses of locally applied retinoic acid could respecify the neural crest-d erived, skeletal components of the beak in a reproducible manner. Retinoic acid-soaked beads were positioned at the presumptive mid-hindbrain junction in stage 9 chicken embryos. Two ectopic cartilage elements were induced, t he first a sheet of cartilage ventral and lateral to the quadrate and the s econd an accessory cartilage rod branching from Meckel's cartilage. The acc essory rod resembled a retroarticular process that had formed within the fi rst branchial arch domain. In addition the quadrate was often displaced lat erally and fused to the retroarticular process. The next day following bead implantation, expression domains of Hoxa2 and Hoxb1 were shifted in an ant erior direction up to the mesencephalon and Msx-2 was slightly down-regulat ed in the hindbrain. Despite down-regulation in neural crest cells, the ons et of Msx-2 expression in the facial prominences at stage 18-20 was normal. This correlates with normal distal beak morphology, Focal labeling of neur al crest with DiI showed that instead of migrating in a neat group toward t he second branchial arch, a cohort of labeled cells from r4 spread anterior ly toward the proximal first arch region. AP-2 expression data confirmed th e uninterrupted presence of AP-2-expressing cells from the anterior mesence phalon to r4. The morphological changes can be explained by mismigration of r4 neural crest into the first arch, but at the same time maintenance of t heir identity. Up-regulation of the Hoxa2 gene in the first branchial arch may have encouraged r4 cells to move in the anterior direction. This combin ation of events leads to the first branchial arch assuming some of the char acteristics of the second branchial arch. (C) 2000 Academic Press.