Epithelium is required for maintaining FGFR-2 expression levels in facial mesenchyme of the developing chick embryo

Abstract

In the developing chick embryo, fibroblast growth factor-2 (FGFR-2) expression patterns correlate with outgrowth of facial prominences. Frontonasal mass prominences that form the pre-nasal cartilage and upper beak express high levels of FGFR-2 receptor, whereas maxillary prominences that form the flattened corners of the beak and palatal shelves express low FGFR-2 transcript levels. Facial epithelium is an abundant source of FGFs and is required to support outgrowth of mesenchymal tissue, including cartilage rod formation. Because FGFR-2 is highly expressed in regions of facial outgrowth and because epithelium is required for outgrowth of facial prominences, epithelium could be required to maintain FGFR-2 transcripts in facial mesenchyme. To test this hypothesis, we removed epithelium to inhibit outgrowth of regions of the embryonic face, grafted frontonasal mass and maxillary prominences into a host limb bud, and then examined changes in FGFR-2 expression usingin situ hybridization. We also hybridized adjacent sections with collagen II probe to identify regions undergoing chondrogenesis. Our results indicate that removal of epithelium from frontonasal mass led to a decrease in FGFR-2 and collagen II expression 24 hr after grafting to host and that neither FGFR-2 nor collagen II expression increased to expected levels at 48 hr. These results suggest that there are signals in the epithelium required for increasing FGFR-2 and collagen II gene transcription, and the expression of these genes are linked to outgrowth of facial prominences. Dev. Dyn. 1997;210: 407–416. © 1997 Wiley-Liss, Inc.