EPITHELIUM IS REQUIRED FOR MAINTAINING FGRF-2 EXPRESSION LEVELS IN FACIAL MESENCHYME OF THE DEVELOPING CHICK-EMBRYO

In the developing chick embryo, fibroblast growth factor-2 (FGFR-2) ex pression patterns correlate with outgrowth of facial prominences. Fron tonasal mass prominences that form the pre-nasal cartilage and upper b eak express high levels of FGFR-2 receptor, whereas maxillary prominen ces that form the flattened corners of the beak and palatal shelves ex press low FGFR-2 transcript levels. Facial epithelium is an abundant s ource of FGFs and is required to support outgrowth of mesenchymal tiss ue, including cartilage rod formation. Because FGFR-2 is highly expres sed in regions of facial outgrowth and because epithelium is required for outgrowth of facial prominences, epithelium could be required to m aintain FGFR-2 transcripts in facial mesenchyme. To test this hypothes is, we removed epithelium to inhibit outgrowth of regions of the embry onic face, grafted frontonasal mass and maxillary prominences into a h ost limb bud, and then examined changes in FGFR-2 expression using in situ hybridization. We also hybridized adjacent sections with collagen II probe to identify regions undergoing chondrogenesis. Our results i ndicate that removal of epithelium from frontonasal mass led to a decr ease in FGFR-2 and collagen II expression 24 hr after grafting to host and that neither FGFR-2 nor collagen II expression increased to expec ted levels at 48 hr. These results suggest that there are signals in t he epithelium required for increasing FGFR-2 and collagen II gene tran scription, and the expression of these genes are linked to outgrowth o f facial prominences. (C) 1997 Wiley-Liss, Inc.