In the developing chick embryo, FGFR-2 expression patterns correlate
with outgrowth of facial prominences: frontonasal mass prominences which form
the prenasal cartilage and upper beak express high levels of FGFR-2 receptor
while maxillary prominences which 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. Since FGFR-2 is highly expressed in
regions of facial outgrowth and 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 using in 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 lead to a
decrease in FGFR-2 and collagen II expression 24 hours after grafting to host
and that neither FGFR-2 nor collagen II expression increased to expected levels
at 48 hours. 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. We localized FGF8
transcripts in the developing chick face to determine whether FGF8 is a
putative epithelial signal required for FGFR-2 expression. Our results indicate
that FGF8 ectodermal expression does not overlap FGFR-2 expression in
mesenchymal tissue and therefore is not a signal required for the expression of
this receptor.Dentistry, Faculty ofGraduat
