| Summary: | Retinoids have been known to influence cell migration, interaction, and
differentiation, but their role in skeletal development is still unclear. We investigated the
role of retinoic acid receptor (RAR) signaling in facial patterning in chicken embryos.
Beads soaked in RAR pan-antagonist, AGN 193109, and implanted into the right nasal
pit of stage 15 to 20 embryos induced mild skeletal abnormalities. In addition, Msx-1,
Msx-2, Shh, and Fgf-8, expression were not altered, implying that AGN 193109 was not
potent enough to completely inhibit all endogenous retinoid signaling in vivo. We
indirectly studied the in vivo effectiveness of AGN 193109 by implanting embryos with
both RAR antagonist and exogenous retinoic acid (RA) soaked beads to determine if the
RAR antagonist could antagonize the RA induced teratogenic effect. We demonstrated
that a low concentration of exogenous RA could result in failure of outgrowth of the
prenasal cartilage and premaxilla. Moreover, the RA induced defects could be rescued
by the cotreatment of RAR antagonist, suggesting that AGN 193109 was effective in
antagonizing retinoid signaling in vivo for 18 hours and could not be dislodged from
RARs by exogenous RA. RA treated embryos did not show any changes in Fgf-8 and
Shh expression, implying that Fgf-8 and Shh were independent of retinoid signaling.
However, ectopic expression of Msx-2 was induced by RA treatment and was not
downregulated by the cotreatment of the RAR antagonist. This data indicated for the first
time that Msx-2 was regulated by retinoid signaling but was not important for the
patterning of the chicken upper beak. Most importantly, we demonstrated that a
temporary downregulation of Msx-1 in stage 24 RA treated embryos resulted in failure of
outgrowth of the prenasal cartilage and premaxilla, but this downregulation was
recovered by the cotreatment of RAR antagonist. Thus, our study revealed that
maintenance of Msx-1 between stage 20 and stage 24 was essential for the outgrowth of
the chicken upper beak.
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