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Fig. 7. Shh signalling along the dental axis during initiation of odontogenesis.
(A) In the presence of epithelium, Shh transcription is limited to
the incisor and molar-forming epithelium, and is established by autoregulation
(dashed line). Shh protein diffuses from epithelium into mesenchyme, where it
induces both Ptc1 and Hhip1. Ptc1 and Hhip1 protein restrict
Shh signalling to the sites of incisor and molar odontogenesis, but some Shh
protein is able to diffuse into the diastema mesenchyme. In the presence of
diastema epithelium, Gas1 sequesters and masks the activity of Shh protein in
the diastema mesenchyme, thus maintaining restriction of Shh signalling to
odontogenic regions. Ptc1 and Hhip1 transcription has not
been shown in the tooth germs for simplicity. (B) In the absence of
epithelium, Gas1 is lost in diastema mesenchyme, leaving localised
Shh signalling in this region, which activates transcription of the target
gene Ptc1.
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