Expression of Sonic Hedgehog and pathway components in the embryonic mouse head: anatomical relationships between regulators of positive and negative feedback

Abstract

Objective: The Hedgehog pathway is a fundamental signaling pathway in organogenesis. The expression patterns of the ligand Sonic Hedgehog (Shh) and key pathway components have been studied in many tissues but direct spatial comparisons across tissues with different cell compositions and structural organization are not common and could reveal tissue-specific differences in pathway dynamics.

Results: We directly compared the expression characteristics of Shh, and four genes with functional roles in signaling and whose expression levels serve as readouts of pathway activity in multiple tissues of the embryonic mouse head at embryonic day 15.5 by serial in situ hybridization. The four readout genes were the positive feedback regulator Gli1, and three negative feedback regulators, Patched1, Patched2, and Hedgehog Interacting Protein. While the relative abundance of Gli1 was similar across tissues, the relative expression levels and spatial distribution of Shh and the negative feedback regulators differed, suggesting that feedback regulation of hedgehog signaling is context dependent. This comparative analysis offers insight into how consistent pathway activity could be achieved in tissues with different morphologies and characteristics of ligand expression.

Keywords: Embryonic day 15.5; Eyelid; Gene expression; Gli1; Hair follicle; Hedgehog interacting protein; Molar; Palatal rugae; Patched; Retina; Sonic hedgehog.

Fig. 1

Expression patterns for Shh and Hh pathway components in developing organs of the embryonic mouse head. A Simplified schematic of Hh signaling. See "Introduction" section for details. B–F in situ hybridizations for Shh (B), Gli1 (C), Ptch1 (D), Ptch2 (E), and Hhip (F) in adjacent coronal sections of the mouse at E15.5. Arrowheads in F denote additional hair follicles

Fig. 2

Close up comparisons of expression patterns. A, B Schematics of upper (A) and lower (B) molars with buccal side to the left, lingual side to the right. C–G Schematics of stage 3 hair follicle (C), palate (D) eyelid (E), eye (F), and retina (G). H–PP) Expression patterns of Shh (H–N), Gli1 (O–U), Ptch1 (V–BB), Ptch2 (CC–II), and Hhip (JJ–PP) in each structure. See abbreviations list for descriptions. White asterisks indicate histological artifacts where tissues are lacking. The asterisks for the eye structures also indicate the pigmentation of the RPE and is not mRNA staining. Black arrowheads in LL point to Hhip in the condensing mesenchyme surrounding the follicle. White arrows in eyelid panels point to the eyelid signaling field, and the arrowhead denotes a hair follicle. White dashed lines in eye panels denote the iris stroma (below line). White dashed lines in retina panels denote the RPE (below lines) and extraocular mesenchyme including the condensing scleral mesenchyme (above lines)