Fig. 9. Models for the role of Hh signaling in ANC development. (A) Diagrams illustrate trabecular cartilage (blue). (Model 1) Classical model: trabecular rods fuse anteriorly to form the trabeculae communis and ethmoid plate (arrows). (Model 2) New model: separate origins for the medial ethmoid (red) and trabeculae (except a small trabeculae communis, arrows). (B) Diagrams of transverse sections illustrate three proposed steps by which Shh controls ANC patterning and chondrogenesis. (Step 1) Shh from ventral neuroectoderm (purple) prevents mixing of migrating NC cells (blue ovals) at the midline. (Step 2) Shh from facial ectoderm (F.E.) promotes trabecular chondrogenesis (arrows). At this stage, shh expression in the CNS does not extend into the ventral hypothalamus, and is therefore unlikely to influence the ANC. (Step 3) Ethmoid precursors (red and blue circles) are induced to chondrify by Hh secreted from both the ventral hypothalamus, and the facial ectoderm (F.E).