Uncoupling Sonic Hedgehog Control of Pattern and Expansion of the Developing Limb Bud

Abstract

Sonic hedgehog (Shh), which regulates proliferation in many contexts, functions as a limb morphogen to specify a distinct pattern of digits. How Shh's effects on cell number relate to its role in specifying digit identity is unclear. Deleting the mouse Shh gene at different times using a conditional Cre line, we find that Shh functions to control limb development in two phases: a very transient, early patterning phase regulating digit identity, and an extended growth-promoting phase during which the digit precursor mesenchyme expands and becomes recruited into condensing digit primordia. Our analysis reveals an unexpected alternating anterior-posterior sequence of normal mammalian digit formation. The progressive loss of digits upon successively earlier Shh removal mirrors this alternating sequence and highlights Shh's role in cell expansion to produce the normal digit complement.

The chicken polydactyly (Po) locus causes allelic imbalance and ectopic expression of Shh during limb development

“…The growth-morphogen model predicts that an increase in posterior Shh expression, resulting in an increase in the range of Shh signalling across the limb, would lead to an increased proliferation in the posterior of the limb and, therefore, an increase in the area of the limb specified to form digits in the Slk leg. Zhu et al (2008) also predict that an increased range of Shh signalling would result in extra anterior digit ''1''s, which we observe occasionally in Slk embryos (e.g., 1, 1, 2, 3, 4 or 1, 1, 1, 2, 3, 4, Fig. 1C, D).…”

Mechanisms of digit formation: Human malformation syndromes tell the story

“…The hand plate of the vertebrate limb (the autopod) is particularly suited for the analysis of such intricate events, because the main signaling centers controlling morphogenesis are well defined and the digit condensations that will differentiate to a unique digit identity provide a well-defined readout. As a prerequisite for the formation of the digit condensation the mesoderm of the autopod undergoes proliferative expansion governed by signaling from ZPA-derived Sonic hedgehog (Shh), which in this context has a dual role coupling the expansion of the limb mesenchyme with the specification of anterior-posterior digit identities (Towers et al, 2008;Zhu et al, 2008). Proliferative expansion of the mesenchyme is of vital importance, because deletion of Mycn from the limb mesenchyme in the mouse thus decreasing mesenchymal proliferation leads to severely reduced size of the skeletal elements (Ota et al, 2007).…”

Vertebrate Limb Development: Moving from Classical Morphogen Gradients to an Integrated 4-Dimensional Patterning System

“…This study indicates that SHH-mediated specification of digit identities occurs within the first 12 hours of SHH signaling and that SHH activity is continuously required to generate the required number of cartilage progenitor cells (Fig. 3E) (Zhu et al 2008).…”

“…As a consequence, only digits with posterior identities form because of exposure of the remaining small autopod territory to high levels of SHH signaling (Towers et al 2008). In addition, genetic inactivation of Shh from defined developmental time points onwards induces digit losses in a sequence not consistent with a morphogen gradient type patterning mechanism (Zhu et al 2008). Rather, the sequence of digit loss reflects the sequence by which the digits normally condense.…”