Molecular signaling pathways are communication systems that transmit messages within and between cells. According to a new study by University of Geneva scientists, inhibiting one such system, called the sonic hedgehog (Shh) pathway, strongly perturbs feather development in chickens by restricting feather bud outgrowth, invagination and branching.
Cooper & Milinkovitch demonstrate that an intravenous injection of sonidegib, a potent Shh pathway inhibitor, at embryonic day 9 (E9) temporarily produces striped domains (instead of spots) of Shh expression in the skin, arrests morphogenesis, and results in unbranched and non-invaginated feather buds — akin to proto-feathers — in embryos until E14. Image credit: Fabrice Berger & Michel Milinkovitch.
Avian feathers are intricate appendages whose forms vary substantially across species and body areas, and between juvenile and adult stages.
Understanding both the developmental and evolutionary mechanisms underpinning this morphological diversity has long fascinated biologists.
The morphological intricacies of avian feathers make them an ideal model for investigating embryonic patterning.
In particular, the Shh pathway is an important mediator of feather outgrowth and branching.
However, functional in vivo evidence regarding its role during feather development remains limited.
To fill this knowledge gap, University of Geneva researchers Rory Cooper and Michel Milinkovitch used light sheet fluorescence microscopy imaging to study the normal patterning of embryonic feathers and how their shape develops.
They also used precise intravenous injections of sonidegib to pharmacologically inhibit Shh pathway signaling during feather development at embryonic day 9, which precedes feather-bud outgrowth on the wings.
This treatment temporarily modified Shh expression to produce striped domains instead of spots on the skin, temporarily stopped feather development, and resulted in unbranched and non-invaginated feather buds — akin to putative proto-feathers — until embryonic day 14.
Although feather development partially recovered later during development, hatched sonidegib-treated chickens exhibited naked regions of the skin surface with perturbed follicles.
Remarkably, these follicles were subsequently reactivated by seven weeks post-hatching, highlighting the robustness of feather patterning as a developmental process.
Overall, the study provides comprehensive functional evidence for the role of the Shh pathway in mediating feather development in chickens, supporting the idea that modified Shh signaling has contributed to the evolutionary diversification of feathers and other skin appendages such as feet scales.
The study also demonstrates the importance of in vivo experiments for obtaining a comprehensive understanding of developmental systems.
“Since the Shh pathway plays a crucial role in feather development, we wanted to observe what happens when it is inhibited,” Dr. Cooper said.
“By injecting a molecule that blocks the Shh signaling pathway on the 9th day of embryonic development — just before feather buds appear on the wings — we observed the formation of unbranched and non-invaginated buds, resembling the putative early stages of proto-feathers.”
“However, from the 14th day of embryonic development, feather morphogenesis partially recovered.”
“Furthermore, although the chicks hatched with patches of naked skin, dormant subcutaneous follicles were autonomously reactivated, eventually producing chickens with normal plumage.”
“Our experiments show that while a transient disturbance in the development of foot scales can permanently turn them into feathers, it is much harder to permanently disrupt feather development itself,” Dr. Milinkovitch added.
“Clearly, over the course of evolution, the network of interacting genes has become extremely robust, ensuring the proper development offeathers even under substantial genetic or environmental perturbations.”
“The big challenge now is to understand how genetic interactions evolve to allow for the emergence of morphological novelties such as proto-feathers.”
The study was published online today in the journal PLoS Biology.
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R.L. Cooper & M.C. Milinkovitch. 2025. In vivo sonic hedgehog pathway antagonism temporarily results in ancestral proto-feather-like structures in the chicken. PLoS Biol 23 (3): e3003061; doi: 10.1371/journal.pbio.3003061
