Initiation of dorso-ventral axis during chick limb development

Area of Science:

• Developmental biology
• Molecular genetics
• Embryology

Background:

• Limb development is a complex process regulated by precise gene expression.
• Understanding the spatio-temporal dynamics of key developmental genes is crucial for deciphering limb formation mechanisms.
• Fibroblast growth factor (FGF) signaling plays a pivotal role in initiating limb bud formation.

Purpose of the Study:

• To analyze the spatio-temporal expression of dorso-ventral genes (Wnt-7a, En-1, Lmx-1, Fgf-8) during normal and ectopic chick limb development.
• To investigate the role of FGF signaling in inducing gene expression patterns related to limb formation.
• To determine the origin of cells contributing to ectopic limb formation.

Main Methods:

• Analysis of spatio-temporal gene expression patterns.
• Application of fibroblast growth factor (FGF) to the flank to induce ectopic limb formation.
• Comparison of gene expression kinetics between normal and ectopic limb development in chick embryos.

Main Results:

• Wnt-7a is the earliest dorsal ectoderm gene expressed in limb-forming regions.
• FGF application rapidly and locally induces Wnt-7a expression.
• Ectopic Lmx-1 induction in dorsal mesoderm is delayed, occurs distally to FGF-2, and appears initially independent of Wnt-7a.
• Flank cells, not normal limb mesoderm, contribute to ectopic limb formation.
• A potential inhibitor in the flank restricts Lmx-1 expression to limb regions.

Conclusions:

• Wnt-7a acts as an early marker and potential initiator of dorsal limb ectoderm development.
• FGF signaling directly induces Wnt-7a, while Lmx-1 induction is a slower, secondary event.
• Ectopic limb formation originates from flank cells, regulated by localized signaling and potential inhibitory factors.