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Biology · Glossary

What is The limb bud?

Definition 11.1 University Biology — Year 2 · Chapter 11 — Organogenesis: Building the Tetrapod Limb

The limb bud is a swelling of lateral plate mesoderm covered by ectoderm, arising at a fixed position along the flank (the limb field) where the Hox code of the body axis permits it; its mesoderm will make bone, cartilage, tendon and dermis, and cells that migrate into it from the somites will make the muscles. The bud has three axes, each governed by a signalling centre. Proximo-distal (shoulder to fingertip): the apical ectodermal ridge (AER), a thickened strip of ectoderm along the tip, secretes fibroblast growth factors (FGFs) that keep the mesoderm beneath it dividing and undifferentiated. Antero-posterior (thumb to little finger): the zone of polarising activity (ZPA), a block of mesoderm at the posterior margin, secretes the morphogen Sonic hedgehog (Shh). Dorso-ventral (back of the hand to palm): the dorsal ectoderm secretes Wnt7a. Forelimb and hindlimb buds look alike and use the same signals; they differ in a transcription factor expressed before the bud appears (Tbx5 in the forelimb, Tbx4 in the hindlimb), which is why a wing is a wing.

The chick limb bud from above, with its three signalling centres: the AER along the tip drives outgrowth, the ZPA at the posterior margin patterns the digits, the dorsal ectoderm sets back from palm.
The chick limb bud from above, with its three signalling centres: the AER along the tip drives outgrowth, the ZPA at the posterior margin patterns the digits, the dorsal ectoderm sets back from palm.
A chick embryo under the scanning electron microscope: the row of somites along the trunk and, on the flank, the paddle-shaped limb bud with the thickened ridge along its rim.
A chick embryo under the scanning electron microscope: the row of somites along the trunk and, on the flank, the paddle-shaped limb bud with the thickened ridge along its rim.
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