Limb development

The vertebrate limb arises out of a general morphogenetic area called a limb field. The position of the limb buds correlates with the anterior boundary of the Hoxc6 gene expression.

Limb formation occurs out of a reciprocal interaction between the mesenchymal cells and the overlying ectodermal cells. The lateral plate mesodermal cells express FGF10 and induce the apical ectodermal ridge (AER) cells to express FGF8 and FGF4 which maintains the FGF10 signal. The position of FGF10 expression is regulated by Wnt8c in the hindlimb and Wnt2b in the forelimb. The forelimb and the hindlimb are distinguished by two T-box containing transcription factors: Tbx5 and Tbx4 respectively.

Programmed cell death removes the spaces between the digits and joints. BMP signaling induces cell death and Noggin block cell death in the digits.

Proximal/Distal Patterning

The limb is made up of three sections: stylopod, zeugopod, and autopod (in order from proximal to distal). There are two competing models for explaining the patterning of these sections.

Progress Zone Model The AER creates a zone of cell proliferation and lays down the limb from proximal to distal. The time cells leave the AER determines their positional value. Proximal structures are formed earlier than distal structures.

The Progress Zone Model was proposed 30 years ago but recent evidence has conflicted with this model.

Early Allocation and Progenitor Expansion Model or Prespecification Model Cells are specified for each segment in the early limb bud and this population of cells expand out as the limb bud grows.

Anterior/Posterior Patterning

The Zone of Polarizing Activity (ZPA) in the limb bud has pattern-organizing activity by action of a morphogen gradient of Sonic hedgehog (Shh). Shh is both sufficient and necessary to create the ZPA and specify the anterior/posterior pattern in the distal limb. Digits 3,4 and 5 are specified by a temporal gradient of Shh. Digit 2 is specified by a diffusible form of Shh and Digit 1 does not require Shh. Shh acts on the transcription factor Gli3 which activates the transcription of HoxD genes along the anterior/posterior axis. Shh is turned on in the posterior through the early expression of Hoxd genes, the expression of Hoxb8, and the expression dHAND. Shh is maintained in the posterior though a feedback loop between the ZPA and the AER. Shh induces the AER to produce FGF4 and FGF8 which maintains the expression of Shh.

Dorsal/Ventral Patterning

Dorsal/Ventral Patterning arises from Wnt7a signals in the overlying epidermis. Wnt7a dorsalizes the limb. Loss of Wnt7a causes the dorsal side of limbs to become ventral sides and causes missing posterior digits. Wnt7a is also required to maintain expression of Shh.