Fibroblast growth factor (FGF) induces the formation of an organizer at the end of the limb bud, called the apical ectodermal ridge (AER), which guides further development and controls cell death.

Limb formation results from a series of reciprocal tissue interactions between the mesenchyme of the lateral plate mesoderm and the overlying ectodermally derived epithelial cells.

The lateral plate mesodermal cells secrete fibroblast growth factors (FGF7 and FGF10) to induce the overlying ectoderm to form an organizer at the end of the limb bud, called the apical ectodermal ridge (AER), which guides further development and controls cell death.

[9][10] The limb's skeletal elements are prefigured by tight aggregates known as cellular condensations of the pre-cartilage mesenchymal cells.

[12] In the process of chondrogenesis, chondrocytes differentiate from the condensations to form cartilage, giving rise to the skeletal primordia.

For example, the radius and ulna of the forelimb, and the tibia and fibula of the hindlimb of the zeugopod are distinct from one another, as are the different fingers or toes in the autopod.

[1] There have been many studies aimed at elucidating specific genes and transcription factors that are found responsible for limb development (See Table 1 below).

In particular, studies have shown that SHH, DACH1, and the BMP, HOX, T-box, FGF, and WNT gene families all play a critical role in successful limb development and formation.

[1] Many of the genes listed in Table 1 play an important role in embryonic development, specifically during skeletal patterning and limb bud formation.

One theory suggests that the degradation of enhancer sequences may have contributed to the progressive limb loss in snake evolution.

In particular, many studies have focused on the ZPA Regulatory Sequence (ZRS) - the enhancer of the Sonic Hedgehog gene (Shh).

Comparisons of the core ZRS in several snake species to the mouse and lizard sequences shows the presence of snake-specific alterations.

[23] The core ZRS proved to be mainly conserved in basal snakes like the boa constrictor and python, which still contain pelvic girdle bones.

[22] In contrast, advanced snakes such as the viper and cobra, in which no skeletal limb structures remain, have a much higher rate of nucleotide changes when compared to the mouse and lizard ZRS.

[24] This degradation in ZRS suggests that this enhancer may be important in further exploring the molecular mechanisms that propelled the morphological evolution of snakes.

These factors make it difficult to understand the mechanism of snake limb loss using a genetic approach, targeting and observing the presence and activity of these genes and their regulatory enhancers.

[25] Many investigations into the development of the limb skeletal pattern have been influenced by the positional information concept proposed by Lewis Wolpert in 1971.

[26] In tune with this idea, efforts have been made to identify diffusive signaling molecules (morphogens) that traverse orthogonal axes of developing limbs and determine locations and identities of skeletal elements in a concentration-dependent fashion.

Proximodistal and craniocaudal axes are not considered to be independently specified, but instead emerge by transitions in the number of parallel elements as the undifferentiated apical zone of the growing limb bud undergoes reshaping.

Other factors like sonic hedgehog (Shh) and Hox proteins, primary informational molecules in the other models, are proposed instead to play a fine-tuning role.

There are other factors thought to control the DV patterning; Engrailed-1 represses the dorsalizing effect of Wnt7a on the ventral side of the limbs.