death. experimental evidence: labeled cells in different position of an early limb bud were restricted to single segments of the limb. limbs lacking expression of required fgf4 & fgf8 showed all structures of the limb and not just the proximal parts. more recently, however, the investigators primarily responsible for both the progress zone and prespecification models have acknowledged that neither of these models accounts adequately for the available experimental data. turing-type reaction–diffusion model this model, a reaction–diffusion model first proposed in 1979, is based on the self-organizing properties of excitable media described by alan turing in 1952. the excitable medium is the limb bud mesenchyme, in which cells interact by positively autoregulatory morphogens such as transforming growth factor beta (tgf-β) and inhibitory signaling pathways involving fibroblast growth factor (fgf) and notch signalling. 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. this model only specifies a "bare bones" pattern. 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. experimental evidence: limb mesenchymal cells, when dissociated and grown in culture or reintroduced within ectodermal "hulls" can recapitulate essential aspects of pattern formation, morphogenesis and differentiation. peculiarities of the limb skeletal pattern in the mouse doublefoot mutant are predicted outcomes of a turing-type mechanism. progressive reduction in distal hox genes in a gli3-null background results in progressively more severe polydactyly, displaying thinner and densely packed digits, suggesting (with the aid of computer modeling) that the dose of distal hox genes modulates the period or wavelength of digits specified by a turing-type mechanism. craniocaudal patterning early signals that define the craniocaudal (anterior-posterior), and proximodistal axes in vertebrate limb development. the discovery, in 1957, of the zone of polarizing activity (zpa) in the limb bud provided a model for understanding the patterning activity by the action of a morphogenic gradient of sonic hedgehog (shh). shh is recognised as a limb-specific enhancer. shh is both sufficient and necessary to create the zpa and specify the craniocaudal pattern in the distal limb (shh is not necessary for the polarity of the stylopod). 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 through a feedback loop between the zpa and the aer. shh induces the aer to produce fgf4 and fgf8 which maintains the expression of shh. digits 3,4 and 5 are specified by a temporal gradient of shh. digit 2 is specified by a long-range diffusible form of shh and digit 1 does not require shh. shh cleaves the ci/gli3 transcriptional repressor complex to convert the transcription factor gli3 to an activator which activates the transcription of hoxd genes along the craniocaudal. loss of the gli3 repressor leads to the formation of generic (non-individualized) digits in extra quantities. dorsoventral patterning dorsoventral patterning is mediated by wnt7a signals in the overlying ectoderm not the mesoderm. wnt7a is both necessary and sufficient to dorsalize the limb. wnt7a also influences the craniocaudal and loss of wnt7a causes the dorsal side of limbs to become ventral sides and causes missing posterior digits. replacing wnt7a signals rescues this defect. wnt7a is also required to maintain expression of shh. wnt7a also causes lmx1b, a lim homebox gene (and thus a transcription factor), to be expressed. lmx1b is involved in dorsalisation of the limb, which was shown by knocking out the lmx1b gene in mice. the mice lacking the lmx1b produced ventral skin on both sides of their paws. 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.