Limb Initiation and Development Is Normal in the Absence of the Mesonephros

AbstractWith rapid progress in understanding the genes that control limb development and patterning interest is becoming focused on the factors that permit the emergence of the limb bud. The current hypothesis is that FGF-8 from the mesonephros induces limb initiation. To test this, the inductive interaction between the Wolffian duct and intermediate mesoderm was blocked rostral to the limb field, preventing mesonephric differentiation while maintaining the integrity of the limb field. The experimental outcome was monitored by following expression ofcSim1andLmx1,molecular markers for the duct and the mesonephros, respectively. Evidence is presented that the intermediate mesoderm undergoes apoptosis when the inductive interaction with the Wolffian duct is blocked.fgf-8expression was undetectable in the mesonephric area of embryos with confirmed absence of mesonephros; nevertheless, limb buds formed and limb development was normal. The mesonephros in general, and specifically itsfgf-8expression, was shown to be unnecessary for limb initiation and development; the hypothesis linking the mesonephros and limb development is not supported. Further studies of axial influences on limb initiation should now concentrate on medial structures such as Hensen's node and paraxial mesoderm; the alternative that no axial influences are required should also be examined

Manifestation of the Limb Prepattern: Limb Development in the Absence of Sonic Hedgehog Function

AbstractThe secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior–posterior limb polarity. In the Shh−/− mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. In this study, we report the analysis of Shh−/− mutant limbs in detail. Each mutant embryo has four limbs with recognizable humerus/femur bones that have anterior–posterior polarity. Distal to the elbow/knee joints, skeletal elements representing the zeugopod form but lack identifiable anterior–posterior polarity. Therefore, Shh specifically becomes necessary for normal limb development at or just distal to the stylopod/zeugopod junction (elbow/knee joints) during mouse limb development. The forelimb autopod is represented by a single distal cartilage element, while the hindlimb autopod is invariably composed of a single digit with well-formed interphalangeal joints and a dorsal nail bed at the terminal phalanx. Analysis of GDF5 and Hoxd11–13 expression in the hindlimb autopod suggests that the forming digit has a digit-one identity. This finding is corroborated by the formation of only two phalangeal elements which are unique to digit one on the foot. The apical ectodermal ridge (AER) is induced in the Shh−/− mutant buds with relatively normal morphology. We report that the architecture of the Shh−/− AER is gradually disrupted over developmental time in parallel with a reduction of Fgf8 expression in the ridge. Concomitantly, abnormal cell death in the Shh−/− limb bud occurs in the anterior mesenchyme of both fore- and hindlimb. It is notable that the AER changes and mesodermal cell death occur earlier in the Shh−/− forelimb than the hindlimb bud. This provides an explanation for the hindlimb-specific competence to form autopodial structures in the mutant. Finally, unlike the wild-type mouse limb bud, the Shh−/− mutant posterior limb bud mesoderm does not cause digit duplications when grafted to the anterior border of chick limb buds, and therefore lacks polarizing activity. We propose that a prepattern exists in the limb field for the three axes of the emerging limb bud as well as specific limb skeletal elements. According to this model, the limb bud signaling centers, including the zone of polarizing activity (ZPA) acting through Shh, are required to elaborate upon the axial information provided by the native limb field prepattern