The frontonasal prominence of the developing avian embryo contains an organizing center, defined by juxtaposition of the Sonic hedgehog (Shh) and Fibroblast growth factor 8 (Fgf8) expression domains. This molecular interface presages any detectable growth of the frontonasal prominence, and experiments involving transplantation of this boundary epithelium have demonstrated it is a source of dors...

We review investigations that have lead to a model of how the ventral spinal cord of higher vertebrate embryos is patterned during development. Central to this model is the secreted morphogen protein, Sonic hedgehog. There is now considerable evidence that this molecule acts in a concentration-dependent manner to direct the development of the spinal cord. Recent studies have suggested that two ...

A crucial issue in limb development is how a correct set of precisely shaped digits forms in the digital plate. This process relies on patterning across the anterior-posterior axis of the limb bud, which is under the control of Sonic hedgehog emanating from the zone of polarizing activity. Recently, Sonic hedgehog function in the limb bud has been shown to have a dual character controlling both...

Throughout the developing nervous system, neural stem and progenitor cells give rise to diverse classes of neurons and glia in a spatially and temporally coordinated manner. In the ventral spinal cord, much of this diversity emerges through the morphogen actions of Sonic hedgehog (Shh). Interpretation of the Shh gradient depends on both the amount of ligand and duration of exposure, but the mec...

Little is known about how patterns of cell types are organized to form brain structures of appropriate size and shape. To study this process, we employed in vivo electroporation during midbrain development to create ectopic sources of Sonic Hedgehog, a signaling molecule previously shown to specify different neuronal cell types in a concentration-dependent manner in vitro. We provide direct evi...

Mice deficient for the homeobox gene Six1 display defects in limb muscles consistent with the Six1 expression in myogenic cells. In addition to its myogenic expression domain, Six1 has been described as being located in digit tendons and as being associated with connective tissue patterning in mouse limbs. With the aim of determining a possible involvement of Six1 in tendon development, we have...

Sonic hedgehog is involved in vertebrate tissue interactions during development. During early feather development, Sonic hedgehog appears very early in epithelial placodes. During late feather development, Sonic hedgehog expression precedes the development of the marginal plates and is specifically localized in the marginal plate epithelium, which will later undergo cell death. By using retrovi...

The earliest known left-right asymmetric genes are expressed at Hensen's node during chick gastrulation. Gene expression following reorientation of the node shows asymmetry is instructed by adjacent tissue, hence left-right information originates outside the node. Subsequently, the node signals back to the lateral tissue, initiating a cascade leading to left-sided expression of nodal in the lat...

Axons must switch responsiveness to guidance cues during development for correct pathfinding. Sonic Hedgehog (Shh) attracts spinal cord commissural axons ventrally toward the floorplate. We show that after crossing the floorplate, commissural axons switch their response to Shh from attraction to repulsion, so that they are repelled anteriorly by a posterior-high/anterior-low Shh gradient along ...