DISTRIBUTION OF SONIC-HEDGEHOG PEPTIDES IN THE DEVELOPING CHICK AND MOUSE EMBRYO

Sonic hedgehog (Shh) encodes a signal that is implicated in both short - and long-range interactions that pattern the vertebrate central nerv ous system (CNS), somite and limb, Studies in vitro indicate that Shh protein undergoes an internal cleavage to generate two secreted peptid es, We have investigated the distribution of Shh peptides with respect to these patterning events using peptide-specific antibodies, Immunos taining of chick and mouse embryos indicates that Shh peptides are exp ressed in the notochord, floor plate and posterior mesenchyme of the l imb at the appropriate times for their postulated patterning functions , The amino peptide that is implicated in intercellular signaling is s ecreted but remains tightly associated with expressing cells, The dist ribution of peptides in the ventral CNS is polarized with the highest levels of protein accumulating tow ards the luminal surface. Interesti ngly, Shh expression extends beyond the floor plate, into ventro-later al regions from which some motor neuron precursors are emerging, In th e limb bud, peptides are restricted to a small region of posterior-dis tal mesenchyme in close association with the apical ectodermal ridge; a region that extends 50-75 mu m along the anterior-posterior axis, Te mporal expression of Shh peptides is consistent with induction of scle rotome in somites and floor plate and motor neurons in the CNS, as wel l as the regulation of anterior-posterior polarity in the limb, Howeve r, we can find no direct evidence for Long-range diffusion of the 19x1 0(3) M(r) peptide which is thought to mediate both short- and long-ran ge cell interactions, Thus, either long-range signaling is mediated in directly by the activation of other signals, or alternatively the low levels of diffusing peptide are undetectable using available technique s.