Vertebrate nervous system posteriorization: Grading the function of Wnt signaling

The establishment of anteroposterior identity in the vertebrate neural plate has been a subject of investigation for decades, but molecular explanations of posteriorization were only revealed beginning in the late 1980s. A model has emerged from several key studies that identifies Wnt signaling as a key posteriorizing agent, which evidence suggests specifies anteroposterior fates in a concentration-dependent manner. In this review, we consider the historical context of posteriorization studies and evaluate models for Wnt-dependent posteriorization. With new information about the mode of delivery of many signaling ligands, we propose alternative scenarios to reconcile the Wnt gradient model with the complex process of gastrulation and potential non-secretory mechanisms of Wnt delivery.