A local Wnt-3a signal is required for development of the mammalian hippocampus

The mechanisms that regulate patterning and growth of the developing cerebr al cortex remain unclear Suggesting a role for Wnt signaling in these proce sses, multiple Wnt genes are expressed in selective patterns in the embryon ic cortex. We have examined the role of Wnt-3a signaling at the caudomedial margin of the developing cerebral cortex, the site of hippocampal developm ent. We show that Wnt-3a acts locally to regulate the expansion of the caud omedial cortex, from which the hippocampus develops. In mice lacking Wnt-3a , caudomedial cortical progenitor cells appear to be specified normally, bu t then underproliferate, By mid-gestation, the hippocampus is missing or re presented by tiny populations of residual hippocampal cells. Thus, Wnt-3a s ignaling is crucial for the normal growth of the hippocampus. We suggest th at the coordination of growth with patterning may be a general role for Wnt s during vertebrate development.