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

The mechanisms that regulate patterning and growth of the developing cerebral cortex remain unclear. Suggesting a role for Wnt signaling in these processes, multiple Wnt genes are expressed in selective patterns in the embryonic cortex. We have examined the role of Wnt-3a signaling at the caudomedial margin of the developing cerebral cortex, the site of hippocampal development. We show that Wnt-3a acts locally to regulate the expansion of the caudomedial cortex, from which the hippocampus develops. In mice lacking Wnt-3a, caudomedial cortical progenitor cells appear to be specified normally, but then underproliferate. By mid-gestation, the hippocampus is missing or represented by tiny populations of residual hippocampal cells. Thus, Wnt-3a signaling is crucial for the normal growth of the hippocampus. We suggest that the coordination of growth with patterning may be a general role for Wnts during vertebrate development.

The generation of cellular diversity in the cerebral cortex

We have studied cell lineage in the rat cerebral cortex using retroviral vectors. With this technique, the virus is used to introduce a marker gene into dividing precursor cells such that their fate can be followed. We have studied cell lineage by using this method in the following ways. First, we have labelled germinal cells of the cerebral cortex in vivo during the period of neurogenesis. Second, we have grown cortical precursor cells in dissociated cell culture, and used the viral labelling technique to follow their development in vitro. Both types of study have shown that by the time neurogenesis is under way, the majority of precursor cells are restricted to the production of a single cell type: neurones, astrocytes, or oligodendrocytes. The only exception is a cell we call the N-O cell because it has the ability to generate both neurones and oligodendrocytes. These data suggest that the ventricular zone, the germinal layer of the embryonic cortex, is a mosaic of different precursor cells each with a different restricted potential. However, this restriction of potential of cortical precursor cells does not extend to their ability to contribute to more than one cortical lamina or cytoarchitectonic area. The precursors of both neurones and grey matter astrocytes contribute cells to multiple layers of the cortex. Moreover, in the hippocampal formation, neuronal precursors can contribute cells to more than one hippocampal field.