Emx2 is required for growth of the hippocampus but not for hippocampal field specification

The vertebrate Emx genes are expressed in a nested pattern in early embryon ic cerebral cortex, such that a medial strip of cortex expresses Emx2 but n ot Emx1. This pattern suggests that Emx genes could play a role in specifyi ng different areas or fields of the cortex along the mediolateral axis. Suc h a role has been supported by the observation that in mice lacking functio nal Emx2 the hippocampus is shrunken and the most medial field of the corte x, the hippocampal dentate gyrus, appears by cytoarchitecture to be missing (Pellegrini et al., 1996; Yoshida et al., 1997). Use of region-specific mo lecular markers shows, however, that hippocampal fields are specified and c orrectly positioned in the Emx2 mutant. In particular, a dentate cell popul ation is generated, although it fails to form a morphological gyrus. This f ailure may be part of a more widespread medial cortical defect in the mutan t. Examination of cortical cell proliferation and differentiation indicates a disruption of the maturation of the medial cortex in the absence of Emx2 . Thus, Emx2 is required for normal growth and maturation of the hippocampu s but not for the specification of cells to particular hippocampal field id entities.