Fig. 8. Preliminary model of signaling interactions that pattern the mouse cerebral cortex. Transcription factors EMX2, LHX2 and FOXG1 expressed in the early cortical primordium regulate Bmp or Nog expression and thereby influence BMP4/7 activity. BMP signaling is active early in choroid plexus development, perhaps specifying the choroid plexus epithelial cell fate. BMP4 signaling further regulates expression of Fgf8/17. FGF8/17 signaling is crucial for neocortical area patterning by regulating the AP axis of the cortical primordium, and at least indirectly, for hippocampal development. FGF8/17 inhibits WNT gene expression; canonical WNT signaling at the cortical hem is required for hippocampal development (Boncinelli et al., 1993; Bulchand et al., 2001; Dou et al., 1999; Fukuchi-Shimogori and Grove, 2001; Fukuchi-Shimogori and Grove, 2003; Galceran et al., 2000; Garel et al., 2003; Hebert et al., 2002; Lee et al., 2000; Mallamaci et al., 1998; Monuki et al., 2001; Panchision et al., 2001; Shimamura et al., 1995; Shimamura and Rubenstein, 1997; Storm et al., 2003).