Figure 1.

Cleavage plane changes as neurogenesis proceeds. As VZ cells near mitosis, they round up at the VZ surface (A). During early neurogenesis, most divisions occur with vertical cleavage planes and presumably yield two symmetrical progenitor daughter cells (B and D). Later in neurogenesis, horizontal cleavage planes are thought to generate basal daughters that are postmitotic neurons (C and E). The VZ surface is at the bottom of the images in D and E. A scheme proposed by Chenn and McConnell (10) correlates the orientation of mitotic cleavage with daughter cell fate (F). Live imaging experiments (see Movie 3) confirm both the relationship between final cleavage plane and daughter cell behavior as well as the existence of boundaries at the 45° and 135° orientations.