Asymmetric divisions are key to regulating the number and patterning of stomata in Arabidopsis thaliana (L.) Heynh. Many formative asymmetric divisions take place in neighbor cells (NCs), cells adjacent to a stoma or stomatal precursor. TOO MANY MOUTHS is a receptor-like protein required for the correct plane of NC division, resulting in the placement of the new precursor distal to the pre-existing stoma. Because plant cells usually become polarized before asymmetric division, we studied whether NCs display a cytological asymmetry as a function of cell stage and of possible division behavior. Cells that divided in the developing leaf epidermis were smaller than 400 micro m(-2) in area and included NCs as well as isolated cells. All NCs in the youngest complexes divided with comparable frequencies, but divisions became restricted to the smaller and most recently produced NCs as the stomatal complex matured. The majority of developing NCs had distally located nuclei, suggesting that nuclear position is actively regulated in NCs. NC stages exhibiting distally located nuclei were the likeliest to divide asymmetrically. However, a distal nucleus did not necessarily predict an asymmetric division, because more NCs had distal nuclei than were likely to divide. No defect was detected in nuclear distribution in tmm NCs. These data suggest that TMM uses intercellular signals to control the plane of asymmetric division after or independently of nuclear positioning.