There are sensors that tell cells where they are,

who they are and how big.

http://www.nature.com/ncb/journal/v6/n9/abs/ncb1166.html

http://www.eurekalert.org/pub_releases/2010-05/ciot-foc051710.php

http://www.eurekalert.org/pub_releases/2010-03/jhmi-hda032510.php

http://www.sciencedaily.com/releases/2007/09/070928213650.htm

http://www.eurekalert.org/pub_releases/2007-07/wuis-rfp073007.php

http://www.eurekalert.org/pub_releases/2007-03/uof-llc030807.php

http://www.eurekalert.org/pub_releases/2003-08/bc-cbs080503.php

Regards,

Joachim