Here is a hypothesis.  Fig.6A diagrammatically shows the developing premaxilla, which consists of the right and left portions, separated by the midline suture (solid arrow) and the incisal canal (I).  As well-known, the developing deciduous incisor tooth buds (A, B) give rise palatally to permanent tooth buds (1,2), respectively (open arrows).  For unknown reasons, the developing permanent central incisor tooth bud produces medially an extra tooth bud (mesiodens, green circle in Fig. 6B).  The cortices of the developing midline suture appears to be a barrier for the developing mesiodens to cross over.  Unlike deciduous or permanent teeth, the development mesiodens apparently does not have its definitive growth pattern (probably due to its absence of deciduous precursor).  It may grow upward (Fig.1), downward (Fig.2) or palatally (Fig.4,6C) along suture cortex, return to main article

Xin Wei, DDS, PhD, MS 1st edition 12/16/2011, last revision 12/17/2011