Evolutionary implications of the occurrence of two vestigial tooth germs during early odontogenesis in the mouse lower jaw.

The study of closely-spaced developmental stages reveals the occurrence of three distinct dental segments during early odontogenesis in the ICR mouse lower jaw: the mesial (MS), the second rudimentary (R2), and the molar segments. At embryonic day (ED) 12.5, the MS displays an accessory bud, which regresses rapidly and disappears at ED 13.5. The R2 segment reaches a wide bud stage at ED 13.5 and then merges with the mesial end of the emerging first lower molar (M1) cap before ED 15.0. The MS and R2 segments never develop into functional teeth and are classified as vestigial tooth germs. Depending on their developmental chronology and on the position they occupy along the prospective mandibular tooth row, MS and R2 segments are putatively assigned to primordia of a third (dP3) and fourth (dP4) lower deciduous premolar, respectively. Evolutionary implications of these developmental data are discussed.