The Tabby mutation leads to abnormal crown morphology in the developing molars. To identify cusps which were altered in number, size, and position in the first lower molars of mutant mice, we analyzed the patterning of odontoblast differentiation using morphological criteria on serial sections and 3D reconstructions. In wildtype mice, polarized and functional odontoblasts were first observed in the median L2 and B2 cusps, then in the distal cusps L3 and B3, and finally in L1, B1, and 4. In Tabby mice, terminal differentiation of odontoblasts was retarded by 24-36 hours compared with wild-type mice. Polarized odontoblasts first appeared in the most mesial part of the tooth and progressively extended distally. The mesial part of the M1 in Tabby fetuses may correspond to the L2, B2 area from wild-type mice. The ante-molar dental primordium observed in some samples would thus represent remnants of cusps L1 and B1.