Figure 3. Altered morphology in MUT-M² and MUT-M₂ was even more evident at E18 (A–H). (A color version of this Fig. is included in the APPENDIX.) Histological images of frontal sections are also displayed for MUT-M¹ (I) and MUT-M₁ (J). By E18, WT epithelia of first (M¹ and M₁) and second (M² and M₂) molars had undergone morphological refinement to form the bell and early cap stages, respectively. Compared with preceding timepoints, the cervical loops (cl) of MUT-M₁ were more developed, resulting in a more substantive region of dental papilla (dp), yet lateral cervical loops were underdeveloped (arrowhead) compared with WT (G). Epithelium posterior to MUT-M₁ that normally gives rise to M₂ was severely hypoplastic (##), and a distinct MUT-M₂ was not readily apparent (G). MUT-M₁ ectopic epithelia (double stars), first seen at E14, was larger and extended along the entire lateral surface (E,H,J). As demonstrated at E15 and E16, MUT-M₁ was enlarged in every dimension compared with its corresponding M¹. Although MUT-M¹ and MUT-M² epithelia had the general outline of WT, these tissues were severely hypoplastic, including the cervical loops (arrowhead) and the epithelial connection between MUT-M¹ and MUT-M². Key: oral cavity (oc), anterior (ant), posterior (pos), medial (me), lateral (la).