Fig. 4. Loss of Spry4 function alone is not sufficient to maintain continuous production of ameloblasts on the lingual side of the incisor. (A-C) Sagittal section of postnatal day (P) 5 Spry4^-/- incisor. The areas boxed in A are shown at higher magnification in B and C. Note that unlike on the labial side, the lingual ameloblast layer does not extend to the posterior end of the incisor. (D-G) RNA in situ hybridization assays for Shh and Fgf3 expression in paraffin sections of P2 incisors of the genotypes indicated. Yellow asterisks indicate mesenchymal expression, and black arrows demarcate the extent of epithelial expression in these panels. Note that no ectopic expression of either gene is detected in the Spry4^-/- incisors at this stage. (H) Frequency of the tusk phenotype (presence of lingual enamel) in Spry4^-/-;Spry2^+/- adult mice, and the effects of reducing the dosage of Fgf9 or Fgf10. The Spry4^-/-;Spry2^+/- animals evaluated were pooled from both crosses used to generate animals with reduced FGF gene dosage. P-values were calculated using Fisher's exact test. Am, ameloblasts; Od, odontoblasts.