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Fig. 3. Tooth phenotype in newborn heads of ActR[IIA+/-;IIB-/-] (A,D) and Smad2+/- (C,F) pups. Corresponding frontal sections of wild-type littermates are shown in B,E. Both maxillary (A,C) and mandibular incisors (data not shown) are missing from the heterozygotes. Mandibular molars are lost from the heterozygotes in D,F, whereas maxillary molars develop normally. mol, molar; nc, nasal cartilages; ns, nasal septum; oc, oral cavity; tg, tongue. Note that the oral cavity in the heterozygotes is grossly mis-shapen. Furthermore, remnants of bone (arrowheads, asterisk) and muscle masses take the place of the mandibular bone.