Fig. 5. Epithelial-mesenchymal interactions regulate Sema3a expression in the mandibular and dental mesenchyme. Expression of Sema3a in the E10.5 head (A1,A2) and E11.5 (E1,E2) and E12.5 (I1-I2) mandibular molar tooth germs, as well as in cultured mandibular and dental mesenchymes and tissue recombinants analyzed by in situ hybridization from frontal sections after 24 hours (B1-D2,F1,F2,H1,H2,J1,J2,K1,K2), 2 days (G1-G2) and 3 days (L1-L2) of culture. Bright- and dark-field images. (B1-B2) No specific Sema3a expression is seen in the E10.5 mandibular mesenchyme cultured alone without epithelium (compare also images in Fig. 5J1-L2 cultured with BSA-soaked beads and Wnt6 and NIH3T3 cells), while in the cultured intact mandible a prominent mesenchymal Sema3a expression is present (C1-C2). (D1-D2) E10.5 oral epithelium has induced Sema3a in the underlying E10.5 molar area of mandibular mesenchyme, which is devoid of Sema3a (arrows in A1). (F1,F2) In cultured E11 mandibular mesenchyme, Sema3a transcripts are largely absent from the presumptive molar area (arrow), whereas the remaining Sema3a is seen in the deep aboral mesenchyme. (G1-G2,L1-L2) Sema3a induction is observed next to the E11 and E12 dental epithelia, which were placed onto the proximal, molar region of E10.5 mandibular mesenchyme. (H1-K2) A prominent Sema3a expression is seen in the E11 jaw and E12 dental mesenchyme under the E11 and E12 dental epithelia, respectively, while some endogenous Sema3a is observed in the dental mesenchyme explant. The expression patterns of Sema3a in these homochronic recombinant explants correlate with the Sema3a expression in in vivo developed teeth (compare E1,E2 with I1,I2). de, dental epithelium; dm, dental mesenchyme; m, mandibular mesenchyme; Md, mandibular process, Mx, maxillary process; oe, oral epithelium. Scale bars: 100 µm (200 µm for D2).