Role of LIM homeobox genes, Lhx6 and L3/Lhx8 in maxillary and mandibular development in chick embryo

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LIM-homeobox containing (Lhx) genes constitute transcriptional regulators that play critical roles in the control of pattern formation and cell type specification. We have identified two genes of chicken Lhx genes, designated Lhx6 and L3/Lhx8, which were widely expressed in the craniofacial region. Whole-mount in situ hybridization showed Lhx6 and L3/Lhx8 were expressed from Stage 14 to 31 in restrict domains of the maxillary and mandibular processes but not in the frontonasal mass. Frozen sections revealed these signals in the mesenchyme underneath the epithelium. To determine whether the expression of Lhx6 and L3/Lhx8 in the maxilla and mandibular primordia requires signals from the overlying oral epithelium, transplant of Stage 25 chick embryo maxilla/mandibular process into the limb bud was performed in which the mesenchyme was grown in the presence or absence of oral epithelium. The grafts of maxilla and mandibular mesenchyme with the epithelium resulted in significant levels of Lhx6 and L3/Lhx8 expressions. No expression of Lhx6 and L3/Lhx8 was still shown in those grafts of epithelium-free mesenchyme. In order to further examine the distribution of signaling molecules from oral epithelium, soaked bead in Fgf-8 or Tgf-β3 was implanted into either maxillary mesenchymal graft from which the epithelium had been removed. Tgf-β3 beads as well as Fgf-8 induced expressions of L3/Lhx8 in the grafts. Overall our data suggested that expression of Lhx6 and L3/Lhx8 in the facial primordia required the epithelial-mesenchymal interactions. Fgf-8 and Tgf-β3 may be capable of regulating the expression in the maxillary processes.