Fgf10 Gene Dosage from a Single Allele Is Insufficient for Forming Multilayered Epithelial Cells in the Murine Lacrimal Gland

Mutations in the fibroblast growth factor 10 (<i>FGF10</i>) gene in humans cause aplasia of the lacrimal and salivary glands (ALSG). In patients with ALSG, heterozygous loss-of-function mutations are found, and <i>FGF10</i> haploinsufficiency results in the absence of these secretory organs. Lacrimal glands (LGs) are formed through epithelial thickening, budding, and branching morphogenesis. To compare the variable phenotypes of the <i>Fgf10^+/-</i> Harderian glands (HGs) previously reported, we examined the development of LGs in wild-type (WT), <i>Fgf10^+/-</i>, and <i>Fgf10-</i>null mice. Pax6 immunostaining was performed to visualize the LG primordia from embryonic day 15.5 (E15.5) onwards. In situ hybridization of the genes encoding the epithelial receptor of FGF10, FGFR2b, and its other ligands was performed to determine their potential involvement in LG development. LG primordia were not observed in <i>Fgf10^+/-</i> mice bilaterally at E16.5 or later stages. At E15.5, budding from the developing conjunctival epithelium (CE) was observed in a small fraction of the <i>Fgf10^+/-</i> LG primordia. In contrast, the <i>Fgf10</i>-null CE failed to promote budding. Among <i>Fgf1</i>, <i>Fgf3</i>, <i>Fgf7</i>, <i>Fgf10</i>, and <i>Fgf22</i>, <i>Fgf10</i> was expressed in the mesenchyme surrounding developing LG epithelial cells, whereas <i>Fgf1</i> was expressed in the LG epithelium of WT mice. <i>Fgf7</i> was initially expressed in the mesenchyme surrounding the nascent LG epithelium, but its expression subsequently became diffused. Thus, we conclude that among the FGFR2b ligands, initial LG formation is dependent on the mesenchymal factors FGF10 and FGF7, and FGF1 is likely to function as an epithelial factor in the LG primordia. A single allele of <i>Fgf10</i> was found to be insufficient to support the budding process during LG morphogenesis.