Distribution of F-actin during mouse facial morphogenesis and its perturbation with cytochalasin D using whole embryo culture

Histological and experimental studies were performed in mouse embryos to elucidate possible roles of actin filaments in the nasal epithelium during facial morphogenesis. C57BL/6 mouse embryos (8.5-11.5 days of gestation) were fixed and frozen sections were stained with rhodamine-phalloidin. Before formation of the nasal placode, there was no specific localization of F- actin. After the nasal placode was formed, intense staining of F-actin was observed at the apical side of the placode. Conversely, it was located at the basal side of the epithelium of developing nasal prominences. By using the whole embryo culture system, perturbation experiments were conducted with cytochalasin D (CD), which inhibits the polymerization of actin filaments. When day-10 embryos were exposed to CD at several concentrations for 24 hr, fusion of nasal prominences was inhibited in a dose-dependent manner. Treatment with a high dose of CD for 2 hr also prevented the same development irreversibly. In contrast, when day-9 embryos were exposed to CD at several concentrations for 24 hr, invagination of the nasal placode was not perturbed at all. The results suggest that apical F-actin plays an essential role in maintaining the close apposition state of the nasal prominences and in the following fusion. During the invagination stage, F-actin might be important in maintaining the epithelial structure, but is not crucial to the initiation of placode invagination.