The mouse F9 teratocarcinoma cell line is used to mimic primitive endoderm differentiation and the subsequent parietal endoderm differentiation seen in vivo during early mouse development. This is achieved by treating undifferentiated F9 cells with the morphogen retinoic acid (RA) to form primitive endoderm and with RA and dibutyryl cyclic AMP to form parietal endoderm. The in vitro F9 model has been studied extensively because the differentiation from primitive to parietal endoderm is one of the earliest epithelial-to-mesenchymal transitions seen in mouse development. RA has been shown to upregulate Wnt6 in F9 cells, which signals through the canonical (3-catenin pathway. The receptor responsible for binding Wnt6 is unknown, but it is hypothesized that Frizzled 7 transduces the Wnt6 signal. Fzd.7 expression was shown to be upregulated in primitive and parietal endoderm and over-expression of Fzd7 in undifferentiated cells was able to induce biochemical, molecular and morphological markers of primitive endoderm. The over-expression of Fzd7 was able to activate the canonical (3-catenin pathway as seen by an increase in phospho-GSK3 levels, but not the non-canonical planar cell polarity pathway. Furthermore, these Fzd7 treated cells were competent to complete EMT and form PE upon subsequent treatment with db-cAMP. These results show that Fzd7 is sufficient to induce PrE through the canonical (3-catenin Wnt signaling pathway and is a likely candidate to be the receptor for Wnt6
