Oct4 expression during the mouse life cycle. Cells and tissues expressing Oct4 are marked in green. Oct4 is expressed in mouse oocytes as a maternal transcript and protein. Zygotic Oct4 expression is activated prior to the 8-cell stage and is abundant and uniform in all cells of the embryo throughout the morula stage. However, as the outer cells of the embryo differentiate into the TE, Oct4 expression is restricted to cells of the ICM in the blastocyst. After implantation, Oct4 expression is maintained in the epiblast. Finally, Oct4 expression becomes restricted to primordial germ cells (PGCs), which are first specified in the extraembryonic mesoderm at the base of the allantoic bud during gastrulation. PGCs give rise to gametes, which following fertilization will develop into a new organism of next generation. Oct4 expression is based on data previously reported by several studies
[6, 7, 9–11].