Loss of Oct4 Expression During the Development of Murine Embryoid Bodies

We describe the internal organization of murine embryoid bodies (EBs) in terms of the structures and cell types formed as Oct4 expression becomes progressively lost. This is done by making the EBs from iPS cells carrying a novel Oct4 reporter (Oct4-MerCreMer;mTmG) which is inducible, sensitive, and permanent in all cellular progeny. When these EBs are treated with tamoxifen, the Oct4 expressing cells switch from a red to a green fluorescence color, and this is maintained thereafter by all their progeny. We show that there is no specific pattern in which Oct4 is downregulated, rather it appears to be spatially random. Many of the earliest cells to lose Oct4 expression stain positive for markers of visceral endoderm (DAB2, α-fetoprotein (AFP), HNF4). These are randomly located, although if endoderm differentiation is allowed to commence before EB formation then an external layer is formed. This is true both of EBs made from the reporter iPS cells, or from an embryo-derived mouse ES line (R1 cells). Markers of the early body axis, Brachyury (BRA) and FOXA2, usually showed a concentration of positive cells in one region of the EB, but the morphology is not predictable and there are also scattered cells expressing these markers. These patterns are similar in R1 cells. Use of the Oct4 reporter showed a difference between BRA and FOXA2. BRA, which marks the early mesoderm, node and notochord, arises in Oct4 expressing cells on days 3-4. FOXA2, which marks the floor plate of the neural tube and definitive endoderm, as well as the node and notochord, arises at the same time but mostly in cells that have already lost Oct4 expression. Several clumps of cardiomyocytes are visible by days 7-8 of EB development, both in our iPS cells and in R1 cells. Using the Oct4 reporter we show that the cells forming these clumps lose Oct4 expression between days 3 and 5. Overall, our results indicate that EBs recapitulate normal development quite well in terms of the tempo of events and the appearance of specific markers, but they do not resemble embryos in terms of their morphology.