*2.5. Spongiotrophoblast Cells and Glycogen Trophoblasts are Reduced in KITD816V Placentas*

Next, we analyzed the development of the spongiotrophoblast layer in more detail. In situ hybridization revealed that the spongiotrophoblast marker Trophoblast Specific Protein Alpha (TPBPA) is hardly present in KITD816V placentas compared to WT placentas at both timepoints analyzed (Supplementary Materials Figure S1B). Also, expression of *Tpbpa* was significantly reduced at E10.5 and E11.5. Further, we investigated the expression of Gap Junction Beta 3-protein (*Gjb3)* and Procadherin 12 (*Pcdh12)*, both markers for glycogen trophoblasts (GlyT). They were significantly decreased in KITD816V placentas on days E10.5 and E11.5 (Supplementary Materials Figure S1C–E). Thus, these results suggest that expression of KITD816V not only blocks cellular proliferation but also heavily impinges on the fine-balanced differentiation of the various subtypes essential for placental proper development. The differentiation to P-TGCs seems increased at the expense of other TGC subtypes such as C-TGCs and SpA-TGCs as well as spongiotrophoblast and GlyT cells.
