Hubrecht Institute-Royal Netherlands Academy of Arts and Sciences and University Medical Center Utrecht, Uppsalalaan 8, Utrecht, The Netherlands.
Development. 2012 Mar;139(5):871-82. doi: 10.1242/dev.071688. Epub 2012 Jan 25.
At the blastocyst stage of mammalian pre-implantation development, three distinct cell lineages have formed: trophectoderm, hypoblast (primitive endoderm) and epiblast. The inability to derive embryonic stem (ES) cell lines in a variety of species suggests divergence between species in the cell signaling pathways involved in early lineage specification. In mouse, segregation of the primitive endoderm lineage from the pluripotent epiblast lineage depends on FGF/MAP kinase signaling, but it is unknown whether this is conserved between species. Here we examined segregation of the hypoblast and epiblast lineages in bovine and human embryos through modulation of FGF/MAP kinase signaling pathways in cultured embryos. Bovine embryos stimulated with FGF4 and heparin form inner cell masses (ICMs) composed entirely of hypoblast cells and no epiblast cells. Inhibition of MEK in bovine embryos results in ICMs with increased epiblast precursors and decreased hypoblast precursors. The hypoblast precursor population was not fully ablated upon MEK inhibition, indicating that other factors are involved in hypoblast differentiation. Surprisingly, inhibition of FGF signaling upstream of MEK had no effects on epiblast and hypoblast precursor numbers in bovine development, suggesting that GATA6 expression is not dependent on FGF signaling. By contrast, in human embryos, inhibition of MEK did not significantly alter epiblast or hypoblast precursor numbers despite the ability of the MEK inhibitor to potently inhibit ERK phosphorylation in human ES cells. These findings demonstrate intrinsic differences in early mammalian development in the role of the FGF/MAP kinase signaling pathways in governing hypoblast versus epiblast lineage choices.
在哺乳动物胚胎着床前发育的囊胚阶段，已经形成了三个不同的细胞谱系：滋养外胚层、下胚层（原始内胚层）和上胚层。在多种物种中，无法获得胚胎干细胞（ES 细胞）系，这表明在早期谱系特化中涉及的细胞信号通路方面，物种之间存在分歧。在小鼠中，原始内胚层谱系与多能上胚层谱系的分离取决于 FGF/MAP 激酶信号，但尚不清楚这是否在物种间保守。在这里，我们通过在培养的胚胎中调节 FGF/MAP 激酶信号通路，检查了牛和人类胚胎中原始内胚层和上胚层谱系的分离。用 FGF4 和肝素刺激的牛胚胎形成完全由原始内胚层细胞组成而没有上胚层细胞的内细胞团（ICM）。在牛胚胎中抑制 MEK 会导致 ICM 中增加上胚层前体而减少原始内胚层前体。MEK 抑制后，原始内胚层前体并未完全被消除，这表明其他因素也参与了原始内胚层的分化。令人惊讶的是，在上游抑制 MEK 的 FGF 信号对牛发育中的上胚层和原始内胚层前体数量没有影响，这表明 GATA6 的表达不依赖于 FGF 信号。相比之下，在人类胚胎中，尽管 MEK 抑制剂能够强烈抑制人 ES 细胞中 ERK 的磷酸化，但抑制 MEK 并没有显著改变上胚层或原始内胚层前体的数量。这些发现表明，在早期哺乳动物发育中，FGF/MAP 激酶信号通路在调控原始内胚层与上胚层谱系选择中的作用存在内在差异。
