SummaryBMP is thought to induce hESC differentiation toward multiple lineages including mesoderm and trophoblast. The BMP-induced trophoblast phenotype is a long-standing paradox in stem cell biology. Here we readdressed BMP function in hESCs and mouse epiblast-derived cells. We found that BMP4 cooperates with FGF2 (via ERK) to induce mesoderm and to inhibit endoderm differentiation. These conditions induced cells with high levels of BRACHYURY (BRA) that coexpressed CDX2. BRA was necessary for and preceded CDX2 expression; both genes were essential for expression not only of mesodermal genes but also of trophoblast-associated genes. Maximal expression of the latter was seen in the absence of FGF but these cells coexpressed mesodermal genes and moreover they differed in cell surface and epigenetic properties from placental trophoblast. We conclude that BMP induces human and mouse pluripotent stem cells primarily to form mesoderm, rather than trophoblast, acting through BRA and CDX2.

Recently, various approaches for controlling the embryonic stem (ES) cell microenvironment have been developed for regulating cellular fate decisions. It has been reported that the lineage specific differentiation could be affected by the size of ES cell colonies and embryoid bodies (EBs). However, much of the underlying biology has not been well elucidated. In this study, we used microengineered hydrogel microwells to direct ES cell differentiation and determined the role of WNT signaling pathway in directing the differentiation. This was accomplished by forming ES cell aggregates within microwells to form different size EBs. We determined that cardiogenesis was enhanced in larger EBs (450 m in diameter), and in contrast, endothelial cell differentiation was increased in smaller EBs (150 m in diameter). Furthermore, we demonstrated that the EB-size mediated differentiation was driven by differential expression of WNTs, particularly noncanonical WNT pathway, according to EB size. The higher expression of WNT5a in smaller EBs enhanced endothelial cell differentiation. In contrast, the increased expression of WNT11 enhanced cardiogenesis. This was further validated by WNT5a-siRNA transfection assay and the addition of recombinant WNT5a. Our data suggest that EB size could be an important parameter in ES cell fate specification via differential gene expression of members of the noncanonical WNT pathway. Given the size-dependent response of EBs to differentiate to endothelial and cardiac lineages, hydrogel microwell arrays could be useful for directing stem cell fates and studying ES cell differentiation in a controlled manner.hydrogel microwells ͉ stem cell differentiation ͉ WNT signal pathway T he developmental versatility of embryonic stem (ES) cells offers a powerful approach for directing cell fate and is a promising source of progenitors for cell replacement therapy and tissue regeneration (1). ES cells can differentiate into a wide spectrum of cell types, such as cardiomyocytes and endothelial cells, by forming embryoid bodies (EBs) (2). Those lineages arise from distinct mesoderm subpopulations that develop sequentially from premesoderm cells (3). Such lineage specification is highly coordinated with differential changes in gene expression (4-8).Despite the therapeutic potential of ES cells, one of the significant challenges to their widespread clinical use, is the inability to homogeneously direct ES cell differentiation into specific lineages. One reason for the heterogeneity in EB differentiation is caused from variations in EB size (9, 10). To address this challenge for controlling the differentiation of ES cells, various microscale technologies (i.e., surface patterning, hydrogel microwells, and microfluidic systems) have been developed for directing the stem cell fate (11-15). Micropatterning techniques have been used to evaluate the effect of EB size on ES cell differentiation. For instance, microfabricated adhesive stencils were used to pattern ES cells for controlling initial ES cell aggregate sizes...