Protocol for Placental Precursors

Abstract

The cells that give rise to the embryonic portion of the placenta, trophoblast stem cells, have been created from mouse fibroblasts, according to two new reports. The first differentiation event of the mammalian embryo separates the inner cell mass of the blastula—the embryo proper—from the outer layer of trophectoderm, which can give rise to trophoblast stem cells in culture. Studies of such stem cells have implicated a growing suite of transcription factors in trophoblast cell fate. In the new studies, combinations of these transcription factors were expressed ectopically in embryonic and adult mouse fibroblasts, generating a cocktail that could convert them into trophoblast stem cells. Caroline Kubaczka et al. [1] homed in on four of these factors (Tfap2c, Eomes, Gata3, and Ets2), and Hana Benchetrit et al. [2] used three (Tfap2c, Eomes, and Gata3), although a fourth factor—Myc—seemed to boost the efficiency of their protocol. Both groups converted embryonic and mouse fibroblasts into cells that expressed markers of trophoblast cells and had correct morphology; these stem cells could be maintained in culture for many generations after the initiating factors were withdrawn. Both groups analyzed, in detail, the stem cells derived from embryonic fibroblasts. These stem cells had gene expression profiles, and DNA methylation and other epigenetic characteristics—similar to trophoblast stem cells derived from blastocysts—could differentiate into trophoblast tissue and form placental tissue when implanted into blastocysts. The cellular conversions occurred in the absence of a common middle step: the generation of an induced pluripotent stem cell. The findings show that it is possible to convert, directly, a differentiated cell type (fibroblast) into another cell type (trophoblast stem cell), apparently without transiting through a pluripotent cell state. The research may also lead to protocols that generate human trophoblast stem cells—an elusive goal.