Biomedical engineering: In vitro amniogenesis.

The amniotes are a vertebrate clade including reptiles, birds and mammals, whose embryos develop from eggs that are either laid on land or nurtured within the maternal environment. The production of large eggs with stores of fluid and nutrients and, later, placentation were evolutionary breakthroughs that allowed for more extended and complex embryonic development in the terrestrial environment. These innovations were enabled by the emergence and specialization of the extra-embryonic membranes — tissues derived from the zygote that support the embryo, allowing respiration, the uptake and processing of nutrients and fluids, and the elimination of waste. The amnion (the origin of the term amniote) is one of the extra-embryonic membranes, consisting of an epithelium and an outer mesodermal layer, which encapsulates a fluid-filled cavity surrounding the embryo and fetus (Fig. 1). The amnion allows fluid exchange, and the sac it forms facilitates movement of the developing fetus within a protective cushion. In humans, the amniotic epithelium appears early during the second week of development, deriving directly from the pluripotent cells of the epiblast. Owing to the inaccessibility of this stage of the human lifecycle to experimentation, the functions of the amnion early in development and the mechanism whereby it differentiates from the epiblast are unknown. A recent study presented compelling evidence showing that primordial germ cells (the precursors of eggs and sperm) arise from the amnion in monkeys1, an unanticipated finding that is indicative of the gaps in our understanding of primate embryology. Recent advances in three-dimensional organoid culture systems have enabled researchers to produce remarkable replicas of brain, gut, kidney and other tissues in the laboratory2. Through engineering of the microenvironment in vitro, several groups have even been able to mimic some early stages of post-implantation development of the human embryo3–5. Deborah Gumucio, Jianping Fu and colleagues now report in Nature Materials6 the use of a bilayered extracellular matrix culture system to allow in vitro formation of the amnion from human pluripotent stem cells, thus modelling a key event in early embryogenesis. They grew human pluripotent stem cells in a sandwich of extracellular matrices designed to mimic the physical environment of the implanting blastocyst. Cells grown between two thick layers of a matrix made up of basement membrane extract lost pluripotency markers and formed cysts composed of cells with morphology resembling the amniotic epithelium (Fig. 2). Control cultures grown on a bed of basement membrane extract alone, or between a bed of extract and a rigid glass surface coated with extract, formed structures with a lumen. They did not differentiate, however, suggesting that the elastic modulus of the surrounding basement membrane extract was critical for differentiation. Further proof of the importance of matrix rigidity is shown in experiments using an elastomer fabricated into an artificial matrix array of microposts, which allowed for precise control of the elastic modulus of the surface. Only those arrays that mimicked the ‘soft’ environment of the extracellular matrix gel supported differentiation. To identify the nature of the differentiated squamous epithelial cells, Amnion