Human embryonic stem cells and their spontaneous differentiation.

Human embryonic stem cells (hESCs) usually grow in saucer-shaped colonies with thickened rims and can form spherical human embryoid bodies (hEBs) under non-adherent conditions. A problem associated with ES cell culture is the spontaneous differentiation of cells into a variety of cell types representing all three germ layers, which is evident in both hESC colonies and hEBs. This presentation deals with the precise origins of hESCs and their spontaneous differentiation in vitro. We have used advanced digital microscopy, including transmission electron microscopy (TEM) to define the fine structure of these cells. We present images of undifferentiated hESCs and their spontaneous differentiation into basic embryonic cell types such as nerve, muscle, connective tissue, epithelium, and digestive tract progenitors, representing all three primary germ layers: embryonic ectoderm, mesoderm and endoderm. It appears that hESCs work in concert and interact with one another, as in tissue formation of the embryo. Our fine structural observations agree mostly with those of the Thomson group. Digital microscopy of plastic sections and TEM are invaluable tools in the precise characterization of cells forming these tissues and a combined study with immunofluorescent markers is most desirable.