will proliferate in an undifferentiated state in the presence of a biochemical called leukemia inhibitory factor (LIF), but the culture conditions required to keep human ESCs from differentiating include growing them in petri dishes on a layer of mouse embryonic fibroblasts (referred to as “feeder cells”) in a medium containing serum from cows. The feeder cells are inactivated, so they are not dividing and expanding, but they produce growth factors that sustain the ESCs. The mechanism of how feeder cells maintain the proliferation of undifferentiated ESCs is unknown. Such in vitro culturing presents certain theoretical hazards to the use of stem cells for regenerative medicine, such as the spread of viruses and other infectious agents not normally found in humans. When removed from feeder cells and grown in suspension (in liquid), human ESCs form aggregated balls of cells called “embryonic bodies,” which have been reported to give rise to a multiplicity of cell types representing all three layers of embryonic tissue development (Itskovitz-Eldor et al., 2000; Reubinoff et al., 2000; Schuldiner et al., 2000). Evidence of the differentiation in culture includes detection of the products of genes associated with different cell types and in some cases by the characteristic shapes that are peculiar to different cell types. Cells derived from human embryonic bodies include “rhythmically contracting cardiomyocytes, pigmented and nonpigmented epithelial cells, and neural cells displaying an exuberant outgrowth of axons and dendrites” (Odorico et al., 2001). In other experiments, cells arising from human ESCs have been reported to express genes associated with liver and pancreas function (Schuldiner et al., 2000). Human ESCs grown in coculture with mouse bone marrow stromal cells have been reported to produce colonies of human hematopoietic precursors and ultimately cells from the blood (Kaufman et al., 1999).

Further evidence of the multipotent capability of human ESCs is based on studies in an in vivo setting. Human ESCs injected into mice form a type of benign tumor called a teratoma that is made up of tissues from all three embryonic layers. The tissues that arise in the tumor are often advanced, organized, and complex, and include teeth, gut, hair follicles, skin, epithelium, muscle, bone, cartilage, lung tissue, and neural

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