Differentiation of mouse embryonic stem cells in vitro: III. Morphological evaluation of tissues developed after implantation of differentiated mouse embryoid bodies

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Abstract
Mouse embryonic stem cells (ES) were allowed to differentiate in a liquid culture system. After 2–3 weeks, complex cystic embryoid bodies developed. These bodies were composed of several structures identified as cardiac muscle and yolk sac blood islands as well as cupshape compartments containing a mixed population of hematopoietic stem cells. When these cystic embryoid bodies were implanted into adult mice, either subcutaneously or under the kidney capsule, they developed into various tissues. These included bone, blood vessels, cardiac muscle, nerves, and skin with hair follicles. In addition, highly differentiated, complicated tissues resembling intestinal epithelium with mucus glands or salivary glandular tissue were derived. The ES tissues from these in vitro developed embryoid bodies developed quickly within 2 to 3 weeks of implantation. This is in contrast to a minimal of 6 weeks for teratocarcinomas derived from embryonic carcinoma cells and/or the direct implantation of undifferentiated embryonic stem cells. Moreover, we found that there are different types of tissue developed upon different sites of implantation. The data suggest a local environment and/or growth factors are influential for ES tissue development. This system provides a possible means to purify and identify stem cells that give rise to specific tissues, and to study the factors regulating the commitment of these stem cells.