This chapter focuses on recognition and identification of the tissues arising from hESCs in experimental teratomas.

For experimental teratoma production, hESCs are typically transplanted to skeletal muscle, testes, or kidney capsule of immunodeficient mice (see Chapter 12). Teratomas are usually harvested 6-7 weeks after the injection of hESCs and are histologically composed of tissues derived from all three germ layers. The histology of teratomas is remarkable; the teratoma tissues often have the microscopic features of equivalent adult human somatic tissues, or fetal.

Most notably, many of the neural components of hESC-derived teratomas have the features of fetal neural tubes and immature brain tissue. An organoid arrangement of tissues may be seen occasionally but overall organogenesis is relatively uncommon in teratomas derived from hESCs. Most hESC teratomas do not contain undifferenti-ated hESCs, but occasionally foci of hESCs can be found in some xenografts. The significance of prolonged immaturity of neural components and the persistence of undifferentiated hESCs in some xenografts remains to be determined.

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