摘要：

Fertilization or parthenogenetic activation of the mouse egg initiates a series of cell divisions, which in the case of the fertilized egg ultimately results in the development of an adult individual. As the cell divisions proceed and cell numbers increase, differential gene expression is responsible for the phenotypic divergence of cells resulting in the formation of discrete cell types. This change is known as "differentiation." As the expression of specific genes becomes fixed within certain cell types, it is accompanied by the commitment, to a greater or lesser degree, of the differentiated cell types to specific lineages within the developing organism. Although the use of embryonal carcinoma (EC) cells as a model of early mammalian development has been well justified, their use should not preclude an appreciation of two important limitations of the experimental material. First, although most EC cell lines are phenotypically very similar, they encompass a wide spectrum of differentiation potentials, often associated with a variety of karyotypic changes. Second, EC cells are derived from tumors and they should, therefore, be considered as neoplastic, or at any rate an abnormal, cell type. The recent development of techniques that have facilitated the direct isolation and culture of pluripotential stem cells from the mouse embryo has presented a number of novel approaches for the study of early mammalian development, using some aspects of the EC cell system. The applications that are considered in the chapter focus on the current direction of research—that is, the differentiation of these recently isolated stem cells in an embryonic environment.

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