Nuclear Differentiation Starts in Early Development

In the mature adult state, a multicellular eukaryote contains many cells, each with a potential for the constitutive or inducible expression of a unique subset of genes belonging to the total genome. 

Nuclear transplantation technique. The nucleus of an unfertilized egg is mechanically removed with a micropipette (.left), and a nucleus from a blastula cell is removed and microinjected into the enucleated egg. 

In 1962, J. Gurdon took this type of experiment one step further by showing that adult frogs could be developed by injecting enucleated eggs with nuclei from the intestinal epithelium of tadpoles. The success frequency was much lower than when nuclei from earlier stages of development were used. Nevertheless, such results showed the influence of the cytoplasm on nuclear expression they also demonstrated that in certain cases nuclei already tentatively com- 

Manipulation of mouse teratoma cells. The cells from a teratocarcinoma may be dispersed and grown in tissue culture. These cells can be injected into an embryo (a), in which case the resulting animal is a chimera in which some cells come from the original parents and others arise from the cells injected into the blastocyst. 

The most convincing evidence for the importance of cytoplasm on nuclear development comes from studies on teratoma, a unique type of tumor found in many kinds of mammals. It is composed both of neoplastic cells, like other tumors, and of many kinds of differentiated cells. A typical teratoma may include nerve, muscle, blood, and skin cells, and other differentiated cells all mixed together with undifferentiated neoplastic stem cells. Only the stem cells are neoplastic, producing more stem cells or more differentiated cells, usually both. In many ways the stem cells behave like 

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Nuclear Differentiation Starts in Early Development Chromosome Structure Varies with Gene Activity Giant Chromosomes Permit Direct Visualization of Active Genes... 

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