FIG. 20.2

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How do stem cells lose their ability to differentiate into any cell type?

BACKGROUND During differentiation, cells become progressively more specialized and restricted in their fates. Early studies left the mechanisms of differentiation unclear.

HYPOTHESIS One hypothesis is that differentiation occurs as a result of changes in gene expression. A second hypothesis is that differentiation occurs as a result of genome reduction, in which genes that are not needed are deleted.

EXPERIMENT John Gurdon carried out experiments in the amphibian Xenopus laevis to test these hypotheses. He transferred nuclei from differentiated cells into unfertilized eggs whose nuclei had been inactivated with ultraviolet light. If differentiation is due to changes in gene expression, then the differentiated nucleus should be able to reprogram itself in the egg cytoplasm and differentiate again into all the cells of a tadpole. If differentiation is accompanied by loss of genes, then differentiation is irreversible and development will not proceed.

RESULTS The experiment was carried out 726 times. In 716 cases, development did not occur; in 10, development proceeded normally.

CONCLUSION Although the experiment succeeded in only 10 of 726 attempts, it showed that the nucleus of an intestinal cell and the cytoplasm of the unfertilized egg are able to support complete development of a normal animal. This result allows us to reject the hypothesis that differentiation occurs by the loss of genes. The first hypothesis—that cells become differentiated as a result of changes in gene expression—was supported. But, because of the small number of successes in reprogramming, additional experiments were needed to validate the conclusions.

FOLLOW-UP WORK Gurdon’s work was controversial. Some critics argued that the successful experiments resulted from a small number of undifferentiated cells present in intestinal epithelium. Others accepted the conclusion but expressed misgivings about possible applications to humans. Later experiments that succeeded in cloning mammals from fully differentiated cells confirmed the original conclusion.

SOURCE Gurdon, J. B. 1962. “The Developmental Capacity of Nuclei Taken from Intestinal Epithelium Cells of Feeding Tadpoles.” Journal of Embryology & Experimental Morphology 10:622–640.