Probing the program of gene expression utilized in early development.

Somatic cell nuclei injected into amphibian oocytes enlarge, exchange protein with the surrounding cytoplasm, and actively synthesize RNA for weeks. The message activity of the RNAs synthesized within the oocytes can be detected as new proteins made a few days after the injection of somatic nucleic. The oocyte cytoplasm seems to reprogram the injected nuclei, allowing the expression only of those genes which are normally active in oocytes (De Robertis and Gurdon, 1977). Genes which are unexpressed in somatic cells can be activated by injecting Xenopus laevis somatic nuclei into oocytes of Pleurodeles waltlii (Urodela). The genes that were activated are normally expressed in Xenopus oocytes but not in somatic cells. Conversely, genes which are normally expressed in somatic cells but not in oocytes become inactive after injection into oocytes. We conclude that genes which become inactive during cell differentiation can be reactivated, in the absence of cell division, by normal components of oocyte cytoplasm. These components could turn out to be examples of the "determinants" of egg cytoplasm responsible for nuclear activity in early development.