Nongonococcal Urethritis and Related Infections

explain the nature of cell differentiation, going back to Weissman's theory of selective loss of hereditary determinants, and brings us up to date with the latest views on differential genetic programming. He argues persuasively that cloning of embryonic and adult cells is an appropriate assay system for determining totipotency, genetic equivalency, and reversibility of differentiation in nuclei. The results of a wide range of experiments on cloning amphibians indicate that, while embryonic nuclei implanted into enucleated eggs will support normal development, i.e., can be cloned, nuclei from adult differentiated cells will not. Among the few exceptions to this rule are the experiments of Gurdon on Xenopus laevis where intestine nuclei implanted into eggs gave rise to a few normal cloned adults. The nature of the apparently irreversible changes that take place in adult cell nuclei is unclear, but subjecting the nuclei to various treatments and then using them for cloning may enable us to find out. Another major issue addressed by McKinnell is the nature of transformation and malignancy. Cloning may be used as a technique for studying this change in cells. If the cells of a malignant tumor are irreversibly altered, e.g., by somatic mutation, it will probably prove impossible to use them for successful cloning. If, on the other hand, cancer is mainly a disease of abnormal and reversible gene expression, malignant cell nuclei placed in the normal cytoplasmic environment of an egg may be persuaded to function normally. Preliminary results from amphibian cloning experiments, as well as the formation of mammalian chimeras with malignant cells, indicate that in a normal embryonic environment cancer cells can give rise to a variety of normal, differentiated, embryonic, and adult tissues. This would suggest that some cancers at least arise from curable defects in gene functioning. Finally, for anyone interested in working with amphibian embryos, McKinnell's book is a gold mine of information on how to obtain, care for, and manipulate these creatures. There is also a useful section on making micro-instruments and setting up the mechanics required for successful cloning.