One durable news topic which has been making recent headlines again is the potential cloning of domestic animals and human beings. Part of the allure of the notion may be its utter simplicity. Because the idea is so easily grasped,one can readily imagine fantastic and terrifying scenarios. A tyrant arranges his cloning so as to pass on his rule to a person even more closely related to him than his offspring. Or the tyrant orders his court cloners to turn out thousands of carbon copies of some particularly desirable genetic type-perhaps muscular, dumb, but soldierly-an army of zombie clones.
Such is the stuff of bad novels and television screenplays. In fact, the current interest in the subject may well be traced back to the publication three years ago ofIn His Image--the Cloning of a Man, by David Rorvik, a wholly fictional tale which masqueraded, apparently convincingly, as the truth. Rorvik quoted genuine research in cell and developmental biology in support of his claim that a human had been successfully cloned. But by hitching a ride on the credentials of others, his book did disservice to the scientific community; it implied that studies which seek to understand the control of the expression of the genes in higher organisms were actually concerned with the technically infeasible and, to most scientists, intellectually trivial problem of cloning people.
The book created such an uproar that the House Subcommittee on Health and the Environtment called a hearing to decide whether bona fide research should continue. Though this was a wonderful dose of free publicity for the hoax, the biologists who were called upon to testify made a good case for the importance of their work and won the right to continue unimpeded. Rorvik, although offered two opprotunities to speak before the committee, never showed up.1
Cloning, defined as the asexual reproduction of an organism into one or more gentically identical individuals, has been much in the news again lately, amid talk of making little Hitlers in multiplicate, running "prize bulls" off a biological assembly line, and other such fantasies. The spark which set off this flash of excitement was the discovery by the press that a team of investigators lead by Peter C. Hoppe at the Jackson Laboratories in Bar Harbor, Maine, and by Karl lllmensee at the University of Geneva, had appareiitly cloned some mice. TheNew York Times got hold of the story before its publication in the prestigious professional journalCell, and consequently, Cell was so besieged for information that it issued a press kit announcing "the successful cloning of a mouse." Many newspapers and magazines picked up on the theme and published stories that reinforced the myth that we are just a step away from human cloning. Yet nowhere in the January 1981Cellarticle is the word "cloning' mentioned. Was the excitement just another case of irresponsible journalism? Were these scientists being coy? Or is cloning so incidental or obvious a part of their work that it needn't be mentioned?
In essence, this is what Hoppe and Illmensee did: Embryos in the very early stages of development, consisting of only a few hundred cells, were removed from the uterus of a pregnant mouse. Then, the inner cell mass-the cells that go on to make the mouse, as opposed to the placenta-were withdrawn from the embryo. These cells were separated and, by extremely delicate micromanipulation, the nucleus (which contains the genetic material DNA) was extracted from each cell and transplanted into a recently fertilized mouse egg(called a zygote). In a process called "enucleation," the two nuclei already present in the zygote--one from the egg and one from the recently admitted sperm--were removed with the same microscopic needle that introduced the transferred nucleus.
These zygotes with their foreign nuclei spent the next few days in a growth medium, dividing and expanding into new embryonic cell masses. They were then reimplanted into the wombs of what are known as "pseudo-pregnant" mice, females which had been tricked into readiness by mating with a vasectomized male. Of the 363 nuclear transplants Hoppe and Illmensee attempted, only three yielded healthily newborn mice, although now that they have the procedure down, the investigators claim their success rate can be vastly improved.
This work can be called "cloning" only if that term is used to mean the transplantation of nuclei from more than one cell of a single embryo and tile successful production of more than one offspring. Note that in this case, if anything has been cloned it is the embryo-- product of two adult mice-- and not either of the individual parents. This is an important point if one is concerned that these techniques might be applied to humans. For those who speak of cloning as a potential danger usually mean tile propagation of genetically predetermined individuals who have been chosen for one or more desirable traits. But the cloning of embryos would be worthless to someone with such ambitions. Embryos are necessarily the result of sexual reproduction, which mixes genes in the offspring in novel and unpredictable ways--thereby maintaining genetic variability. The cloning of embryos, therefore, is useless to tyrant and animal breeder alike: it contains no way to predict in advance the complete genetic composition of the progeny.
So what, then, was the purpose of this research? Imagine yourself a cell biologist, fascinated by the question of how such a magnificently complex animal as the mouse develops from a single-celled zygote. You already know that in the cooperative processes called "determination" and "differentiation" dividing cells in the growing embryo become committed to more and more specialized physiological roles as they begin to form the tissues of the body. (File notable subversion of this rule is the cancer cell. Seebelow.) Cells in the very early embryo are "totipotential." This means that they may give rise to daughter cells which have the potential to formanypart of the body. But as these daughter cells divide in their turn, and as the embryo begins to take shape, the number of cell types that can be descended from them becomes increasingly restricted. For example, once an embryonic cell becomes a blood stem cell, it can only beget blood cells. And once one of these daughters of a blood stem cell becomes awhite blood cell, it can never give rise to ared blood cell, although its descendents may differentiate into a variety of still more specialized white blood cells.
As a researcher, you are aware that the developmental blueprint carried by the DNA directs this orderly, branching differentiation of embryonic cell lineages into blood, nerves, sinew, and so on. Is, then, the irreversibility of detemination the result of chemical modifications to the DNA, and if so, can we discover when and how this change takes place?
Were you a biologist to whom this latter question had occurred you might well design an experiment in nuclear transplantation in order to begin to find an answer. By placing the nucleus of a cell which has already made developmental progress into an enucleated zygote you would force that nucleus, if it were still competent, to begin embryonic growth all over again. The question is, how long can we permit the nuclear donor to develop before transplantation and still observe complete, normal development?
Ultimately, the nuclear transplantation experiment is a step toward determining just how late in development the DNA of a cell retains the totipotential character of the germ cell or zygote-that ability to follow any and all of the paths of cellular and tissue specialization. For at some stage in the determination and development of a cell lineage, that ability is lost: the parts of the genetic blueprint no longer needed by that cell type may be thought of as biochemically closed off. For example, a muscle cell might not need some of the genes essential to a blood cell- those coding for hemoglobin, perhaps- and would either discard or permanently shut down these genes. The nucleus from a muscle cell, when transplanted into an enucleated zygote, might direct incomplete or defective development; the hybrid cell might not even grow at all.
What then, to a cell biologist, is the significance of the report from Geneva and Bar Harbor? Nuclear transplantation experiments had in fact been carried out with frog eggs over twenty years ago at the University of Oxford, the laboratory of J.B. Gurdon. Gurdon discovered that although nuclei from embryonic frog cells were capable of promoting the entire developmental repertoire, the transplanted nuclei from fully differentiated cells, such as skin cells, had a limited developmental capability. Hoppe and Illmensee's experiments with mice are important decendants of Gurdon's pioneering studies, for several reasons. First, they are a great technical achievement. Years of work went into developing and perfecting the beautifully precise microsurgical methods for transplanting nuclei without destroying the cells. Second, the mouse is a mammal, and much closer to man on an evolutionary scale than the frog. Third, the sophistication of mouse genetics amplifies the potential knowledge to be gained, through genetic manipulation of the donor animals. Fourth, mice have much shorter reproductive cycles than frogs; experiments with mice can be performed and repeated in much less time.
The experiments that captured the attention of the press, then, are only preliminary. Truly illuminating results will arise from later experiments involving transplantation of nuclei from cells further and further along in carrying out the developmental program. And when biologists at last discover at what stage an embryonic cell's nucleus is no longer able to direct the development of an entire organism, they will turn to yet another task: finding out precisely the nature of the restriction- what it is that makes one cell competent while a slightly more mature cell is incompetent. This is really the inverse of the objective of cloning; it is a fascination with the point at which cloning becomes impossible.
But what if- what if someone decided that this research was interesting and important enough to try on humans? Here is how it would work. Hundreds of fertile women, and at least a few men, would have to be enlisted to mate at appropriate times so as to furnish abortable early stage embryos; hundreds more would have to line up to produce recipient zygotes; yet another group of volunteers would be required to gestate the experimental embryos. This last group would have to be willing to incubate foreign fetuses that would most likely miscarry or, if born, might well be deformed by horrible birth defects. Any volunteers?
Ten years ago, the eminent and gloomily eloquent biochemist Erwin Chargaff
attacked the sensationalization of science by the press, and accused scientists
of complicity in the distortions.
Not infrequently, quite banal discoveries... are being celebrated with a newspaper and television ballyhoo that would have deserved a better soap. Although these are mostly pseudo discoveries, not even applicable to unicellular organisms, one uses press conferences, interviews, and the like, to allude to the imminent 'synthesis of life'or to impending cures by 'genetic engineering.' The public, which forgets rapidly, retains the pleasant taste of the greatness and vigor of the sciences.Yet although many scientists are often too eager to obtain a broader forum for their work, the biologists who pursue the research popularly known as "cloning" actually shun press coverage. Indeed, the writing of this article was hampered by the unwillingness of several leaders in the field to answer questions. For these cell biologists find themselves faced with an uncomfortable dilemma. On the one hand, they feel that any public mention of cloning serves only to further sensationalize and misinterpret their work. (And perhaps they are right: you, the reader, were probably drawn to this story by the presence of cloning in the title.) But on the other hand, if the recent spate of misleading news articles suggests anything, it is that even silence cannot protect scientists and their work from misrepresentation.
1The proceedings of this hearing contain a splendid, intelligible summary of the most valuable and interesting work in this field. They are contained in "Developments in Cell Biology and Genetics," Serial No. 95-105, U.S. Government Printing Office, Washington, D.C., 1978.
DIFFERENTIATION AND CANCER