The Viability of Frozen Human Embryos: Lessons From Animal Research
In the long debate over the ethical permissibility of destroying embryonic human beings in research, it has sometimes been suggested that frozen embryos are somehow less human, or less alive, than are unfrozen embryos. For instance, in January 2000, Senator Arlen Specter of Pennsylvania introduced S. 2015, a bill providing for federal funding of human embryonic stem cell research. The bill provided, in pertinent part, that:
The human embryonic stem cells involved shall be derived only from embryos that otherwise would be discarded that have been donated from in-vitro fertilization clinics with the written informed consent of the progenitors.
In a statement to the press, the senator said, "[T]he discarded embryos are not going to be used for human life. If there was any possibility, I would be the first to oppose their use for scientific research."
Though his statement is ambiguous, it appears Sen. Specter meant to suggest that, once frozen, the embryo was either dead or could not be unfrozen, implanted, and brought to live birth. However, frozen embryos remain alive, and can be thawed, implanted and brought to a live, healthy birth. This has, in fact, been accomplished several times, both in the United States and elsewhere. For instance, on February 5, 2004, BBC News reported that an Israeli woman gave birth to twins from embryos frozen twelve years ago.
While such reports conclusively rebut Specter's suggestion that frozen embryonic human beings cannot be brought to live birth, there is significant scientific research examining other animals--such as invertebrates, insects, frogs, and turtles--which reinforces the point. In this paper, we summarize research concerning how these creatures survive in low temperatures. Freezing places these creatures in suspended animation, but it does not kill them. We hope this will conclusively rebut any belief that frozen human embryos are either no longer alive or cannot be brought to a live birth. Frozen embryonic human beings, like the frozen animals discussed in this paper, can survive freezing. Indeed, whether frozen embryonic human beings will, as Specter put it, "be used for human life" (or, more accurately, whether they will be born alive) depends solely upon the will, good or ill, of the human beings into whose hands fate has placed them. Nothing about their dilemma robs them of the humanity they share with us.
Freeze Survival Techniques in Animals
Animals in their natural environments have to face many situations that, if not handled properly, will cause their deaths. One of these life threatening phenomena is cold weather. In the majority of cases, freezing of animal body tissues or organs is lethal. However, there are some species that can undergo some degree of freezing without damage. In order to prevent injuries due to low temperatures, two basic strategies have been developed:freeze-avoidance and freeze-tolerance. In this paper, I will indicate the mechanisms that are necessary to prevent damage by freezing. At the end, I will provide some data for freeze tolerance of intertidal marine invertebrates, insects, frogs, and turtles respectively.
Freeze-avoidance and Freeze-tolerance
Low temperatures are lethal to cold-blooded animals. These species can adopt two basic strategies to face temperatures below the freezing point of their body fluids (FP): freeze-avoidance and freeze-tolerance. Both strategies involve adaptations on behavioral, physiological, and biochemical levels. Freeze-avoidance is the safest way to prevent the lethal effects of freezing. The most effective strategy is to find a place for hibernation where the temperature does not fall bellow the freezing point (FP). However, some animals have developed another strategy. It enables them to keep their body fluids liquid even at temperatures far below FP.
The contact between environmental ice and a supercooled animal is often lethal, because so-called "inoculative freezing"
Freeze-tolerance is the most challenging hibernation strategy.
Because an animal in a frozen state is completely helpless, there must be good reasons why freeze-tolerance has been developed. There are at least three important advantages achieved by this hibernating strategy. The first advantage is the possibility of early spring emergence. Hibernating in less-protected hibernation sites, animals can detect the warmer spring temperatures sooner than those which hibernate in more protected places. This prolongs the time for their growing season. The second advantage has to do with the protection against predators. Turtles that hatch late in the season stay in place and first surface the following spring. But at that time the hatchlings are older and stronger, and thus, more prepared for facing the challenge of predators. The third main advantage of adopting the freeze tolerance is range extension. The species can penetrate into areas where freeze avoidance is not possible.
Freeze tolerance requires several mechanisms for preventing possible freezing injuries:
Freezing in general must be carefully controlled in order to ensure survival. Ice growth is initiated in two ways. It may be inoculated, i.e., body fluids start to freeze when brought in contact with environmental ice at or below FP. Or it may occur spontaneously in supercooled body fluids. The slower the freezing occurs the more time the cells have to prepare for freezing. The lower the temperature at which ice formation begins, the faster the ice will be formed, and thus less time will be available for implementing cryoprotective means.
The animals that can survive freezing of their body fluids belong to various species. So far, freeze tolerance is observed with some insects (members of Coleoptera, Hymenoptera, Diptera, and Lepidoptera), marine invertebrates (bivalves--Mytilus edulis, Modiolus demissus, Cardium edule, Venus mercenaria; gastropods--Littorina littorea, Nassarius obsoletus, Acmea digitalis, Melampus bidentatus; and barnacles--Balanus balanoides), four species of land hibernating frogs (wood frog--Rana sylvatica, gray tree frog--Hyla versicolor, spring peeper--Hyla crucifer, and chorus frog--Pseudacris triseriata), reptiles (box turtles--Terrapene carolina, and painted turtles--Chrysemis picta, etc., garner snakes, some lizards).
A. Marine Invertebrates
In the intertidal zone, marine invertebrates can be exposed to sub-zero temperatures twice a day. Because of the environment, marine invertebrates cannot use dehydration to protect themselves from the impacts of freezing. The factors influencing freezing survival are: salinity, temperature, anaerobis,
Examples of freeze tolerance in insects can be found at all stages of life. Some species
C. Terrestrially Hibernating Frogs
The limits for frog survival are narrow but still sufficient for chosen hibernation sites. Frogs usually supercool to 28.4 or 26.6 degrees Fahrenheit (-2 or -3 degrees Celsius) and can survive freezing at 21.2 to 17.6 degrees Farenheit (-6 to -8 degrees Celsius). It is important to note that the survivable temperatures vary with individual species and location. However, it seems that survival ranges are well matched to the needs of the species. Long-term survival is sufficient--animals of all four species that have been studied survived three days frozen at 26.6 degrees Farenheit (-3 degrees Celsius). When frozen, frogs have stiffened limbs, ice crystals under the skin and interspersed with skeletal muscles, and ice filling the abdominal cavity and surrounding all organs. There is no breathing, no heartbeat, and no bleeding when the aorta is severed. Organs such as the liver and the heart are pale, because of blood withdrawal. The heart is the first organ to start functioning again when thawed. Frogs, in order to secure freeze tolerance, also use cryoprotectants (glucose, glycerol, etc.). They might die from dehydration if hibernating in direct contact with air because so-called "freeze drying" can occur.
The above-mentioned species of turtles remain in their nest after hatching. This may cause the hatchlings to be exposed to sub-zero temperatures. Many experiments were made to determine the characteristics and limits of freeze tolerance of turtles. Turtles before freezing in laboratory conditions usually supercool. When freezing is just about to occur, the body temperature rises slightly below the FP and controlled ice formation starts.
It is difficult to determine exact data for freezing tolerance in individual animal species. These data depend on the way the laboratory experiments are conducted. For example, the lowest survivable temperature (or the length of time animals can be exposed to freezing) depends on how fast the temperature was lowered during the experiments.
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