Parabiosis

Parabiosis, meaning "living beside", is a technical term in various contexts in fields of study related to ecology and physiology. It accordingly has been defined independently in at least three disciplines, namely experimental or medical physiology, the ecology of inactive physiological states, and the ecology of certain classes of social species that share nests.

Parabiosis derives most directly from new Latin, but the Latin in turn derives from two classical Greek roots. The first is παρά (para) for "beside" or "next to". In modern etymology, this root appears in various senses, such as "close to", "outside of", and "different".

The second classical Greek root from which the Latin derives is βίος (bios), meaning "life".

In the field of experimental physiology, parabiosis is a class of techniques in which two living organisms are joined together surgically and develop single, shared physiological systems, such as a shared circulatory system. Through surgically connecting two animals, researchers can prove that the feedback system in one animal is circulated and affects the second animal via blood and plasma exchange. Total blood volume is exchanged approximately ten times per day in rat experiments using parabiosis. One limitation of the experiments is that outbred rats cannot be used because it can lead to a significant loss of pairs due to intoxication of the blood supply from a dissimilar rat.

In the mid-1800s, parabiotic experiments were pioneered by Paul Bert. He postulated that surgically connected animals could share a circulatory system. Bert was awarded the Prize of Experimental Physiology of the French Academy of Science in 1866 for his discoveries. Parabiotic experiments were scarcely revisited until the 20th century.

Many of the parabiotic experiments since 1950 involve research regarding metabolism. One of these experiments was published in 1959 by G. R. Hervey in The Journal of Physiology. This experiment was to support the theory that damage to the hypothalamus, particularly the ventromedial hypothalamus, leads to obesity caused by the overconsumption of food. This results from the ventromedial hypothalamus failing to respond to physiological signals that suppress appetite. The result is attributed to the feedback control system in the brain. Rats in the study were from the same litter, which had been a closed colony for multiple years. The two rats in each pair had no more than 3% difference in weight. Rats were paired at four weeks old. Unpaired rats were used as controls. The rats were conjoined in three ways. First, the peritoneal cavities were opened and connected between the two rats. Later, to avoid the risk of tangling the two rats’ intestines together, smaller cuts were made. After more refinement of the experimental procedure, the abdominal cavities were not opened and the rats were conjoined at the hip bone with minimal cutting. In order to prove that the two animals were sharing blood, researchers injected dye into the veins of one rat and the pigment would show up in the conjoined rat. It was necessary to verify the exchange of blood and plasma. The scientists refined the lesion placement by practicing the procedure on other rats. The rats were killed with ether and weighed at the conclusion of the experiment and the amount of fat in each animal was quantified.

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