Glass frogs have transparent tissues and translucent skin, adaptations that they use as camouflage against potential predators while they sleep during the day on green leaves.
For many vertebrates in nature, especially terrestrial ones, achieving the level of transparency of this species of amphibian is a challenge, as the multitude of red blood cells continually circulating through the body darkens and opaques even the most transparent tissues.
Now, Argentinian biologist Carlos Taboada, from Duke University in the United States, along with other colleagues from the North American country, have studied how glass frogs overcome this physiological barrier.
Using calibrated photographs to measure their transparency and photoacoustic imaging to track the movement of red blood cells in living specimens, the researchers found that glass frogs become on average 34-61% more transparent while sleeping. The study is on the cover of Science magazine.
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Glass frogs hide blood in the liver
According to the authors, this exceptional transparency is achieved by removing about 89% of its red blood cells from circulation and “hiding” them in the liver during sleep, with no deleterious vascular or metabolic effects on this frog.
However, it is possible that your metabolism is affected: “Normally, vertebrates need a good supply of oxygen to meet our metabolic needs. Every tissue in our body needs it. Somehow glass frogs need to lower their oxidative metabolism and one way to do this is to remain completely still for hours and hours during the day.”.
“We know there is a relationship with the circadian cycle of these frogs,” he adds. When sleeping during the day, glass frogs remain motionless in vegetation where they slow down their metabolism and camouflage themselves.”. Later, when they wake up and become active, the number of circulating red blood cells increases dramatically, as well as their opacity.
“Frogs wake up at night and their metabolism increases as they move to hunt insects, vocalize, mate, etc. In these cases, they need a greater supply of oxygen, and the way to do this is to mobilize more red blood cells.“, explains the biologist, and comments: “We are currently investigating some of the biochemical and physiological mechanisms that regulate the sequestration and “packaging” of red blood cells in the liver and hope to have more data soon.”.
Implications in blood clotting
In most vertebrates, such a high local concentration of red blood cells such as that accumulated by the glass frog usually causes coagulation processes, so the new findings could help to better understand the mechanisms involved in the prevention of these pathologies. vascular and others in humans.
“The living organisms we know are the result of millions of years of evolution, which involves physiological changes and novelties that can offer biochemical responses that other organisms like humans do not have.», underlines Taboada.
“One of the aspects that we are studying – he continues – is how glass frogs manage to have normal coagulation processes when faced with injuries, but at the same time they do not generate pathological clots by completely stopping the flow blood and compressing nearly 90% of their red blood cells into a very small volume”.
“We are studying these local anticoagulation mechanisms, something very difficult to achieve in humans with conventional treatments, and many fields of research are opening up, for example in the field of thrombosis.“says the biologist.
Although points remain to be clarified, for example, whether these frogs can control changes in the circulation of red blood cells in the presence of a predator, the results of the study provide insight into a unique adaptation within vertebrates, in more to serve to better understand the blood circulation and to develop new anticoagulants or other cardiovascular drugs.
Camera, photoacoustics and action
The study’s lead author, Carlos Taboada, explains how they did it:We used a combination of techniques including calibrated photographs, various optical devices such as integrating spheres – which are spheres whose interior is lined with a highly reflective material and allows us to capture the light passing through the frogs – and spectrometers to measure the properties of this light”.
“We also use photoacoustic microscopy -he continues-, which is a technique that uses light and sound. Essentially, we use one or more specific colored lasers to detect red blood cells, taking advantage of their natural light-absorbing properties – which turn them red, by the way. When these cells absorb light, some of it emerges as ultrasound which we can detect and therefore map to the exact point in the tissues where the red blood cells that absorbed the light were. Using this non-invasive technique, we were able to track their location in the liver as the frogs slept during the day.”.
Reference: Carlos Taboada et al. “Glass frogs conceal blood in their livers to maintain transparency”. Science, 2022
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