The glass frog (Hyalinobatrachium fleischmanni) is a species that lives in tropical forests from southern Mexico to Ecuador. It has transparent fabrics and translucent skin, adaptations that it uses as camouflage against possible predators while it sleeps during the day on green leaves.
For many vertebrates, especially terrestrial ones, reaching the level of transparency of this amphibian species is a challenge, as the multitude of red blood cells that continuously circulate through the body darkens and opaques even the most transparent tissues.
These frogs become 34-61% more transparent during sleep, removing 89% of their red blood cells from circulation and “hiding” them in the liver.
Argentine biologist Carlos Taboada, from Duke University, in the United States, along with other colleagues from the North American country, investigated 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 34 to 61 percent more transparent, on average, while they sleep. O studying It’s on the cover of Science magazine.
Photoacoustic microscopy images showing circulating red blood cells inside a sleeping glass frog and under anesthesia (which breaks down blood storage). / Junjie Yao, Duke University
According to the authors, this exceptional transparency is achieved by removing about 89% of its red blood cells from the circulation and ‘hiding’ them in the liver during sleep, with no deleterious vascular or metabolic effect on this frog.
The study is the cover of the magazine ‘Science’
However, Taboada points out to SINC that their metabolism is possibly affected: “Normally, vertebrates need a good supply of oxygen to meet our metabolic needs. Every tissue in our body needs it. In a way, glass frogs need to reduce their oxidative metabolism, and one way to do that is to remain completely still for hours and hours during the day.”
“We know that there is a relationship with the circadian cycle of these frogs”, he adds. When they sleep during the day, glass frogs lie motionless in vegetation where they slow down their metabolism and are camouflaged.” Later, when they wake up and are activated, the number of circulating red blood cells increases considerably, as does 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 so is by mobilizing more red blood cells”, explains the biologist, and comments: “Right now we are studying some of the biochemical and physiological mechanisms that regulate the sequestration and ‘packaging’ ‘ of red blood cells within the liver and we hope to have more data soon.”
Implications on blood clotting
In most vertebrates, a local concentration of red blood cells as high as that accumulated by the glass frog tends to trigger coagulation processes, so the new discoveries could help to better understand the mechanisms involved in the prevention of these and other vascular pathologies in people.
“The living organisms that we know are the result of millions of years of evolution, which implies physiological changes and novelties that can offer biochemical responses that other organisms such as human beings do not have”, points out Taboada.
We study these local anticoagulation mechanisms, something very difficult to achieve in humans with conventional treatments, and many research fields are open, for example in the area of thrombosis.
Carlos Taboada (Duke University)
“One of the edges we are studying – he continues – is how glass frogs manage to have normal coagulation processes in the face of injuries, but at the same time do not generate pathological clots, completely interrupting blood flow and compressing almost 90% of their red blood cells in a very small volume.
“We are studying these local mechanisms of anticoagulation, something that is very difficult to achieve in humans with conventional treatments, and many fields of investigation are opening up, for example in the area of thrombosis”, advances 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 help to better understand a unique adaptation within vertebrates, in addition to serving to better understand the flow and to develop new anticoagulants or other cardiovascular drugs.
Camera, photoacoustics and action
The main author of the study, Carlos Taboada, explains how they did it: “We used a combination of techniques that included everything from calibrated photographs, various optical devices such as integrating spheres – which are spheres whose interior is covered by a highly reflective material and allows us to capture the light that passes through frogs – and spectrometers to measure the properties of that light.”
“We also use photoacoustic microscopy –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 makes them red, by the way. When these cells absorb light, some of it appears as ultrasound that we can detect and therefore map to the exact spot in the tissues where the red blood cells that absorbed the light were located. Thanks to this non-invasive technique, we were able to track its location in the livers while the frogs were sleeping during the day.”
Reference:
Carlos Taboada et al. 🇧🇷Glassfrogs hide blood in the liver to maintain transparency”. Science, 2022
