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Waiting to exhale

作者:终牺矍    发布时间:2019-03-07 05:17:00    

By Matt Walker PEOPLE and lungfish have the same protein in their lungs to stop them collapsing when they breathe out—though 300 million years of evolution separates us from the fish. So say Australian biologists who have traced the origins of lung cells that let vertebrates conquer the land. Air breathing was crucial for the success of vertebrate land animals and appeared in an ancient group known as sarcopterygiian fish, which evolved into amphibians, reptiles, birds and mammals. But three species of sarcopterygiians remain relatively unchanged. Two of these, the South American and South African lungfishes, are well adapted to air breathing and can survive on just their lungs for extended periods. The Australian lungfish, however, relies on its gills and only uses its lungs during periods of high activity, according to John Power of Flinders University in Adelaide. “It’s the most primitive lungfish and holds a special place in the evolution of the transition to land,” says Power. By examining this evolutionary throwback, he and his colleagues hoped to find clues about how the earliest air breathers functioned. “The fundamental problem with air breathing is that of surface tension,” says Power. As animals breathe out, the lung’s internal surfaces, which are moist, come into contact. If the surface tension is too high, these surfaces stick together, making it almost impossible to inhale again. To overcome this, all vertebrate air breathers seem have two types of cells in their lungs, known as alveolar type I and II cells. Type I cells form the gas diffusing barrier and type II produce a mixture of lipids and proteins that work as a surfactant, regulating surface tension in the lungs. The researchers studied lung samples from a female Australian lungfish, Neoceratodus forsteri, under an electron microscope. “The lining comprises one cell type unlike any in other species we have seen,” says Power. “It looks like an ancestral cell.” Type I and II cells could have evolved from these cells, as they have both the general architecture of type I cells and form the gas-diffusing barrier. At the same time, the organelles inside them looked like those in type II cells. (Journal of Experimental Biology, vol 202, p 2543). The team also found that antibodies to human surfactant proteins called SP-A and SP-B reacted with proteins taken from the fish, implying that both use the same “anti-glue” to keep their lungs open. These proteins must have been conserved for 300 million years, says Power. The findings strongly suggest that all air breathing vertebrates use the same cellular mechanism to keep their lungs open,

 

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