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Metamorphosis of a Frog and Apoptosis

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Frogs are among the largest living order of amphibians. Cold blooded and incapable of regulating their body temperatures, amphibians are the most primitive class of land-living vertebrates, and the first to emerge from an aquatic environment to live on land for much of their adult lives. After hatching from an egg as larva, frogs go through an elaborate metamorphosis that takes up to three months, during which the young differ in appearance and structure from the adult. This fascinating process includes the programmed death of cells, or apoptosis.


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Frogs and toads were the first animals on earth to develop vocal cords, and only the male has vocal sacs that fill with air and act as a resonator to amplify their mating calls, much like the box on a guitar. Each species of frog has its own distinctive call by which it can be identified. Mating choruses begin in March and continue into May, depending on the species. About a week later, the female frogs arrive at the breeding pond and the mating ritual of amplexus begins. The male fertilizes the eggs externally as they are laid in the water.


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The eggs form a globular mass that attaches itself to submerged vegetation or rests on the muddy pond floor. Each egg contains a small amount of yolk that nourishes the developing embryo until the larva breaks out of the egg and develops into a tadpole. By 7 to 8 weeks, tadpoles have fully developed hind legs that propel them through the water. At 9 to 10 weeks, the front limbs begin to emerge. It is at this stage the tadpole gradually begins to lose its tail through the process of apoptosis.


Apoptosis, originally a Greek word referring to the falling of leaves in autumn, is the term given to the death of living cells through programmed cell suicide, so called because the cell takes an active role in its own disappearance. The phenomenon of apoptosis was first discovered when batrachologists (biologists who specialize in amphibians) who were studying the metamorphosis of certain frogs observed the gradual disappearance of the tadpole's tail as it morphed into a frog -- a programmed cell death essential to frog development. During apoptosis, enzymes called caspases are activated and destroy essential components that would otherwise keep the cell functional.


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Tadpoles typically lose about one-quarter of their weight during their transformation into froglets. At 10 to 13 weeks, shortly before the froglet leaves the water in which they have developed, their tail has completely disappeared through the process of apoptosis and their forelegs emerge. At this stage, frogs have lungs instead of gills and loose-fitting, permeable skin through which they absorb both water and oxygen directly from the environment. Body fluids are easily lost through their thin skin if they are not in a damp environment.

By September the froglets are nearly two inches long and have doubled in weight. As the weather becomes cooler they move to deeper hibernation ponds where they congregate around the edges, then disappear into the water after the first cold night to hibernate until spring.

Adult Frogs

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All adult frogs are carnivores and feed on moving prey, consuming huge quantities of insects. Their life span in the wild is difficult to determine, but batrachologists believe that most common frog species live from three to seven years.