{When it comes to genitalia, the shape, size, location and even quantity varies across the species -snakes and lizards have two penises, while birds and humans have one.}<\/p>\n
Now researchers have discovered not only how such external genitalia forms, they have identified why the genitals grow in different regions of the body in different animals.<\/p>\n
During tests, a team of geneticists were even able to change the location of where the penis grew by tweaking the signals sent by cells<\/p>\n
Snake and lizard genitalia is derived from the same tissue that gives creates hind legs, while mammalian genitalia derives from the tail bud, at the bottom of the trunk.<\/p>\n
They may appear in different locations, but they have similar functions and genetics.<\/p>\n
Reporting in the journal Nature, researchers from Harvard Medical School\u2019s Department of Genetics, led by Clifford Tabin, found that a particular part of the embryo is responsible for this location difference.<\/p>\n
The embryonic cloaca \u2013 which eventually develops into the urinary and gut tracts \u2013 sends signals that tell nearby cells and tissues to form into external genitalia.<\/p>\n
The cloaca\u2019s location determines which tissues receive the signal first.<\/p>\n
In snakes and lizards, the cloaca is located closer to the lateral plate mesoderm, the same tissue that makes the paired limbs.<\/p>\n
But, in mammals, the cloaca is closer to what\u2019s known as the \u2018tail bud\u2019, found at the bottom of the animal\u2019s trunk, or torso.<\/p>\n
To confirm these findings, the researchers grafted cloaca tissue next to the limb buds in one group of chicken embryos, and beside the tail buds in a second.<\/p>\n
The researchers found that in both cases, cells closer to the grafted cloaca responded to the signals and began forming genitals.<\/p>\n
While mammal and reptile genitalia are not the same, in that they are formed from different tissue, Professor Tabin said, they do share a \u2018deep homology\u2019 \u2013 in that they develop from a similar genetic reaction, and are created by the same molecular signals.<\/p>\n
\u2018Here we see that an evolutionary shift in the source of a signal can result in a situation where functionally analogous structures are carved out of nonhomologous substrate,\u2019 said study author Patrick Tschopp.<\/p>\n
\u2018Moreover, this might help to explain why limbs and genitalia use such similar gene regulatory programs during development.\u2019<\/p>\n
In a separate study, researchers from University of Florida marked a variety of cells in a chick embryo with a fluorescent marker. They were then able to follow how those cells developed.<\/p>\n
From this, they found the cells that turned into either a penis or a clitoris start out as two groups of cells, found on opposite sides of the embryo.<\/p>\n
As the embryo curls and forms, and it changes from a flat sheet to a 3D embryo, the cells meet.<\/p>\n
They then form a bud and merge to become a penis.<\/p>\n
In snakes and other reptiles, the researchers suggest these buds don\u2019t merge in the same way, so the animals form dual penises.<\/p>\n
They added that genitals defects in humans may be caused by the embryo not curling and closing correctly.<\/p>\n
Daily Mail<\/p>\n","protected":false},"excerpt":{"rendered":"
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