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Speaker 1: Welcome to Brainstuff production of I Heart Radio. Hey brain Stuff,

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Speaker 1: Lauren foke obamb here, I want to talk to you

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Speaker 1: about one way that mice could be considered more highly

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Speaker 1: evolved than humans. Nipples. Whereas human males develop a pair

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Speaker 1: of nipples or sometimes more that generally serve no biological function,

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Speaker 1: male mice exit the womb with their bellies smooth and

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Speaker 1: nipple free. So why do men have nipples? Come along

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Speaker 1: with me. As I explore embryonic development, three or four

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Speaker 1: weeks after conception, all human embryos develop parallel mammary ridges

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Speaker 1: called milk lines. They extend from the top of the

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Speaker 1: chest to the lower abdomen. At that stage, the embryo

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Speaker 1: still has what scientists referred to as indifferent gonads, meaning

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Speaker 1: they're still capable of developing either into testes or ovaries.

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Speaker 1: It's not until week seven or so that genes and

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Speaker 1: the embryos sex chromosomes kick in, you know, usually xx

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Speaker 1: or x Y, but there's a bunch of variations. They're

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Speaker 1: what caused the formation of sexual dimorphisms, the physical traits

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Speaker 1: that distinguished the typical biological male from the typical biological female. Again,

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Speaker 1: there are variations, but pertinent to our discussion today, those

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Speaker 1: genes don't tinker with the already developing milk lines. The

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Speaker 1: milk lines recede naturally as the fetus grows, leaving behind

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Speaker 1: nipples and milk producing glands called lobules and ducks, and

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Speaker 1: some fatty tissue in between in both males and females.

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Speaker 1: Now in mice, memmory tissue also forms in both male

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Speaker 1: and female embryos during early pregnancy, but according to Yale

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Speaker 1: University research first published in a particular protein stops the

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Speaker 1: process in male mice just a few days after the

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Speaker 1: mammary tissue starts to form. It produces a protein known

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Speaker 1: as parathyroid hormone related peptide, or pt RP. In male embryos,

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Speaker 1: this protein signals the mammary cells to form hormone receptors

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Speaker 1: that attract the male hormones already circulating in the embryo's blood.

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Speaker 1: Those hormones shut down the mammary growth process and degenerate

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Speaker 1: what tissue has already formed, leaving the hey rodents nipple

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Speaker 1: lists by the time of birth. Mice are among an

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Speaker 1: elite group of mammals with such efficient male nipple des

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Speaker 1: drawing genes, horses, and platypuses are in there too. Human

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Speaker 1: males lacking any such system are left with tissue that's

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Speaker 1: even capable of producing milk under the right circumstances. So

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Speaker 1: that's how men have nipples. But why well? Nipples and

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Speaker 1: healthy breast development are closely linked with female reproductive success,

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Speaker 1: so closely linked that biologists figure it was more evolutionarily

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Speaker 1: advantageous for all embryos to develop breast tissue, whether they'd

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Speaker 1: wind up using it or not. Today's episode was written

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Speaker 1: by me and produced by Tyler Clang. Brain Stuff is

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Speaker 1: a production of iHeart Radio's How Stuff Works from are

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Speaker 1: in this a bunch of other curious topics. Visit our

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Speaker 1: home planet, how stuff Works dot com, and for more

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Speaker 1: podcasts for my heart Radio, visit the iHeart Radio app,

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Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.