Women’s nipples have long been a source of fascination and controversy, from celebrity gossip stories of wardrobe malfunctions and “nip slips” to feminist movements for gender equality. Nipples even became a fashion accessory.
Men’s nipples are a different story.
While they don’t tend to attract the same type of controversy, people have long been fascinated about why men have them. The question even made it into a popular science book.
So, if (most) men don’t breastfeed, why do men have nipples? The answer lies partly in how we develop in the womb. We’ll get to male breast milk later.
Several genes determine whether the baby ends up with either male or female reproductive organs. A gene called SRY (sex-determining region Y) on the short arm of the Y chromosome is considered the master gene.
This is activated when the embryo is around seven weeks old. When activated, it eventually leads to the development of male reproductive organs and the disappearance of the primitive female reproductive duct.
As females don’t have a Y chromosome, the primitive female reproductive duct continues to develop into female reproductive organs while the primitive male reproductive duct disappears.
But breasts and nipples start to form before the SRY gene has been activated, between weeks four and six. This is when two ridges called mammary crests or milk lines extend between the primitive armpit and the groin.
So, later in male development, even as most of the mammary crest disappears, the cells around the chest that form primitive nipples and areola smooth muscle remain. These remaining cells go on to form the final breasts and nipples.
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