Wednesday, 25 March 2015

Dig a Little Deeper: A natural history of male nipples

You may recall that last October we took a tour around the human body to tally up the spare parts. During that discussion we broached the subject of male nipples, but perhaps we didn’t give them their full due. The truth is, when you dig a little deeper, these useless chest nubs are a good jumping off point to learn about everything from embryology to hormones to evolution. With that in mind let’s take a clothing optional journey into the natural history of these intriguing appendages.
As the Bloodhound Gang taught us all so many moons ago “You and me baby ain’t nothing but mammals…” and that is the short answer for why we all still have nipples. Part of what has made our order of the animal kingdom so successful is the fact that all of our mothers for millions of years have been able to produce milk for us. Without going into too much detail – because what milk is made of could be an article onto itself – milk is a mixture of water, fat and other chemicals found in a woman’s body. It provides a readily available food source for babies and gives their immune systems a boost as antibodies are passed down through a substance called colostrum.

When a human embryo begins to form there is a specific order in which different processes take place. Around week 4 the development of sexual systems begins, but curiously the system that takes shape is not gender differentiated. It is not until week 8 that the male Y chromosome kicks into gear and starts producing testosterone, which initiates the development of male parts and blocks the development of female ones.

The thing about nipples is that the basic structures are past the point of no return at that point. In the end, the milk-producing systems of the body fully form in both men and women, but men usually lack the hormonal trigger to lactate. Milk is produced in cavities in breast tissue called alveoli which are lined with milk producing cells, men’s alveoli are just ridiculously under-worked.

Interestingly, there are certain circumstances under which some men can produce milk. That isn’t really news to male dayak fruit bats, who routinely do so, but it is pretty uncommon in the human world. Milk production is triggered by a hormone called prolactin, which is usually lacking in men’s bodies. However, when something happens to throw off hormone production like starvation followed by access to food (as has been seen in POWs), or liver cirrhosis the pituitary gland can go haywire and flood the body with prolactin.

A not-so-fun fact that derives from the fact that both men and women have the necessary hardware to produce milk is that both men and women can get breast cancer. Cases of male breast cancer are rare, accounting for less than 1% of all cases, but they do happen. Risk factors for male breast cancer are age (older is worse), alcohol abuse, radiation exposure, family history and an inherited genetic condition called Klinefelter Syndrome where lower levels of androgens (male hormones) and higher levels of estrogen are produced in the body.
Not all male mammals are burdened as humans are with nipples. Male mice, horses and platypuses are born with the creepily smooth chests all men who have been purple-nurpled dream of. The problem with our nipples though, is that they aren’t that costly to produce in terms of the raw materials your body needs. That is why millions of years of evolution have failed to select against male nipples. We would need to produce a whole new protein during embryonic development (PTHrP in mice) to stop the process. The upshot is that men are stuck with nipples for at least another few million generations.