Human embryos all start out pretty much the same, until the fourth week of gestation when the genes on chromosome 23 kick in, creating sex differences. Cut to puberty, when the pituitary gland helps those mammaries mature in females. The result is that female mammaries become primed for milk production at puberty, so that any spike in the hormone prolactin causes them to lactate.
So men come into this world with all the right hardware for the job, but puberty sets dads-to-be down the wrong path for milk production. When a man does lactate and yes, it happens it is almost always a symptom of an underlying medical problem or thanks to injections of estrogen and prolactin, which come with a slew of side effects. Male lactation is a known side effect of anabolic steroids and Thorazine , an antipsychotic drug.
Pituitary tumors and cirrhosis of the liver can also make men produce milk from their nipples. Soon after being rescued from POW camps in World War II, some men lactated because their hormones resumed producing prolactin once they were fed, but their livers had still not recovered from the trauma.
All male and female mammals have mammary glands, which is the organ that produces milk. When mammary glands mature fully in females during puberty, they develop into a state where a hormonal spike—most notably of prolactin—can easily induce lactation. So the difference is a matter of hormonal influence, first during puberty and then again after pregnancy. Provide the right hormones, though, and male lactation is absolutely possible.
One way to do this is through medical intervention. There are a number of ways this can be done, but generally, you need to introduce both estrogen and prolactin into the system, often by injection. Some drugs, like Thorazine—an antipsychotic popular in the midth century—and the heart medication digoxin have induced milk from men as a side effect. The circumstances, as you might guess, are rare. One way to induce lactation from men is via starvation.
The ability to lactate exists in a variety of male animals, though male milk production is typically a rare occurrence.
However, male Dayak fruit bats are commonly known to lactate. So what about human male lactation? Believe it or not, men can, in fact, lactate. Milk is produced in small hollow cavities in breast tissue called alveoli, where the linings contain cells that secrete milk when properly triggered.
While there are a variety of things that go into someone lactating, including various hormones, the main trigger for milk production is sufficient amounts of the hormone prolactin, which is primarily produced by the pituitary gland, though also by the prostate in men and a layer of the uterine wall known as the myometrium in women, among other places. Both men and women have alveoli, but typically have markedly different levels of prolactin.
In fact, a normal non-pregnant woman has around one third to twice the hormone than the average man at any given time. So how can men lactate without the necessary processes involved in child birth triggering a ramp up of prolactin production? Prolactin is ultimately metabolized by the liver helping to regulate its concentration in the body in conjunction with certain mechanisms in the hypothalamus.
More recently a report in the Canadian Medical Association Journal documented a case of a man who had a pituitary tumor that resulted in, among other things, overproduction of prolactin, causing him to lactate. Have I begotten them, that thou shouldest say unto me, Carry them in thy bosom, as a nursing father beareth the sucking child, unto the land which thou swarest unto their fathers?
And a similar much more modern and well documented report was made in of a Sri Lankan man, Mr. B Wijeratne, who took up breastfeeding his 18 month old daughter after his wife died giving birth to their second child. This process can commonly be observed in cases of women adopting babies and wanting to breastfeed them.
They are able to achieve this via regularly having the child suckle and further using a breast pump every few hours over the course of several weeks.
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