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Forming Brains with Estrogen

Updated: Nov 5, 2019

The debate about female Vs male brains and their abilities, capabilities and tendencies has been painfully long and often grossly ill-informed, but the consensus is pretty clear. Brains are plastic.

We form the brain more than it forms us.

The brain difference at birth is small, and I'm not about to use all the big words required to describe the finicky differences. I'm here for the basics. Hormones in the womb may not dictate our lives but they must do something, right?

Yes, they do. But what they do is... well... kinda backwards.

Baby brains all start out the same

A sperm meets an egg, and by six months later there are 86 billion neurons inside a tiny fetal head. That's a production rate factories can't even aspire to. But apparently the human brain has a pretty serious quality control policy, and it's implemented after those cells are made and arranged.

After the six month mark, half of the brain cells die. Boom! Just like that.

But what about the hormones, I hear you mutter. What about all that maternal estrogen, or those tiny little fetal testes that start forming around six weeks into gestation if Dad donated a Y chromosome?

Are babies swimming in Mum's estrogen?

Here's some basic biology that often gets missed: Mum's estrogen doesn't enter the baby's brain. Nope. Nada.

The baby makes a molecule called alpha-fetoprotein, and that attaches to maternal estrogen and stops it entering the brain. (This applies to estrogen produced by the placenta too, just in case anyone was thinking deeply.)

Estrogen from the mother does not make an XX fetus into a girl. It doesn't need to. Brains don't have to be changed to become 'female'.

What we call female, in a brain, is actually just a brain that nothing got changed in. It's the original design. Destiny, if you like.

How, then, does a brain become 'male'?

This is the backwards part.

I love it.

Estrogen cannot enter the female brain, but testosterone can. Doesn't sound backwards yet, does it? Fetal testes make testosterone, so surely that makes the changes to the forming brain?

Well... no.

You see, there's an enzyme called aromatase in that little fetal brain, and it nabs the testosterone and turns it into estradiol. FYI, there are three hormones that are lumped together and called Estrogen, and one of them is estradiol.

What I'm saying is that in a little XY fetus, the developing brain is bathed in estrogen that can only be made in the brain if there is testosterone to make it from.

XX (female) fetal brain: no exposure to estrogen

XY (male) fetal brain: estrogen party zone

The standard issue human brain becomes masculinised by the actions of the 'female' hormone estrodial.

Evolution has pulled some almighty hilarious pranks, and this one is awesome.

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