It Is All In Your Brain

As research continues probing the brain with more and more sophisticated tools differences in the brain between male and female brains are being found.

Sex on the BrainMasculinization of the developing rodent brain leads to significant structural differences between the two sexes.The Scientist
By Margaret M. McCarthy
October 1, 2015

A mammalian embryo is female by default. Males develop when the Sry gene of the Y chromosome is expressed, spurring the development of testes. During fetal development, the testes produce large amounts of testosterone, much of which is converted to estrogen. Both hormones then act on the brain, inducing the cellular process of masculinization.

One of the most well studied brain regions that differs between male and female mice is the preoptic area, a region of the hypothalamus that is essential for sexual behavior in males. The SDN, for example, is bigger in males than females, due to the die off of cells in the female brain in the absence of androgen hormones from the testes. The pBNST is similarly larger in males than in females. Conversely, the AVPV is larger in females than in males, though there are 10-fold more neural projections from the pBNST to AVPV in males.

In addition to the anatomical differences between the male and female brain, researchers have uncovered variation at the cellular level. In the mPN of male mice, neurons have twice as many dendritic spines (potential synapses) as do neurons in females; synapse number correlates with male copulatory behavior in adulthood. The male mPN also has more innate immune cells known as microglia, and these cells are in a more activated state, with shorter and thicker processes. In contrast, the microglia in female brains have long, thin processes indicative of a quiescent state. Astrocytes in this part of the male brain are “bushy,” with more abundant processes than those in the same region of the female brain.

In another article in the Scientist they explore the differences between boys and girls.

But there is now increasing evidence that differences in brain function are prevalent across the sex divide, and that these differences manifest in surprising ways in animal models of both health and disease. (See “Gender bias in neuropsychiatric disorders.”) Many sex differences in adult brain structure and behaviors are the result of in utero organizational effects of gonadal steroid hormones, in particular androgens and their aromatized derivatives, estrogens, both of which are present in substantially higher concentrations in male fetuses due to testicular steroidogenesis. Brain differences between the sexes can also arise from diverse factors, including the expression of genes carried on the sex chromosomes and discrepancies in maternal treatment of male and female progeny. Together, these factors mediate differences in neurogenesis, myelination, synaptic pruning, dendritic branching, axonal growth, apoptosis, and other neuronal parameters.

This is not to say that everything is different. Indeed, much of the brain and its functions are indistinguishable between the two sexes. But when it is different, the question is, how did the differences come about? By what cellular mechanisms did the course of development change in a particular region that differs between males and females?
[…]
Developmental masculinization of the brain leads to significant structural differences in the brains of the two sexes. (See illustration.) Some brain regions are larger in males; others are smaller. Collections of cells that constitute nuclei or subnuclei of the brain differ in overall size due to differences in cell number and/or density, as well as in the number of neurons expressing a particular neurotransmitter. The length and branching patterns of dendrites and the frequency of synapses also vary between males and females—in specific ways in specific regions—as does the number of axons that form projections between nuclei and across the cerebral hemispheres. Even nonneuronal cells are masculinized. Astrocytes in parts of the male brain are more “bushy,” with longer and more frequent processes than those in the same regions of the female brain. And microglia, modified macrophages that serve as the brain’s innate immune system, are more activated in parts of the male brain and contribute to the changes seen in the neurons.

When you couple this with a study in Spain of the brains of trans people it starts to reinforce the reality that being transgender is a form of being intersex. Back in 2011 New Scientist published an article on the transgender brain.

Antonio Guillamon‘s team at the National University of Distance Education in Madrid, Spain, think they have found a better way to spot a transsexual brain. In a study due to be published next month, the team ran MRI scans on the brains of 18 female-to-male transsexual people who’d had no treatment and compared them with those of 24 males and 19 females.

They found significant differences between male and female brains in four regions of white matter – and the female-to-male transsexual people had white matter in these regions that resembled a male brain (Journal of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.05.006). “It’s the first time it has been shown that the brains of female-to-male transsexual people are masculinised,” Guillamon says.

In a separate study, the team used the same technique to compare white matter in 18 male-to-female transsexual people with that in 19 males and 19 females. Surprisingly, in each transsexual person’s brain the structure of the white matter in the four regions was halfway between that of the males and females (Journal of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.11.007). “Their brains are not completely masculinised and not completely feminised, but they still feel female,” says Guillamon.

The thing to remember is that this might be only one of many vectors that lead to a person being transsexual, that there could be many more reasons why we are transgender.