Explainer: How the body sculpts a child

In the womb, hormones direct the development of tissues that will distinguish one biological sex from the other

Early in development, all babies look much the same. They don’t take on features that make them more boy- or girl-like until later, when genes on their sex chromosomes instruct the body about how to finish sculpting its tissues.

Early in development, all babies look much the same. They don’t take on features that make them more boy- or girl-like until later, when genes on their sex chromosomes instruct the body about how to finish sculpting its tissues.

J.K. Califf /Flickr (CC BY-SA 2.0)

In the weeks after a baby is conceived, its brain starts to develop as a tiny ribbon of cells. In the next few weeks, those cells form a tube. The brain is at the top, and the spinal cord is at the bottom. Over the months leading up to birth, those few cells will divide and multiply into billions of nerve cells. They will talk to each other through trillions of intricately wired electrical and chemical connections. Their conversations will be guided by our genes. And those chats can get complicated.

As discussed in part 1 of this series, genetics sets specific pathways in motion. But it’s not as simple as girl or boy. For many reasons that are not well understood, a baby’s genetic inheritance — and events in the womb — can trigger a spectrum of physical changes that will differ in males and females.

It begins early. During the first six weeks of development, genes begin turning on the production of signaling chemicals. These tell the body how to sculpt its anatomy. Some subtle changes will make the tissues in a boy slightly different from those in a girl. Hormones will drive those differences. The body releases these master signaling agents in very tiny amounts. Yet hormones can trigger important activities. And a primary one is determining early on whether a baby will develop the anatomy of a boy or girl.

The brain relays those subtle signals on how the body will take shape.

The brain is a bit like the roads crisscrossing a large, busy and densely populated city. Nerve cells there swap thousands of chemical signals every second. Genes regulate these. And some of the signals also will control other genes, turning some on or off.

In a sense, they tell other cells where to migrate and what shape to take. Some of those cells will go on to form organs, such as the heart, kidneys or lungs. Others will become tissues that make up the muscles, intestines, limbs or reproductive anatomy.

Initially, whether in a girl or boy, these tissues will look quite similar — but not quite identical.

To be or not to be . . . a girl

All of that changes in about the sixth week of pregnancy. Suddenly, a fetus will start to become more girl- or boy-like. Richard M. Sharpe is an expert on the male reproductive system at the University of Edinburgh’s Centre for Reproductive Health in Scotland. He explains that whether a human fetus becomes male or females depends on a complex system of steps and actions by gene pathways and hormones. Researchers long considered the female form to be the default form or “the way the cards are set to fall.” In other words, unless a certain series of switches were flipped on to program the male form, a fetus would default to being female, says Sharpe. But newer research now shows it’s more complicated than that.

To become female, a cascade of genes must be effectively turned on by the active blocking of male genetic pathways, he says. In other words, the body’s form is set through a sort of balancing act between male and female hormones.

If the male pathway becomes active, a child’s reproductive system begins to develop gonads, or sex-specific organs, typical of a boy. These tissues were set to make ovaries (female reproductive organs) — until certain genes informed the body that it was a boy. Then male reproductive organs instead began to emerge.

Among those male organs are testes. Early on, they begin producing large amounts of testosterone (tess-TOSS-tur-own). It’s one in a family of male sex hormones, or androgens. Dihydrotestosterone, or DHT, is another. A male fetus will convert a tiny bit of its testosterone, about 5 percent, into DHT.

The testes make about 95 percent of a boy’s androgens. Adrenal glands that sit atop the kidneys also secrete testosterone.

Testosterone and DHT work as chemical stimulants. They cause a male embryo to create the gonads and other reproductive tissues that will distinguish a boy from a girl, and a man from a woman.

At least outwardly.

In some relatively rare instances, the brains in people with the genes to be a boy may broadcast a message that they are girls. Or a child with a girl’s genes may one day come to identify as a boy. The brain, here, is calling the shots. And sometimes it may be sending a message that differs sharply from the blueprints for life encoded in that person’s genes.

Power Words

(for more about Power Words, click here)

adrenal gland Hormone-producing glands that sit at the top of the kidneys.

androgen  A family of powerful male sex hormones.

dihydrotestosterone (DHT) A male sex hormone, or androgen, that plays an important role in the development of male physical characteristics and reproductive anatomy.

embryo  The early stages of a developing vertebrate, or animal with a backbone, consisting only one or a or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.

feminize    (in biology) For a male person or animal to take on physical, behavioral or physiological traits considered typical of females.

gene   (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genitals or genitalia    The visible sex organs.

gonads  The reproductive organs that make eggs (in females) and sperm (in males).

hormone    (in zoology and medicine)  A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. 

masculinize  (in biology) For a female person or animal to take on physical, behavioral or physiological traits considered typical of males.

ovary (plural: ovaries) The organ in the females of many species that produce eggs.

sex  A person’s biological status, typically male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal reproductive organs, and external genitals.

testis   (plural: testes) The organ in the males of many species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.

testosterone  Although known as male sex hormone, females make this reproductive hormone as well (generally in smaller quantities). It gets its name from a combination of testis (the primary organ that makes it in males) and sterol, a term for some hormones. High concentrations of this hormone contribute to the greater size, musculature and aggressiveness typical of the males in many species (including humans).

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