How Hormones in the Womb Guide the Development of Tissues That Determine Biological Sex
At the very earliest stages of development, all embryos look almost identical. The features that clearly distinguish a baby as male or female are not yet present. These differences only begin to emerge when genes located on the sex chromosomes start to “direct” how the body’s tissues will be formed.
During the first weeks of pregnancy, the embryo’s brain begins to develop as a thin strip made up of only a few cells. In the following weeks, these cells take on a tubular shape: the upper part becomes the brain, while the lower part develops into the spinal cord. Over the months leading up to birth, these cells divide and multiply, eventually forming billions of nerve cells. They communicate with one another through trillions of electrical and chemical connections.
This complex “communication” is guided by our genes. Genetic signals can trigger highly intricate and delicate processes. As emphasized in earlier parts of this series, although genetics sets certain developmental pathways, the process is not simply a binary choice between “male” or “female.” Many factors that are not yet fully understood can cause differences in physical development between male and female bodies as a result of the embryo’s genetic makeup and events occurring in the womb.
Everything Begins Very Early
During the first six weeks of development, genes begin producing specific signaling chemicals. These substances “instruct” the body on how different tissues should form. Small biological differences lead to tissues developing differently in male and female embryos. The primary regulators of these differences are hormones.
Although hormones are released in extremely small amounts, they have exceptionally powerful effects. They determine, at a very early stage, whether the embryo will develop along a male or female anatomical pathway. Many of these subtle signals originate in the brain.
The Brain — The Center of Development
The brain can be compared to a large, densely populated, and constantly active city. Neurons exchange thousands of chemical signals every second. These processes are regulated by genes. Some genes activate or deactivate others, “telling” cells where to move and what form to take.
Some of these cells develop into vital organs such as the heart, kidneys, and lungs. Others form muscles, the intestines, limbs, and the tissues that make up the reproductive anatomy. At this early stage of development, the tissues of male and female embryos appear very similar, though they are not exactly the same.
Female or Male?
From approximately the sixth week of pregnancy onward, this similarity begins to change rapidly. The embryo increasingly develops along either a male or female trajectory. According to Richard M. Sharpe, a specialist at the Scottish Centre for Reproductive Health at the University of Edinburgh, whether an embryo develops as male or female depends on an extremely complex interaction between genetic pathways and hormones.
For a long time, scientists considered the female developmental pathway to be the “default” option. It was believed that unless specific genetic mechanisms were activated to initiate male development, the embryo would automatically develop as female. However, recent research shows that the process is far more complex.
For female anatomy to form, the male developmental pathway must be actively suppressed while female-specific genetic mechanisms are activated. In other words, the final form of the body results from a delicate balance between male and female hormones.
The Role of Hormones
If the male developmental pathway is activated, the embryo’s reproductive system begins to form male-specific sex organs and gonads (the testes). In fact, at early stages, these tissues have the potential to develop into ovaries. However, the activation of certain genes sends a “male” signal to the body, altering the direction of development.
The testes begin producing large amounts of testosterone at an early stage. Testosterone belongs to a group of male sex hormones known as androgens. A small portion of this hormone (about 5 percent) is converted into a more potent hormone called dihydrotestosterone (DHT).
The testes produce approximately 95 percent of androgens in a male embryo. In addition, the adrenal glands located above the kidneys also secrete small amounts of testosterone. These hormones act as chemical “keys,” enabling the formation of male sex organs and other male characteristics — at least in outward appearance.
When the Brain Makes a Different “Decision”
In rare cases, the brain of a genetically male individual may send signals aligned with female development, or vice versa. Sometimes, a genetically female individual may perceive themselves as male. In such situations, the brain acts as the primary “control center,” transmitting signals that may differ from the genetic blueprint.
This demonstrates just how complex and multidimensional human development is. Biological sex is not determined by genes alone — hormones, the brain, and subtle changes during prenatal development all play crucial roles in shaping human identity.