This blog post scientifically examines how human sex is determined, specifically why hormonal signals during a particular stage of fetal development play a decisive role.
Human sex chromosomes consist of the X chromosome and the Y chromosome. A female egg contains only an X chromosome, while a male sperm carries either an X or a Y chromosome. Human sex is determined by whether the female egg is fertilized by a sperm carrying an X chromosome or by a sperm carrying a Y chromosome. In the former case, the individual develops into a female with XX chromosomes; in the latter, into a male with XY chromosomes.
In animals with two sexes, like humans, one sex serves as the basic model for sex determination. This basic model sex varies depending on the animal species. In birds, the male is typically the basic model, while in mammals, including humans, the female is the basic model. The non-basic model sex develops from the basic model during ontogeny through a series of steps regulated by sex chromosome genes. Therefore, male formation requires not only the basic program for female formation but also additional processes regulated by the Y chromosome. If the male hormones produced under the Y chromosome’s instructions do not act, the fetus develops as female.
Early after fertilization, the sex determination process is suppressed and does not manifest. Around 6 weeks, a single pair of gonads, destined to become testes or ovaries, forms. Both the Wolffian duct, which will develop into the male reproductive organs (epididymis, vas deferens, seminal vesicles), and the Müllerian duct, which will develop into the female reproductive organs (fallopian tubes and uterus), are present. The Wolffian duct and Müllerian duct are involved only in the development of part of the male and female reproductive organs, respectively. The external organs distinguishing the two sexes originate from common tissue in both male and female fetuses. Whether this common tissue becomes the penis and scrotum in males or the clitoris and labia in females is determined by whether or not it receives additional male hormone signals during fetal development.
Around the seventh week of pregnancy, the sex-determining gene located on the Y chromosome sends a signal to the single gonad to initiate testis formation, marking the first step in male development. Once the single gonad develops into testes, subsequent male development is regulated by hormones produced in the newly formed testes. Without the hormonal signals secreted by the testes at appropriate times, the fetus cannot develop a male body and will not even form the penis necessary to deliver sperm to a female.
After testicular formation, the testes first secrete anti-Müllerian hormone (AMH), signaling the Müllerian ducts to regress. The window during which the Müllerian ducts can disappear in response to this signal is very narrow during development, so the timing of this signal is precisely regulated. Subsequently, the testes send another signal to the Wolffian duct to promote the development of male reproductive organs, primarily through testosterone, the representative male hormone. When testosterone binds to receptors, the Wolffian duct develops into the epididymis, vas deferens, and seminal vesicles. These organs are all involved in transporting sperm from the testes to the penis. If these hormonal signals from the testes are not delivered to the Wolffian duct at the appropriate time, the Wolffian duct naturally disappears within approximately 14 weeks after conception. Additionally, dihydrotestosterone (DHT), produced when testosterone is converted by specific enzymes, directs the formation of male reproductive organs such as the prostate, urethra, penis, and scrotum. The formed scrotum envelops the testes as they descend from the abdominal cavity during late pregnancy.
In female fetuses, the process of the single gonad transforming into an ovary begins later than in male fetuses, around the 3rd to 4th month of pregnancy. During this period, the Wolffian duct, necessary for male reproductive organ formation, naturally degenerates and disappears without hormonal signals. While female body development does not depend entirely on hormonal signals like the male process, the female hormone estrogen is known to act as an essential factor for the normal development and functional performance of the ovaries.
