There are many cultural and social factors involved in making a baby into a man or a woman. But biologically speaking, sex starts when you’re just a tiny group of cells in your mother’s uterus.
We have a pretty good general idea of how “maleness” or “femaleness” develops in a human embryo, and how this is translated into the capacity to make eggs or sperm.
We’re also beginning to understand how many other genes contribute to the amazing variation in human sexual development, behaviour and identity.
The early flexibility in this system is fascinating. It reminds me of author Hugh Lofting’s iconic Pushmi-pullyu (“push-me-pull-you”), a two-headed character in the Doctor Dolittle stories, who is in a tizz to decide which way to go.
Germ cells and gonads
Most cells in our bodies are destined to die. But set aside in an embryo are a few cells that retain their ability to become a whole person. These cells – called “primoridal germ cells” ultimately develop into sperm or eggs.
But they have a long journey to get there. About three weeks after conception, 50 primordial germ cells are set aside in membranes outside the embryo. They multiply and make an epic march into the embryo, moving right through the embryonic gut. These cells arrive in the embryonic gonads by six weeks.
Later they receive signals that direct them to become sperm (that are made in their billions throughout the life of a man), or to become the 20 000 eggs a girl is born with.
Eggs and sperm are unique in that each has half the number of chromosomes as other cells. People have two copies of the human genome in every body cell, one from mum and one from dad. Germ cells need to cut this back to a single genome which is a mixture of the two parents’ genes. This is accomplished by a clever type of cell division called “meiosis” in which the 46 chromosomes replicate once, but the cell divides twice.
The organs in which all this happens are gonads: testes in men, ovaries in women.
Gonads start off as a ridge of cells on either side of the backbone-to-be at about five weeks after conception. This “genital ridge” starts off the same in all embryos.
But in embryos destined to be boys, the genital ridge receives a signal called the “testis determining factor” at ten weeks after conception. This signal kick-starts development of testes and suppresses ovarian development.
If it doesn’t get the testis signal, the genital ridge waits a few more weeks, and becomes an ovary.
Then factors from the testis or ovary push the germ cells one way or the other, into developing as either sperm or eggs.
The gonads don’t just make sperm or eggs. They also pump out hormones that affect the whole development of the embryo. The embryonic testis makes testosterone which directs male development, fashioning a penis and scrotum. Estrogen has the opposite effect – supporting the development of female genitalia, and priming the future breasts.