Klinefelter syndrome, named after Dr. Harry Klinefelter who first identified it, is a chromosomal problem where a person with an XY genotype - biologically a male - inherits at least one extra X-chromosome, and sometimes a few extra ones.
Having an extra X chromosome makes the testicular cells generate less testosterone, which is the hormone responsible for primary sex characteristics like development of the sex organs as well as secondary sex characteristics like height and body shape.
It’s worth mentioning up front, that we’re using the term male here, rather than boy or man, to talk about the biological category of a person’s sex rather than a person’s gender identity.
Now, in puberty, in both males and females, the hypothalamus starts to release more gonadotropin releasing hormone, which gets the pituitary gland to release luteinizing hormone and follicle-stimulating hormone.
In males, these hormones affect the Leydig cells and the Sertoli cells.
The Leydig cells are in the interstitium of the testes, and in response to luteinizing hormone they convert cholesterol into testosterone.
The testosterone along with follicle-stimulating hormone, then stimulate Sertoli cells in the seminiferous tubules of the testes to make more sperm.
To main balance or homeostasis, testosterone reduces gonadotropin releasing hormone and luteinizing hormone, and Sertoli cells release the hormone inhibin which inhibits release of follicle-stimulating hormone.
In Klinefelter syndrome, this hormone balance is altered.
The extra X-chromosome interrupts the normal function of the Sertoli and Leydig cells.
Starting at puberty and continuing throughout life, Sertoli and Leydig cells don’t produce inhibin or testosterone, respectively.
This means that levels of luteinizing hormone and follicle stimulating hormone increase.
Less testosterone also suppresses testes maturation and sperm production as well as development of secondary male characteristics.
In fact, each additional X-chromosome increases the estrogen to testosterone ratio, making the changes even more striking.
Klinefelter syndrome develops when a gamete, either a sperm or an egg, contains at least one extra X-chromosome.
Typically, a gamete with 23 chromosomes, including one sex chromosome either X or Y, develops when parent germ cells undergo the process of meiosis.
Early on in meiosis the germ cell makes a copy of all of its chromosomes, with each chromosome having sister chromatids at that point.
