This endocrine system involves the gonads and sex hormones, known as the hypothalamic-pituitary-gonadal axis or HPG axis. The HPG axis holds high importance for the survival of the species, without it extinction would be inevitable.
The HPG axis controls development, the reproduction system and aging in animals. It also regulates the immune system and has roles in metabolism, mood regulation and behaviours.
So as you can see, this axis involves the hypothalamus signalling the pituitary which then signals the gonads: the ovaries or testes depending on sex. Bear in mind that the hormone cycle varies significantly depending on species, sex and natural variations between individuals.
Some Notes on Sex, Hormones and Gender
The biological processes described here are the typical and most commonly seen. It does not account for variations of intersex people. Nor how the system works if altered by external factors like hormone replacement therapy, misuse of anabolic steroids (never a good idea) and other medical interventions.
This does of course include transgender people who may be medically transitioning so their body aligns with their gender. A brief reminder that I do not tolerate transphobia on my platforms. Gender and biological sex are not the same thing. Biology always has exceptions to the ‘standard’ and that is why I never use the term ‘normal.’ Interestingly, Harley Quinn from Suicide Squad puts it well: “Normal is a setting on the drier.”
A final note for my international readers: I use British English spelling and that includes certain hormones we are discussing today. I will include the American English spelling in [square brackets] to prevent confusion.
Sex Hormones
Gonadotropin-Releasing Hormone (GnRH)
Starting at the top, gonadotropin-releasing hormone (GnRH) starts the process. The hypothalamus secrets GnRH which travels down to the anterior pituitary to stimulate it to produce the gonadotropin hormones. The clue’s in the name! The sex hormones provide a negative feedback look to inhibit GnRH secretion.
Lutenising Hormone (LH)
The anterior pituitary releases lutenising hormone (LH) in response to GnRH. LH is a gonadotropin, which means it stimulates the gonads: the ovaries and the testes. LH and FSH work synergistically. In males LH stimulates the Leydig cells, part of the testes, to produce testosterone.
In females an acute surge of LH triggers ovulation and formation of the corpus luteum. Corpus luteum translates from Latin as the ‘yellow body’ and is what is left of the follicle once ovulation has occurred. The egg then travels into the Fallopian tube.
Follicle-Stimulating Hormone (FSH)
Unsurprisingly, follicle-stimulating hormone stimulates the ovarian follicles. Sometimes it’s better to use a name that is clear and self-explanatory, you know? FSH regulates grown, puberty and the reproductive processes.
In both sexes, FSH stimulates maturation of the primordial germ cells. The germ cells go on to become the ‘gametes’ i.e. ova (eggs) and sperm. In females it stimulates growth of the follicle around the egg and development in preparation for ovulation.
In males, FSH induces the Sertoli cells within the testes, to produce androgen-binding proteins. These proteins concentrate testosterone within the testes for spermatogenesis, sperm production.
Oestrogens (Estrogens)
Several oestrogens are produced but oestradiol (estradiol or E2) is the preominant one during the reproductive years. One of the sex hormones, oestradiol plays numerous roles in the body.
Oestrogens regulate metabolism and fat distribution in the body. It also increases bone formation, and this is why post-menopausal AFAB people are at a higher risk of osteoporosis. Oestrogens also mediate the formation of secondary sexual characteristics such as breast development.
Oestrogen prepares the reproductive tract for sex and implantation of a fertilised egg. It also triggers the LH surge which then triggers ovulation. In other animals, sexual receptivity during oestrus (estrus/heat) requires oestrogen. It may also play a role in mental health, especially around menstruation and pregnancy. For example, post-partum depression often improves once oestrogen levels stabilise again.
Progestagens
Progesterone is the most important of these sex hormones and has many functions. Progesterone maintains the uterus and placenta during pregnancy. The corpus luteum produces progesterone after ovulation, until the placenta can take over.
In humans, if no pregnancy occurs, progesterone levels drop which causes menstruation. In other animals, different processes occur. Progesterone levels are linked to breast cancer in humans.
In dogs it is well documented as well, because of the way the reproductive cycle works in female dogs. Dogs have a long luteal phase with high progesterone levels for the same length of time regardless of whether they are pregnant.
This is why no blood test can diagnose pregnancy in dogs and why they are so prone to phantom pregnancies as well. In un-spayed female dogs, 50-70% of all tumours are mammary tumours. Early spaying reduces the risk but advice on how early to spay is varied and should account for breed, size and potential risks of spaying too early. Discuss the matter with your vet.
Testosterone
Testosterone is an androgen sex hormone. It manages the development of male reproductive tissues, sexondary sexual characteristics of males and many other functions. It is important for muscle and bone mass, metabolism and behaviour.
While in the uterus at 4-6 weeks gestation, testosterone begins the differentiation of tissues into male parts instead of female. The SRY gene, on the Y chromosome, is necessary for this. Later in pregnancy, testosterone works with anti-Müllerian hormone to further develop male features.
Of course, testosterone manages male puberty and the development of secondary sexual characteristics. It regulates and promotes sperm production. Sufferers of PCOS (polycystic ovarian syndrome) produce excess testosterone, which causes many of the symptoms of the disorder.
Conclusion
This is a brief rundown of the main sex hormones and their effect on the gonads. The reproductive system varies massively between species and humans are very different from other mammals. Remember that all bodies will produce some level of all of these hormones and that hormones are not the whole story.
Many disorders around these hormones exist, which explains the reason for intersex people and people whose chromosomes and hormones don’t line up with their gender presentation. Biology loves variety and there are always exceptions to the rules.
Thanks for reading! Feel free to ask any questions in the comments. See you next time!
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