Reproductive hormones, also known as sex hormones, are steroid hormones that play important roles in the development and maintenance of sexual characteristics, fertility, pregnancy, and menstruation.
Hormones are small signalling molecules that bind to receptor proteins expressed by different cells to trigger a response and help regulate physiological activities. Hormones are produced by several glands in our body as part of the endocrine's system role to maintain homeostasis.
Homeostasis is the maintenance of steady-state conditions in a biological system. Read our article on the Endocrine System to learn more about how this important regulatory system in our body helps regulate us!
The Hypothalamic-Pituitary-Gonadal (HPG) Axis
Before talking about the sex hormones, we first need to introduce two essential endocrine glands found in the brain. They are the hypothalamus and the pituitary glands. Both these glands are important in regulating the endocrine system, as they produce many hormones that target other endocrine glands. The pituitary gland is divided into anterior and posterior segments that secrete different hormones.
Hormones that target and regulate other glands of the endocrine system are called tropic hormones.
The hypothalamic-pituitary-gonadal (HPG) axis consists of the hypothalamus, pituitary gland and the gonads. The hypothalamus secretes gonadotrophin-releasing hormone (GnRH) which acts on the anterior pituitary and stimulates it to produce and release luteinizing hormone (LH) and follicle-stimulating hormone (FSH) into the bloodstream. These two hormones act on male and female gonads and stimulate the release of testosterone and oestrogen, respectively.
Actions that are controlled by a series of glands and hormones are referred to as an axis. Another example of an axis is the HPA axis (Hypothalamic-Pituitary-Adrenal) which plays an important role in our body's response to stressful situations!
Gonads are important reproductive and endocrine glands responsible for the production of gametes or reproductive cells used in sexual reproduction. In males the gonads are called testes and in females they're called ovaries. Check out the Reproduction article to learn more about their role!
Male Reproductive Hormones
FSH targets a series of special cells in the testes which drives spermatogenesis. LH stimulates another group of cells in the testes to produce testosterone, the main male sex steroid hormone. Testosterone has numerous roles in the body. It stimulates spermatogenesis and is essential for maintaining libido. Other actions of testosterone include development of secondary sexual characteristics through puberty (e.g. pubic and facial hair), growth of external genitalia, as well as deepening of the voice. Testosterone also has anabolic effects, stimulating the growth of bone and muscle.
Spermatogenesis is the process through which germ cells in the testis' seminiferous tubules divide via meiosis and differentiate into haploid cells capable of fertilizing an egg cell. Check out our article Meiosis to learn about this special type of cell division.
High testosterone levels have negative feedback on the hypothalamus, inhibiting GnRH synthesis, as well as LH and FSH in the pituitary gland, resulting in lowering testosterone levels.
Negative Feedback systems are homeostatic self-regulation processes whereby changes to a biological system are reversed and returned to the previous optimal level. Read our article Negative Feedback to learn more about them!
Some tissues convert testosterone to dihydrotestosterone (DHT), which accounts for 10% of the total circulating testosterone levels. DHT binds to the same hormone receptors that testosterone does. Some tissues respond solely to DHT, whereas others, such as the prostate, are more responsive to DHT than to testosterone.
Female Reproductive Hormones
FSH and LH stimulate the ovaries to produce mature gametes, as well as to synthesize and secrete oestrogen, the main female reproductive hormone. Women have two ovaries on either side of the uterus. The tubes that connect the ovaries to the uterus are called fallopian tubes or uterine tubes.
Menstrual cycle reproductive hormones functions
The levels of LH, FSH, oestrogen and progesterone in the bloodstream go through a cycle every month called the menstrual cycle. The first menstrual period happens between the ages of 11 and 15 and continues to recur every month until the beginning of menopause between the ages of 45 and 55. Each cycle normally lasts an average of 28 days (21-35 days).
The menstrual cycle refers to a monthly cycle of natural changes that occur in the female reproductive system. These changes are necessary for the production of an egg cell every month, as well as for preparing the uterus for pregnancy.
The menstrual cycle is controlled by feedback systems:
Moderate levels of oestrogen have a negative feedback effect on the HPG axis, lowering the amounts of LH and FSH released to return the oestrogen back to baseline levels. High levels of oestrogen in the presence of progesterone also have negative feedback on the HPG axis. However, high oestrogen levels without progesterone exert a positive feedback effect on the HPG axis, causing more LH and FSH to be released.
A positive feedback system, unlike negative feedback systems, amplify stimuli instead of reversing homeostatic disturbances!
The events that transpire during the menstrual cycle can be described in the context of two separate cycles, the ovarian and the uterine cycles.
The ovarian cycle
The ovarian cycle consists of two phases. The follicular phase, and the luteal phase. Ovulation occurs between these two phases. It marks the end of the follicular phase and the start of the luteal phase. The follicular phase marks the beginning of a new cycle as follicles begin to mature and prepare to release an oocyte.
An ovarian follicle is a group of cells found in the ovaries that surround an egg cell and secrete hormones that regulate the menstrual cycle. Women have roughly 200,000 to 300,000 follicles at the time of puberty, each with the capacity to produce a mature egg cell for fertilization upon ovulation, meaning the release of an egg from the ovaries.
The oocyte is another term for a mature egg cell that is released every month from the ovaries.
There is minimal FSH, LH, and oestrogen in the blood at the start of a new cycle, so many follicles begin to grow independently of these hormones. Because oestrogen levels are low, there is little negative feedback acting on the HPG axis, leading to an increase in FSH and LH levels. These increase the follicle's growth and the production of oestrogen. During each menstrual cycle, only one dominant follicle can continue to grow and be released. As oestrogen levels rise to moderate levels, negative feedback decreases FSH levels, allowing just the dominant follicle to survive, which is now capable to grow independently of FSH. Follicular oestrogen gradually rises and because there is minimal progesterone in the system at this time, it exerts positive feedback on the HPG axis, triggering an upsurge in LH levels. The follicle releases the mature oocyte into the fallopian tube in response to the LH surge. This process is called ovulation. For around 24 hours, the oocyte remains viable for fertilization.
Following ovulation in the luteal phase, the remnant of the ruptured follicle becomes the corpus luteum and continues to secrete oestrogen and, for the first time, progesterone. Oestrogen in presence of progesterone exerts negative feedback on the HPG axis which lowers the LH and FSH levels to prevent the formation of any new follicles in the ovaries. Oestrogen and progesterone also maintain suitable conditions for fertilisation of the oocyte and implantation.
- In the absence of fertilisation, the corpus luteum spontaneously regresses after 14 days at the end of the cycle. This causes a considerable drop in hormone levels which removes the negative feedback on the HPG axis and allows it to reset and prepare for a new cycle to begin anew.
- If fertilisation happens, the embryo's placenta generates a hormone called the human chorionic gonadotropin (hCG), which keeps the corpus luteum intact. After a while, however, the placenta starts making oestrogen and progesterone itself and the corpus luteum eventually regresses.
The uterine cycle
The uterine cycle describes the events that occur inside the uterus during the menstrual cycle, and it consists of three phases: menstruation, the proliferative phase, and the secretory phase.
- Menstruation signifies the start of a new menstrual cycle. It happens without fertilization after the corpus luteum has regressed, and the uterine lining has started to shed. Menstrual bleeding normally lasts 2-7 days, with the average woman losing between 10-80ml of blood.
- Following menstruation, the proliferative phase occurs concurrently with the follicular phase of the ovary, preparing the reproductive tract for fertilization and implantation. Oestrogen causes the uterine lining to thicken, as well as the secretion of a thin alkaline cervical mucus that aids sperms' entrance into the uterus.
- The secretory phase occurs along with the luteal phase. Progesterone promotes the thickening of the uterine lining as well as the development of secretory glands. In the presence of oestrogen, progesterone induces the cervix to generate a thick acidic mucus that inhibits any further sperms from entering. These two hormones also stimulate breast tissue growth and other metabolic changes that slightly raise the average body temperature.
Check out the Menstrual Cycle Hormones article to learn more!
Menstrual cycle hormone graph
The female reproductive hormone cycles outlined above are summarized in the graph below.
Therapeutic Reproductive Hormones Functions
Exogenous hormones can be used in certain scenarios for therapeutic purposes, like Hormone replacement therapy (HRT) or Transgender hormone therapy (THT).
Hormone Replacement Therapy
Hormone replacement therapy (HRT) is an effective treatment for the symptoms of menopause. HRT can also help with other long-term health issues connected with menopause, including lowering the risk of osteoporosis, cardiovascular disease, and stroke. The goal of HRT is to supplement the reduced hormone levels caused by menopause.
When the ovaries cease producing eggs, menopause has started. The hormones oestrogen and progesterone are lowered in the blood as a result of menopause. In addition to subfertility, menopause can cause symptoms such as hot flushes, dryness of the vagina and weakening of the bones. HRT works by replenishing these hormones to some degree to lower menopausal symptoms and health concerns.
Transgender Hormone Therapy
Transgender hormone therapy (THT) is a type of hormone therapy in which sex hormones and other pharmacological drugs are given to transgender people to match their secondary sexual traits more closely with the gender they identify themselves with. This sort of hormone therapy is classified into two groups based on whether the target of the treatment is masculinization or feminization:
- In the form of masculinizing hormone therapy, transgender males or transmasculine persons are administered testosterone and anti-oestrogen drugs.
- In contrast, feminizing hormone treatment involves the administration of oestrogen and anti-testosterone medications to transgender women or transfeminine persons.
Reproductive Hormones - Key Takeaways
- Reproductive hormones, also known as sex hormones, are steroids hormones that play important roles in the development and maintenance of sexual characteristics, fertility, pregnancy, and menstruation.
- The hypothalamus secretes gonadotrophin-releasing hormone (GnRH) which acts on the anterior pituitary and stimulates it to produce and release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
- Testosterone is the main male sex hormone and Oestrogen is the main female sex hormone.
- The ovarian cycle consists of two phases: the follicular phase and the luteal phase. The uterine cycle describes the events that occur inside the uterus during the menstrual cycle, and it consists of three different phases.
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