The Reproductive System: Gametes, Hormones and Reproductive Health
Male and female structures, gamete formation, hormonal control, the menstrual cycle, fertilization and implantation, contraception and fertility treatments.
🎯 By the end of this lesson
- Describe the main structures of the male and female reproductive systems and their functions.
- Explain spermatogenesis and oogenesis and how they differ.
- Describe the hormonal control of the male system using GnRH, LH, FSH, testosterone and inhibin.
- Outline the menstrual cycle, naming the phases and the hormone changes that cause each.
- Distinguish negative from positive feedback within the reproductive cycle and childbirth.
- Describe fertilization, early development and implantation, and the role of hCG.
- Compare groups of contraceptive methods by how they work and the protection they give against STIs.
- Explain how clomiphene and IVF treat infertility and note the associated risks.
1Overview
Almost every other organ system works to keep an individual alive. The reproductive system is different, because its function is to continue the species, and it is controlled by the same feedback principles as the rest. It begins to function at puberty, when the hypothalamus increases its release of gonadotropin-releasing hormone (GnRH), and it links closely to the endocrine, nervous and urinary systems. This lesson covers the organs, gamete production and hormonal control in both sexes, the menstrual cycle, fertilization and early development, and then contraception and fertility treatments, which are part of the course's study of lifestyle and health.
2Male reproductive system
The male gonads, the testes, hang in the scrotum, where the temperature is 2 to 4 degrees Celsius below core body temperature, which sperm production requires. A muscle in the scrotum and a heat-exchanging network of vessels raise or lower the testes to adjust that temperature. Each testis contains coiled seminiferous tubules in which sperm form, and between the tubules are Leydig cells that make testosterone.
Spermatogenesis
Sperm develop by spermatogenesis, which begins at puberty and continues throughout adult life. Diploid stem cells in the tubule wall divide by mitosis, and some of the daughter cells undergo meiosis (Lesson 4) to give haploid cells with 23 chromosomes. These then mature into sperm over about 64 days. Sertoli cells in the tubule nourish the developing sperm and form a barrier that protects them from the immune system. A mature sperm has a head containing the nucleus and an enzyme-filled cap, the acrosome, a midpiece packed with mitochondria that supply ATP (Lesson 2), and a flagellum for swimming.
Ducts and glands
Sperm mature and are stored in the epididymis, pass along the vas deferens and are joined by fluid from the seminal vesicles (fructose for energy), the prostate gland and the bulbourethral glands. The mixture is semen, which leaves through the urethra, the same tube used for urine, though a sphincter prevents both from passing at once. A vasectomy cuts and seals the vas deferens, so sperm cannot reach the semen while testosterone production is unaffected.
Hormonal control in males
GnRH from the hypothalamus stimulates the anterior pituitary to release luteinizing hormone (LH), which acts on Leydig cells to make testosterone, and follicle-stimulating hormone (FSH), which acts with testosterone on Sertoli cells to support sperm production. Testosterone also drives the development of male sex characteristics, bone and muscle growth and the sex drive. Two negative feedback loops regulate the system: testosterone inhibits GnRH and LH, and inhibin from Sertoli cells inhibits FSH. Hormone levels in a healthy adult male remain fairly steady, in contrast to the cyclical pattern in females.
3Female reproductive system
The female gonads are the ovaries. Each is next to an uterine tube (oviduct), which has finger-like fimbriae that sweep up the released egg. The tubes lead to the uterus, a muscular organ whose lining, the endometrium, receives the embryo, and the uterus opens through the cervix into the vagina, which serves as the birth canal. The external genitalia are collectively the vulva. The mammary glands produce milk after birth, stimulated by prolactin and released by oxytocin (Lesson 7).
Oogenesis and the follicle
Egg production, oogenesis, differs from spermatogenesis. A female is born with all the primary oocytes she will have, about one to two million, arrested part way through meiosis, and only about 400 are released over a lifetime. Each month under FSH stimulation, several follicles begin to grow, but normally only one matures, so that one egg is released. Meiosis divides the cytoplasm unequally, so that the egg keeps most of the cytoplasm, and meiosis is completed only if a sperm enters. Menopause, usually between 50 and 52 years of age, follows when the supply of follicles is exhausted, and estrogen and progesterone levels fall.
Hormonal control in females
The same hypothalamic and pituitary hormones as in males are involved, but the pattern is cyclical. FSH stimulates follicle growth, and the follicle makes estrogen. Early on, estrogen has a negative feedback effect, but once it reaches a high level for a few days it switches to positive feedback, triggering a sudden surge of LH that causes ovulation, the release of the egg. After ovulation the empty follicle becomes the corpus luteum, which secretes progesterone and estrogen. These hormones prepare the uterine lining and suppress further LH and FSH release.
4The menstrual cycle
The ovarian cycle and the uterine cycle run together and last about 28 days, although the normal range is wider. By convention, day 1 is the first day of bleeding.
- Menstrual phase (days 1 to 5). The endometrium that built up in the previous cycle is shed as menses, because hormone levels are low.
- Proliferative phase (days 6 to 14). FSH grows follicles; rising estrogen rebuilds the endometrium. A peak of estrogen triggers the LH surge, and ovulation occurs around day 14.
- Secretory phase (days 15 to 28). The corpus luteum secretes progesterone, which makes the endometrium thicker and glandular, ready for an embryo.
- Back to menses. If no embryo implants, the corpus luteum degenerates after about 10 to 12 days, hormone levels fall and the lining is shed.
Ovulation is not always at day 14. For a cycle lasting 32 days, the luteal phase is fairly constant at about 14 days, so ovulation occurs near day 18. The unfertilized egg survives about a day, but sperm can live up to several days in the female tract, so the fertile window begins several days before ovulation. Tracking methods built on this have limited reliability when cycles vary.
The cycle combines both kinds of feedback loop (Lesson 3). Mostly it uses negative feedback, but the LH surge is a positive feedback event, which ends only when the follicle ruptures, like the oxytocin loop in childbirth.
5Fertilization, pregnancy and birth
Fertilization normally takes place in the uterine tube within about a day of ovulation. Sperm undergo capacitation in the female tract, which makes them able to fertilize. When a sperm reaches the egg, the acrosome releases enzymes that digest the egg's coating (the acrosomal reaction). Once one sperm enters, the egg's membrane and coating change to block other sperm, which prevents polyspermy and a lethal extra set of chromosomes. The sperm and egg nuclei fuse to form a diploid zygote with 46 chromosomes (Lesson 4). It divides repeatedly while moving to the uterus, forming a ball of cells, the blastocyst, which implants in the endometrium about 6 to 7 days after fertilization.
The embryo secretes human chorionic gonadotropin (hCG), the hormone detected in pregnancy tests, which keeps the corpus luteum active so that progesterone remains high and menstruation does not occur. Later the placenta takes over hormone production and also exchanges nutrients, oxygen and wastes between mother and fetus. Fraternal (non-identical) twins arise when two eggs are fertilized; identical twins arise when one embryo splits.
At childbirth, the stretch of the cervix triggers oxytocin release, which strengthens uterine contractions, which in turn stretch the cervix further. This is the positive feedback loop of Lesson 3 and ends only when the baby is delivered.
6Reproductive health: contraception and fertility treatments
The course treats contraception and fertility drugs as lifestyle and health topics, because they involve choices that affect the whole body. A decision can be informed by understanding what each method does to the cycle described above.
Contraception
The National Health Service in the United Kingdom lists 17 methods of contraception, which fall into several groups.
- Hormonal methods (the combined pill, the progestogen-only pill, the injection, the implant, the patch and the vaginal ring) mainly prevent ovulation by supplying hormones that suppress the LH surge, and also thicken cervical mucus so sperm cannot pass.
- Intrauterine methods: the copper IUD (non-hormonal) and the hormonal intrauterine system make the uterus hostile to sperm and eggs.
- Barrier methods (male and female condoms, diaphragm and cap) block sperm from reaching the egg.
- Sterilization: tubal occlusion or vasectomy, intended to be permanent.
- Emergency contraception: an emergency pill taken within 3 or 5 days of unprotected sex, depending on the type, or a copper IUD fitted within five days.
Only condoms also help to prevent sexually transmitted infections (STIs), so using a condom together with another method protects against both pregnancy and infection. Methods differ in how well they work in typical use, because some depend on correct and consistent use whereas others, such as implants and IUDs, do not.
Fertility drugs and assisted reproduction
If ovulation does not occur, as in polycystic ovary syndrome, clomiphene citrate may be prescribed. It blocks estrogen receptors in the hypothalamus, so the brain senses low estrogen, and releases more FSH and LH, which stimulates the follicles. It is typically given at 50 mg for 5 days starting on day 2 of the cycle, and about 70% of women ovulate with treatment. Because several follicles may mature, the chance of a multiple pregnancy is 5 to 10%, so treatment is usually limited to six cycles.
In in vitro fertilization (IVF), hormones stimulate the ovaries to mature several eggs, which are collected, fertilized with sperm in a laboratory dish, and grown for a few days. One or two embryos are then transferred to the uterus. IVF treats blocked tubes, low sperm count and unexplained infertility, but it involves medical risk, cost and ethical choices about the fate of unused embryos (Lesson 5).
Sexually transmitted infections
Sexually transmitted infections spread by contact with the genital tract, and they connect the reproductive system to the immune system (Lesson 9). Bacterial infections such as chlamydia and gonorrhea can be treated with antibiotics (see the related article on antibiotics), but are often without symptoms, so untreated infection can scar the uterine tubes and cause infertility. Viral infections such as HIV and human papillomavirus (HPV) are not cured by antibiotics. A vaccine is available that protects against the types of HPV most likely to cause cervical cancer. Testing, condom use and vaccination are the main prevention strategies, and Lesson 14 returns to HIV as an example of a disease of the immune system.
Puberty and the life span of the system
Before puberty, GnRH release is low. The rise at puberty switches on the gonads, whose hormones cause the growth spurt and secondary sex characteristics such as the growth of body hair and, in females, of the breasts. In females the system ends at menopause, when estrogen decline contributes to hot flashes and to a loss of bone density, while in males testosterone declines gradually and sperm production continues, usually, into old age. These changes show that the reproductive system is under lifelong endocrine control (Lesson 7).
7Interdependence and homeostasis
| System | Link to the reproductive system |
|---|---|
| Endocrine | GnRH, LH, FSH, sex steroids, oxytocin, prolactin and hCG control gametes, the cycle and lactation |
| Nervous | hypothalamus controls the pituitary; the reflexes of erection, ejaculation and labour are neural |
| Urinary | shared urethra in males; pregnancy increases the workload of the kidneys |
| Cardiovascular | erection depends on blood flow; blood volume rises in pregnancy and the placenta needs a rich blood supply |
| Skeletal | estrogen and testosterone promote bone growth and maintain bone density; bone loss increases after menopause |
| Immune | the female tract and the testes both have special immune protections; condoms reduce infection risk |
In maintaining homeostasis the reproductive system is unusual, because it serves the species rather than the individual and its hormones act on almost every tissue. Nutrition, stress, extreme exercise and body fat all feed back on GnRH release, which is why severe energy deficiency can stop the menstrual cycle.
Pregnancy can occur only on day 14. The timing of ovulation varies between people and between cycles, and sperm survive for several days. Another error is that a woman is born without eggs and makes new ones monthly. In fact the supply of oocytes is present at birth and falls steadily with age.
A home pregnancy test detects hCG in urine. Because hCG is made only by an embryo after implantation, the test is not positive until several days after conception.
🔑Key terms
?Quick check
Try each question first, then reveal the answer.
1. Why are the testes located in the scrotum?
Sperm production requires a temperature 2 to 4 degrees Celsius below core body temperature, which the scrotal position provides.
2. Compare the number of gametes produced by one meiotic division in males and females.
One male germ cell gives four sperm, whereas in females meiosis divides the cytoplasm unequally, so one germ cell gives one egg.
3. Explain how testosterone production is kept stable.
Testosterone inhibits GnRH and LH release (negative feedback), and inhibin from Sertoli cells inhibits FSH, so rising hormone levels reduce their own stimulus.
4. What triggers ovulation and why is this an example of positive feedback?
A sustained high level of estrogen from the follicle causes a surge of LH, which causes ovulation. High estrogen produces more LH instead of less, a positive feedback effect.
5. Describe what happens to hormone levels and the endometrium if fertilization does not occur.
The corpus luteum degenerates, so progesterone and estrogen fall, and the endometrium is shed as menses.
6. How does hCG prevent menstruation in early pregnancy?
hCG from the embryo keeps the corpus luteum secreting progesterone, so the endometrium is maintained.
7. How does the combined contraceptive pill prevent pregnancy, and what STI protection does it provide?
It supplies hormones that suppress the LH surge and so ovulation, and thicken cervical mucus. It gives no protection against STIs, which only condoms provide.
8. Explain how clomiphene induces ovulation and why it increases the chance of twins.
It blocks estrogen receptors in the hypothalamus, so the brain senses low estrogen and increases FSH and LH. More than one follicle may mature, so a multiple pregnancy occurs in about 5 to 10% of cases.
BC curriculum content covered in this lesson
- Organ systems: reproductive system (structure and function)
- Structural and functional interdependence (reproductive system with the endocrine, nervous, urinary and skeletal systems)
- Lifestyle and health: contraception and fertility drugs
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References
- OpenStax. Anatomy and Physiology 2e, 27.1 Anatomy and Physiology of the Testicular Reproductive System. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 27.2 Anatomy and Physiology of the Ovarian Reproductive System. Accessed October 7, 2026.
- OpenStax. Anatomy and Physiology 2e, 28.1 Fertilization. Accessed October 7, 2026.
- NHS. Methods of contraception. Accessed October 7, 2026.
- Buckinghamshire Healthcare NHS Trust. Clomiphene citrate for ovulation induction. Accessed October 7, 2026.
- BC Ministry of Education and Child Care. Anatomy and Physiology 12 (curriculum). Accessed October 7, 2026.
These lessons follow the content areas listed in the British Columbia curriculum. They are study material written for this site and are not an official document. The official curriculum is the authority on what each course requires. Lessons are general education, not medical advice.