18.09.02 · organismal-bio / reproductive

Gametogenesis: spermatogenesis and oogenesis, and hormonal control by FSH and LH

stub3 tiersLean: nonepending prereqs

Anchor (Master): Guyton, A. C. & Hall, J. E. — Textbook of Medical Physiology, 14th ed. (2021), Ch. 80-83

Intuition Beginner

Sperm are produced continuously in the testes from puberty onward -- about 1000 per second, or roughly 100--300 million per day. Eggs, by contrast, are all present at birth in the ovaries (approximately 1--2 million), and only one matures each month in a cycle controlled by hormones. The asymmetry is fundamental: spermatogenesis is a production line running nonstop; oogenesis is a withdrawal from a fixed bank account that only shrinks over time.

Both processes are driven by the same two hormones from the anterior pituitary: FSH (follicle-stimulating hormone) and LH (luteinizing hormone). In males, FSH supports sperm production and LH drives testosterone secretion. In females, FSH stimulates follicle growth each month and LH triggers ovulation. Both are stimulated by GnRH (gonadotropin-releasing hormone) from the hypothalamus, which pulses every 60--120 minutes.

The hormones also regulate themselves through feedback. When testosterone or estrogen levels rise high enough, they signal back to the hypothalamus and pituitary to slow down FSH and LH secretion. This keeps the system in balance -- except for one dramatic exception: at midcycle, a sustained high level of estrogen switches from negative to positive feedback, triggering a massive LH surge that causes ovulation.

Visual Beginner

Comparison of the two gametogenic pathways:

Feature	Spermatogenesis	Oogenesis
Onset	Puberty	Fetal development
Duration	~64 days (continuous)	Decades of arrest; monthly cycle
Products per meiosis	4 functional sperm	1 ovum + polar bodies
Meiotic arrest	None	Prophase I (fetal) and metaphase II (post-ovulation)
Gamete pool	Continuously renewed	Fixed at birth, declining
Location	Seminiferous tubules	Ovarian follicles
Supporting cells	Sertoli cells	Granulosa cells

Worked example Beginner

Trace the hormonal cascade that produces a mature egg in a typical 28-day menstrual cycle:

Days 1--5 (early follicular phase). Menstruation begins. FSH from the anterior pituitary recruits a cohort of 5--15 ovarian follicles to resume growth. Each follicle contains a primary oocyte arrested in prophase I.
Days 5--7. The recruited follicles grow and their granulosa cells begin producing estrogen under FSH stimulation. One follicle becomes dominant -- it has the most FSH receptors and the best blood supply. The rest undergo atresia (degeneration).
Days 7--13 (late follicular phase). The dominant follicle produces increasing amounts of estrogen. Rising estrogen causes the uterine lining (endometrium) to thicken and proliferate. Estrogen also exerts negative feedback on FSH, helping select the single dominant follicle.
Day ~13 (the estrogen switch). When estrogen from the dominant follicle reaches a critically high sustained level (~36 hours), it switches from negative to positive feedback on the anterior pituitary.
Day ~14 (LH surge and ovulation). The positive feedback triggers a massive LH surge (and a smaller FSH surge). The LH surge causes: (a) the primary oocyte to complete meiosis I, producing a secondary oocyte and the first polar body; (b) enzymatic breakdown of the follicle wall; (c) release of the secondary oocyte from the ovary. The secondary oocyte begins meiosis II and arrests at metaphase II.
Days 15--28 (luteal phase). The ruptured follicle transforms into the corpus luteum, which secretes progesterone and estrogen. Progesterone prepares the endometrium for possible embryo implantation. If fertilization does not occur, the corpus luteum degenerates after ~14 days, progesterone drops, and the endometrium is shed -- beginning the next cycle.

Check your understanding Beginner

Formal definition Intermediate+

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of the testes and proceeds through three phases:

Mitotic proliferation. Spermatogonial stem cells (type A spermatogonia) undergo mitosis to maintain the stem cell pool and produce type B spermatogonia, which are committed to differentiation. Type B spermatogonia undergo DNA replication to become primary spermatocytes (2n, 4C).
Meiosis. Primary spermatocytes undergo meiosis I (reductional division) to produce two secondary spermatocytes (n, 2C), each with 23 chromosomes. Secondary spermatocytes immediately undergo meiosis II (equational division) to produce four spermatids (n, 1C) per original primary spermatocyte. The entire meiotic phase takes approximately 24 days.
Spermiogenesis. Round spermatids undergo morphological transformation into elongated spermatozoa without further cell division. Key changes include: condensation of the nucleus into the sperm head, formation of the acrosome (containing hyaluronidase and acrosin), development of the flagellum from the centriole, and mitochondrial assembly in the midpiece. Excess cytoplasm is shed as residual bodies phagocytosed by Sertoli cells. Spermiogenesis takes approximately 24 days.

The total duration from spermatogonium to spermatozoon is approximately 64 days in humans, with an additional 12 days for epididymal transit and maturation.

Sertoli cells (nurse cells) line the seminiferous tubules and provide essential support:

Form the blood-testis barrier via tight junctions, dividing the epithelium into basal and adluminal compartments. This protects post-meiotic germ cells (which express novel antigens from meiotic recombination) from immune attack.
Produce androgen-binding protein (ABP), which concentrates testosterone in the seminiferous tubule to levels 50--100 times higher than serum, essential for spermatogenesis.
Secrete inhibin B, which provides negative feedback on FSH secretion from the anterior pituitary.
Provide structural support, nutrients (lactate), and paracrine signals (GDNF, SCF) to developing germ cells.
Phagocytose residual bodies and damaged germ cells.

Leydig cells (interstitial cells) reside in the connective tissue between seminiferous tubules and produce testosterone in response to LH. Testosterone acts on Sertoli cells (via androgen receptors) to maintain spermatogenesis and on peripheral tissues to maintain secondary sexual characteristics. Intratesticular testosterone concentration is approximately 50--100 ng/mL, compared to 3--10 ng/mL in serum.

The hypothalamic-pituitary-testicular axis

Hypothalamus --(GnRH, pulsatile)--> Anterior pituitary --(LH)--> Leydig cells --(testosterone)--> Sertoli cells --> spermatogenesis
                                                          \--(FSH)--> Sertoli cells --(inhibin B)--> negative feedback on FSH
                                                                                      \--(ABP)--> concentrate testosterone
Testosterone --> negative feedback on hypothalamus (GnRH) and anterior pituitary (LH/FSH sensitivity)

GnRH pulsatility is essential: continuous GnRH administration paradoxically suppresses gonadotropin release (the basis of GnRH agonist therapy for prostate cancer and precocious puberty). Pulse frequency differentially affects FSH vs. LH: slower pulses (~~every 2--3 hours) favor FSH secretion; faster pulses (~~every hour) favor LH secretion.

Oogenesis

Oogenesis begins during fetal development (weeks 8--20 of gestation). Oogonia undergo mitotic proliferation and then enter meiosis I, arresting as primary oocytes in prophase I (diplotene/dictyate stage). At birth, the ovaries contain approximately 1--2 million primary oocytes, each surrounded by a single layer of flattened follicular cells, forming primordial follicles. No new oogonia are produced after birth.

Follicular development proceeds through defined stages:

Primordial follicle: primary oocyte + single layer of flattened granulosa cells. The resting pool; recruitment is gonadotropin-independent, regulated by intraovarian factors (PI3K-AKT-FOXO3 signaling).
Primary follicle: granulosa cells become cuboidal; the oocyte secretes the zona pellucida (ZP1, ZP2, ZP3 glycoproteins). FSH receptors begin to appear on granulosa cells.
Secondary (antral) follicle: multiple layers of granulosa cells; theca cells differentiate around the granulosa layer; follicular fluid accumulates to form the antrum. This stage is FSH-dependent.
Graafian (preovulatory) follicle: large antrum; the oocyte sits on a stalk of granulosa cells (cumulus oophorus). LH receptors appear on granulosa cells. The follicle is ready for ovulation.

At ovulation, the LH surge causes the primary oocyte to complete meiosis I, producing a secondary oocyte (n, 2C) and the first polar body. The secondary oocyte immediately begins meiosis II and arrests at metaphase II. Meiosis II is completed only if fertilization occurs.

The ovarian cycle

The ovarian cycle is divided into three phases:

Follicular phase (days 1--14): FSH recruits a cohort of antral follicles; one becomes dominant. Granulosa cells produce estrogen via the two-cell, two-gonadotropin model (LH stimulates theca cells to produce androgens; FSH stimulates granulosa cell aromatase to convert androgens to estrogens).
Ovulation (day ~14): the LH surge triggers meiosis I completion, follicle wall degradation (prostaglandins, collagenases, plasmin), and oocyte release.
Luteal phase (days 15--28): the ruptured follicle becomes the corpus luteum, which secretes progesterone and estrogen. Progesterone transforms the endometrium from proliferative to secretory. If no pregnancy, the corpus luteum degenerates after ~14 days into the corpus albicans.

The menstrual cycle

The menstrual cycle coordinates the ovarian cycle with uterine endometrial changes:

Phase	Days	Dominant hormone	Ovarian event	Endometrial event
Menstrual	1--5	Progesterone drop	Corpus luteum regression	Endometrial shedding
Proliferative	6--14	Estrogen (rising)	Follicle growth and maturation	Endometrial proliferation
Ovulatory	~14	LH surge	Oocyte release	--
Secretory	15--28	Progesterone (+ estrogen)	Corpus luteum function	Glandular secretion, spiral artery coiling

Hormonal control: feedback mechanisms

Negative feedback (the default):

Testosterone inhibits GnRH pulse frequency (hypothalamus) and reduces pituitary sensitivity to GnRH (LH and FSH).
Estrogen (at low-moderate levels) inhibits GnRH and gonadotropin secretion.
Progesterone (luteal phase) suppresses GnRH pulse frequency.
Inhibin B (from Sertoli cells in males; from granulosa cells in females) selectively suppresses FSH secretion.

Positive feedback (the midcycle exception):

Sustained high estrogen levels (~>200 pg/mL for >36 hours) from the dominant Graafian follicle switch to positive feedback on the anterior pituitary, triggering the preovulatory LH surge. This is the only physiological example of positive feedback in the HPG axis.

GnRH pulsatility is the upstream regulator. The pulse generator resides in the arcuate nucleus of the hypothalamus. Kisspeptin neurons (KNDy neurons: kisspeptin, neurokinin B, dynorphin) are the key upstream regulators of GnRH neurons. Loss of kisspeptin signaling (e.g., mutations in KISS1 or its receptor GPR54) causes hypogonadotropic hypogonadism (failure to enter puberty).

Key results Intermediate+

Result 1 (Four sperm per meiosis vs. one ovum). Each primary spermatocyte produces four functional spermatozoa through equal cytoplasmic division at both meiotic divisions. Each primary oocyte produces one ovum and two or three polar bodies because cytokinesis is unequal at both meiotic divisions: virtually all cytoplasm is retained by the oocyte, concentrating maternal resources (mitochondria, ribosomes, mRNA, proteins) needed for early embryonic development before the zygotic genome activates.

Result 2 (The 64-day spermatogenesis timeline). The complete process of spermatogenesis in humans takes approximately 64 days (16 days for mitotic proliferation, 24 days for meiosis, 24 days for spermiogenesis). Because the seminiferous epithelium is organized in overlapping waves along the tubule, sperm release (spermiation) occurs continuously rather than in batches. This means that any insult affecting spermatogenesis (fever, toxin, radiation) will not be reflected in semen analysis for approximately 64 days. Conversely, treatments targeting fertility take at least 2--3 months to show effect.

Result 3 (Follicle depletion and reproductive lifespan). The ovarian follicle pool declines continuously from approximately 1--2 million at birth to approximately 400,000 at puberty to fewer than 1,000 at menopause (average age 51). Only about 400 follicles are ovulated over a reproductive lifetime; more than 99.9% of all follicles undergo atresia. The rate of loss accelerates in the late 30s, producing the characteristic decline in fertility and increase in meiotic nondisjunction with maternal age.

Exercise 1

Exercise 1 (medium, short answer).

Explain why continuous (non-pulsatile) administration of a GnRH agonist such as leuprolide initially stimulates but then suppresses gonadotropin secretion.

Hint

GnRH must be pulsatile to maintain pituitary gonadotrope responsiveness. What happens to GnRH receptors when GnRH is constantly present?

Answer

GnRH agonists initially stimulate FSH and LH release because they activate GnRH receptors on pituitary gonadotropes (the "flare" effect, lasting 1--2 weeks). However, continuous receptor activation causes downregulation of GnRH receptors and desensitization of gonadotropes through receptor internalization and post-receptor signaling uncoupling. The result is profound suppression of FSH and LH, and consequently testosterone or estrogen -- a chemical castration.

This paradoxical effect is exploited clinically: GnRH agonists (leuprolide, goserelin) are used to suppress testosterone in prostate cancer, suppress estrogen in endometriosis and uterine fibroids, treat precocious puberty, and prevent premature LH surges during IVF ovarian stimulation protocols (in "long protocol" IVF). GnRH antagonists (cetrorelix, ganirelix) achieve the same suppression immediately by competitively blocking the receptor without the initial flare.

Exercise 2

Exercise 2 (medium, symbolic).

For each stage listed below, state the chromosome number (n), the DNA content (C, where 1C = haploid DNA content), and the number of cells produced from one starting diploid germ cell (2n, 2C). Assume the starting cell has undergone DNA replication.

(a) Primary spermatocyte (b) Secondary spermatocyte (c) Spermatid (d) Primary oocyte (arrested in prophase I) (e) Secondary oocyte (arrested at metaphase II) (f) Ovum (post-fertilization)

Hint

After meiosis I (reductional), chromosome number halves but each chromosome still has two chromatids (2C). After meiosis II (equational), the chromatids separate, giving 1C. Spermatogenesis divides equally at both steps; oogenesis divides unequally but follows the same ploidy rules.

Answer

Stage	Chromosomes	DNA content	Cells from one 2n germ cell
(a) Primary spermatocyte	46 (2n)	4C	1
(b) Secondary spermatocyte	23 (n)	2C	2
(c) Spermatid	23 (n)	1C	4
(d) Primary oocyte	46 (2n)	4C	1
(e) Secondary oocyte	23 (n)	2C	1 (+ 1st polar body)
(f) Ovum	23 (n)	1C	1 (+ 2nd polar body)

The key difference: one primary spermatocyte yields four functional sperm; one primary oocyte yields one functional ovum plus two or three polar bodies. The polar bodies receive minimal cytoplasm and degenerate.

Exercise 3

Exercise 3 (medium, short answer).

Describe the blood-testis barrier. Where is it located, what is its composition, and what is its functional significance?

Hint

Consider why post-meiotic germ cells might be recognized as foreign by the immune system. What structure prevents this immune attack?

Answer

The blood-testis barrier is formed by tight junctions between adjacent Sertoli cells near the basal surface of the seminiferous epithelium. It divides the epithelium into two compartments:

Basal compartment (outside the barrier): contains spermatogonia and early primary spermatocytes. These cells are accessible to circulating factors and immune surveillance.
Adluminal compartment (inside the barrier): contains meiotic and post-meiotic germ cells (secondary spermatocytes, spermatids, spermatozoa). These cells are immunologically privileged.

The functional significance is immune protection. Meiosis generates novel surface antigens through recombination and mutation that the immune system has never encountered during T-cell education in the thymus. Without the barrier, these antigens would trigger an autoimmune response. Disruption of the barrier (by trauma, infection, vasectomy, or toxin exposure) can lead to anti-sperm antibody formation, which impairs sperm motility and fertilization capacity.

The barrier also creates a specialized microenvironment in the adluminal compartment with concentrated testosterone (via ABP from Sertoli cells), which is essential for meiosis and spermiogenesis.

Advanced treatment Master

Clinical disorders of gametogenesis

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder in reproductive-age women (prevalence 6--12%). Diagnostic criteria (Rotterdam, 2003) require two of three: (a) oligo-anovulation, (b) clinical or biochemical hyperandrogenism, (c) polycystic ovarian morphology on ultrasound (>=12 follicles of 2--9 mm diameter in each ovary, or ovarian volume >10 mL). The pathophysiology involves increased GnRH pulse frequency favoring LH over FSH, leading to excessive theca cell androgen production. Hyperinsulinemia (from insulin resistance, present in 70--80% of PCOS patients) worsens hyperandrogenism by reducing sex hormone-binding globulin (SHBG) and directly stimulating ovarian androgen production. Low FSH relative to LH impairs follicle maturation, producing the characteristic multiple small follicles that fail to ovulate. PCOS is the leading cause of anovulatory infertility and is associated with metabolic syndrome, type 2 diabetes, and endometrial hyperplasia (from unopposed estrogen without progesterone).

Male factor infertility accounts for approximately 40--50% of all infertility. Oligospermia (sperm concentration <15 million/mL), asthenospermia (motility <32%), and teratospermia (morphology <4% normal forms) may occur alone or in combination. Varicocele (dilated pampiniform plexus veins in the scrotum) is the most common correctable cause, present in approximately 35% of men with primary infertility and 80% with secondary infertility. The mechanism involves increased scrotal temperature (disrupting spermatogenesis, which requires 2--4 degrees C below core body temperature), oxidative stress, and reflux of adrenal and renal metabolites. Varicocelectomy improves semen parameters and pregnancy rates in selected patients.

Cryptorchidism (undescended testis) is the most common congenital genitourinary anomaly, affecting approximately 3% of full-term male newborns. The testis normally descends from the abdomen through the inguinal canal into the scrotum during the third trimester. An undescended testis exposed to core body temperature suffers impaired spermatogenesis and increased risk of germ cell neoplasia (seminoma). Orchidopexy (surgical repositioning) before 12--18 months of age partially preserves fertility potential and facilitates testicular self-examination. Bilateral cryptorchidism carries a higher risk of infertility than unilateral.

Chromosomal aneuploidies affecting gametogenesis

Klinefelter syndrome (47,XXY) affects approximately 1 in 600 male births and is the most common genetic cause of male infertility. The extra X chromosome causes progressive seminiferous tubule hyalinization and Leydig cell hyperplasia, producing small firm testes, azoospermia (absence of sperm in semen), low testosterone, elevated FSH and LH (due to loss of feedback inhibition), tall stature, gynecomastia, and learning difficulties. Testosterone replacement addresses the hormonal deficiency but does not restore fertility; however, testicular sperm extraction (TESE) with ICSI can retrieve viable sperm in approximately 50% of patients.

Turner syndrome (45,X) affects approximately 1 in 2,500 female births and results from complete or partial absence of one X chromosome. Ovarian dysgenesis leads to streak gonads (fibrous tissue without follicles), primary amenorrhea, and infertility. Estrogen replacement is required for development of secondary sexual characteristics and maintenance of bone health. Spontaneous puberty and menstruation occur in approximately 10--20% of patients with mosaicism (45,X/46,XX), and rare natural pregnancies have been reported. Assisted reproduction with donor oocytes can achieve pregnancy but carries increased cardiovascular risk (aortic dissection).

Assisted reproductive technologies (ART)

In vitro fertilization (IVF) involves: (a) ovarian hyperstimulation with exogenous FSH (recombinant FSH or urinary-derived menotropins) to recruit multiple follicles; (b) prevention of premature LH surges with GnRH agonists or antagonists; (c) final oocyte maturation triggered by hCG (which mimics the LH surge) or a GnRH agonist; (d) transvaginal ultrasound-guided follicular aspiration 34--36 hours later; (e) fertilization in vitro; (f) embryo culture for 2--6 days; (g) transfer of 1--2 embryos to the uterus. Excess embryos may be cryopreserved.

Intracytoplasmic sperm injection (ICSI), developed by Palermo and colleagues in 1992, involves direct microinjection of a single sperm into the oocyte cytoplasm, bypassing the requirement for sperm capacitation, zona pellucida binding, acrosome reaction, and membrane fusion. ICSI is indicated for severe male factor infertility (oligospermia, asthenospermia), previous fertilization failure with conventional IVF, and use of surgically retrieved sperm (TESE, MESA). ICSI fertilization rates are approximately 70--80%, comparable to conventional IVF in non-male-factor cases.

Ovarian reserve testing

Ovarian reserve refers to the quantity and quality of remaining oocytes. Assessment includes:

Anti-Mullerian hormone (AMH): produced by granulosa cells of small pre-antral and antral follicles. AMH levels correlate with the remaining primordial follicle pool and are the most reliable single marker of ovarian reserve. Low AMH (<1.0 ng/mL) predicts diminished reserve and poor response to ovarian stimulation; very high AMH suggests PCOS. AMH is cycle-independent (unlike FSH).
Antral follicle count (AFC): transvaginal ultrasound count of follicles measuring 2--10 mm in both ovaries on days 2--4 of the menstrual cycle. AFC <5--7 predicts poor response.
Basal FSH (measured on day 3): elevated basal FSH (>10--15 mIU/mL) indicates diminished reserve, as the pituitary must secrete more FSH to recruit follicles when the pool is depleted. However, basal FSH has high cycle-to-cycle variability and may be normal even when reserve is declining.
Clomiphene citrate challenge test (CCCT): measures FSH on day 3 and day 10 after clomiphene (100 mg, days 5--9). An exaggerated day-10 FSH rise indicates diminished reserve.

Menopause

Menopause is defined as 12 months of amenorrhea resulting from follicle depletion. The average age is 51 (range 45--55). The menopausal transition (perimenopause) is characterized by:

Variable cycle length (skipped cycles, shorter or longer intervals) due to declining follicle numbers and fluctuating FSH levels.
Rising FSH (often >25 mIU/mL in the late transition) as inhibin B from granulosa cells falls with follicle depletion, removing negative feedback. AMH becomes undetectable.
Vasomotor symptoms (hot flashes, night sweats): the mechanism involves estrogen withdrawal destabilizing the hypothalamic thermoregulatory center. Hot flashes correlate with LH pulses but are not caused by LH itself.
Estrogen deficiency effects: vaginal atrophy and dryness, decreased bone mineral density (accelerated bone loss of 2--5% per year in the first 5 years post-menopause, leading to osteoporosis), increased cardiovascular risk (loss of estrogen's vasodilatory and anti-inflammatory effects on vessels), skin changes, and cognitive effects.
Hormone replacement therapy (HRT): estrogen replacement (with progestogen in women with a uterus, to prevent endometrial hyperplasia) relieves vasomotor symptoms and reduces bone loss. The Women's Health Initiative (2002) demonstrated increased risk of breast cancer, stroke, and venous thromboembolism with combined estrogen-progestin HRT in women over 60, leading to revised guidelines recommending HRT at the lowest effective dose, for the shortest duration, primarily for symptom relief in women under 60 or within 10 years of menopause onset. Transdermal estrogen may have a lower thrombotic risk than oral.

Spermatogenesis toxicology

The testis is vulnerable to environmental and therapeutic toxicants because spermatogenesis requires actively dividing cells, precise hormone signaling, and a specialized microenvironment:

Chemotherapy: alkylating agents (cyclophosphamide, chlorambucil) cause dose-dependent germ cell apoptosis. Permanent azoospermia occurs at cumulative cyclophosphamide doses >7.5 g/m^2. Sperm banking before treatment is standard of care.
Radiation: doses as low as 0.1 Gy cause temporary oligospermia; 2--3 Gy causes permanent azoospermia. Fractionated radiation is more toxic than single-dose exposure.
Heat: the testes reside in the scrotum at 34--35 degrees C (2--4 degrees below core temperature). Experimental warming to 37 degrees C impairs meiosis and spermiogenesis. Hot tub use, tight underwear, and laptop positioning on the lap may modestly affect semen quality.
Environmental endocrine disruptors: phthalates (DEHP), bisphenol A (BPA), pesticides (vinclozolin, DDT), and polychlorinated biphenyls (PCBs) interfere with androgen signaling or estrogen receptors. Animal studies demonstrate decreased sperm counts and increased genital abnormalities; human epidemiological data show secular declines in sperm concentration in Western populations.
Lifestyle: tobacco smoking reduces sperm concentration, motility, and morphology. Alcohol, marijuana, and anabolic steroids (which suppress the HPG axis) also impair spermatogenesis.

Contraceptive mechanisms

Combined oral contraceptive pill (COCP): contains synthetic estrogen (ethinyl estradiol) and progestin. Mechanisms: (a) suppression of the midcycle LH surge, preventing ovulation (primary mechanism); (b) thickening of cervical mucus, impeding sperm penetration; (c) alteration of endometrial receptivity; (d) impairment of tubal motility and oocyte transport. When taken correctly, failure rate is <1% per year; typical-use failure rate is approximately 9%.

Progesterone-only pill (POP/minipill): thickens cervical mucus and suppresses ovulation in approximately 50--60% of cycles. Requires strict timing (within a 3-hour window each day). Failure rate is approximately 0.3--9% per year.

Intrauterine devices (IUDs):

Copper IUD (Paragard): copper ions are spermicidal and inhibit sperm motility and capacitation. Also causes a local sterile inflammatory reaction in the endometrium that is toxic to sperm and oocytes. Prevents fertilization primarily; does not disrupt implanted embryos. Failure rate <1% per year. Effective for 10--12 years.
Hormonal IUD (Mirena, Kyleena, Skyla, Liletta): releases levonorgestrel (a progestin) locally. Thickens cervical mucus, suppresses ovulation in some cycles (depends on dose and individual), and causes endometrial atrophy. Also effective for 3--8 years depending on the device.

Emergency contraception: levonorgestrel (Plan B) inhibits or delays ovulation when taken within 72 hours of unprotected intercourse (most effective within 24 hours). It does not prevent implantation of a fertilized egg. The copper IUD inserted within 5 days of unprotected intercourse is the most effective form of emergency contraception (failure rate <0.1%) and provides ongoing contraception.

Exercise 4

Exercise 4 (hard, analysis).

A 32-year-old woman presents with irregular menses (every 45--90 days), acne, and difficulty conceiving. Laboratory studies show: LH ratio 3:1, total testosterone 85 ng/dL (elevated), fasting insulin 22 uIU/mL (elevated). Pelvic ultrasound shows 15 follicles of 3--8 mm in each ovary.

(a) What is the most likely diagnosis? (b) Explain the pathophysiology connecting insulin resistance to hyperandrogenism in this condition. (c) What is the first-line treatment for her infertility, and what is its mechanism?

Hint

The elevated LH ratio, hyperandrogenism, polycystic ovarian morphology, and insulin resistance are characteristic of a specific syndrome. How does insulin affect ovarian androgen production and SHBG levels?

Answer

(a) Polycystic ovary syndrome (PCOS), meeting all three Rotterdam criteria: oligo-ovulation (irregular menses), hyperandrogenism (elevated testosterone, acne), and polycystic ovarian morphology.

(b) Insulin resistance contributes to hyperandrogenism through several mechanisms:

Increased LH drive: insulin resistance is associated with increased GnRH pulse frequency, favoring LH over FSH. Elevated LH hyperstimulates theca cells to produce excess androgens.
Direct ovarian effect: insulin acts through its own receptor (or IGF-1 receptors) on theca cells to synergize with LH, further amplifying androgen production.
Decreased SHBG: hyperinsulinemia suppresses hepatic production of sex hormone-binding globulin, increasing the free (biologically active) fraction of testosterone.
Impaired follicle maturation: relative FSH deficiency (due to the elevated LH ratio) means granulosa cells cannot adequately aromatize androgens to estrogens. Follicles arrest at the small antral stage, producing the characteristic "string of pearls" appearance on ultrasound.

(c) Lifestyle modification (weight loss, exercise) is first-line, as even 5--10% weight reduction can restore ovulation by decreasing insulin resistance and hyperandrogenism. For ovulation induction specifically, clomiphene citrate (a selective estrogen receptor modulator) or letrozole (an aromatase inhibitor) is used. Clomiphene blocks estrogen negative feedback on the hypothalamus and pituitary, tricking the body into perceiving low estrogen and increasing FSH secretion to stimulate follicle development. Letrozole inhibits estrogen synthesis, achieving the same FSH increase. Current evidence favors letrozole over clomiphene for ovulation induction in PCOS (higher live birth rate, fewer multiple pregnancies). Metformin (an insulin sensitizer) may be added to improve ovulation rates by addressing the underlying insulin resistance.

Exercise 5

Exercise 5 (hard, analysis).

A couple presents with infertility after 18 months of unprotected intercourse. Semen analysis shows: volume 2.5 mL, concentration 3 million/mL (normal >=15 million), motility 25% (normal >=32%), morphology 2% normal forms (normal >=4%). FSH is elevated at 18 mIU/mL (normal 1--12). Testosterone is low-normal.

(a) Classify the semen analysis abnormalities using standard WHO terminology. (b) Explain why FSH is elevated. (c) What investigation should be performed to identify a potentially reversible cause? (d) What assisted reproductive technology would be most appropriate if no reversible cause is found?

Hint

Low sperm concentration = oligospermia. What does elevated FSH tell you about the state of spermatogenesis? Consider anatomical causes of impaired sperm production.

Answer

(a) The findings show oligo-astheno-teratospermia (OAT): oligospermia (concentration <15 million/mL), asthenospermia (motility <32%), and teratospermia (morphology <4% normal). This combined abnormality suggests a global impairment of spermatogenesis rather than an isolated defect.

(b) Elevated FSH indicates primary testicular failure (hypergonadotropic hypogonadism). With impaired spermatogenesis, Sertoli cells produce less inhibin B. The reduced inhibin B removes negative feedback on the anterior pituitary, causing FSH to rise. The pituitary is attempting to stimulate spermatogenesis by increasing FSH, but the testis cannot respond adequately -- the problem is at the gonadal level, not the pituitary.

(c) Scrotal ultrasound (or physical examination) should be performed to evaluate for varicocele, the most common correctable cause of male infertility. A varicocele is identified by palpation of a "bag of worms" in the standing position and confirmed by ultrasound demonstrating dilated veins (>3 mm) with retrograde flow on Valsalva. If a clinically significant varicocele is present, varicocelectomy may improve semen parameters and fertility, particularly when preoperative FSH is not severely elevated and testicular volume is adequate. Karyotype and Y-chromosome microdeletion analysis should also be performed in severe oligospermia (<5 million/mL) to evaluate for Klinefelter syndrome (47,XXY) or AZF (azoospermia factor) deletions on the Y chromosome.

(d) If no reversible cause is found or treatment fails, ICSI (intracytoplasmic sperm injection) with the partner's sperm is the most appropriate ART. ICSI requires only a single viable sperm per oocyte, making it effective even in severe oligospermia. If sperm cannot be found in the ejaculate, testicular sperm extraction (TESE) -- biopsy of seminiferous tubules to retrieve sperm directly from the testis -- can be combined with ICSI. Donor sperm is an option if TESE fails.

Connections Master

Reproductive biology 18.09.01. This unit deepens the gametogenesis overview from 18.09.01, which introduced spermatogenesis, oogenesis, the HPG axis, and the menstrual cycle as part of the broader reproductive biology survey. Here we focus specifically on the cellular stages, hormonal regulation, and clinical consequences of gametogenic dysfunction.
Endocrine hormones and regulation 18.07.01. The HPG axis is a three-tier endocrine control system (hypothalamus--pituitary--gonad) exemplifying the general feedback principles introduced in 18.07.01. The switch from negative to positive estrogen feedback at midcycle is a physiologically rare example of positive feedback in an endocrine axis.
Hypothalamic-pituitary axis 18.07.02 pending. GnRH neurons in the hypothalamus and gonadotropes in the anterior pituitary are part of the hypothalamic-pituitary system covered in 18.07.02 pending. Kisspeptin neurons as upstream regulators of GnRH illustrate the concept of neuropeptide control of pituitary function.
Cell cycle and mitosis 17.08.01. Gametogenesis depends on the specialized meiotic cell cycle modifications introduced in 17.08.01. The prolonged meiotic arrest of oocytes in prophase I (up to 50 years) and the meiotic nondisjunction that increases with maternal age both link directly to cell cycle mechanics.
Immunology 18.10.01. The blood-testis barrier is a specialized immune privilege site. Maternal-fetal immune tolerance (the fetus is semi-allogeneic) is a related concept connecting reproduction to immunology. Anti-sperm antibodies from barrier disruption illustrate immune-mediated tissue damage.
Development 18.11.01. Gametogenesis produces the gametes that fuse at fertilization, initiating the developmental program covered in 18.11.01. Epigenetic reprogramming during gametogenesis establishes the genomic imprints that regulate embryonic development.

Historical & philosophical context Master

The study of gametogenesis spans centuries of observation, technical innovation, and conceptual breakthrough. Antonie van Leeuwenhoek first observed human spermatozoa under the microscope in 1677, though he interpreted them as preformed miniature organisms ("animalcules") contained within semen -- an early version of the preformation theory that held that organisms existed fully formed in either sperm or egg.

The competing theories of ovism (the egg contains the preformed organism; sperm are irrelevant parasites) and spermism (the sperm carries the preformed organism; the egg is merely nourishment) dominated embryological thought for over a century. Both were demolished by the epigenetic view, championed by Caspar Friedrich Wolff (1759), who demonstrated that embryonic structures develop progressively rather than being preformed. The resolution came with the cell theory (Schleiden and Schwann, 1838--1839) and the eventual understanding that both gametes contribute chromosomal material equally.

The chromosomal basis of sex determination was established by Nettie Stevens and Edmund Wilson (1905), who independently discovered that the Y chromosome determines male development in insects. The SRY gene on the Y chromosome was not identified until 1990 (Sinclair et al.), nearly a century later.

The hormonal control of reproduction was elucidated through a series of discoveries. The existence of pituitary gonadotropins was inferred from experiments in the 1920s showing that pituitary extracts stimulated gonadal function. FSH and LH were biochemically purified and characterized in the 1940s--1960s. GnRH was isolated by Schally and Guillemin in 1971 (Nobel Prize, 1977), and its pulsatile nature was demonstrated by Knobil and colleagues in the 1970s, who showed that continuous GnRH infusion paradoxically suppressed gonadotropin release in rhesus monkeys -- a finding that transformed clinical reproductive medicine.

The discovery of kisspeptin as the upstream regulator of GnRH (de Roux et al., Seminara et al., 2003) -- identified through patients with loss-of-function mutations in GPR54 who failed to enter puberty -- opened a new layer of understanding of the HPG axis and potential contraceptive targets.

IVF, first successfully performed by Edwards and Steptoe in 1978 (resulting in the birth of Louise Brown), and ICSI, developed by Palermo and colleagues in 1992, transformed the treatment of infertility. Edwards received the Nobel Prize in 2010; Steptoe had died in 1988 and was ineligible. These technologies raised profound ethical questions about embryo selection, germline modification, and the definition of parenthood that continue to evolve.

The field of gametogenesis also raises the philosophical question of why sexual reproduction exists at all. The twofold cost of sex (Maynard Smith, 1978) -- an asexual female passes 100% of her genes to offspring, while a sexual female passes only 50% -- demands an explanation for the persistence and ubiquity of sex. The leading explanations include: the Red Queen hypothesis (sex generates the genetic diversity needed to evade coevolving parasites), the advantage of recombinational repair of damaged DNA, and the benefits of combining beneficial mutations from different lineages (Fisher-Muller theory). The question remains open and is one of the most debated topics in evolutionary biology.

Bibliography Master

Knobil, E. & Neill, J. D. (eds.), The Physiology of Reproduction, 3rd ed. (Academic Press, 2006), Ch. 1--5.
Sherwood, L., Human Physiology: From Cells to Systems, 9th ed. (Cengage, 2016), Ch. 20.
Silverthorn, D. U., Human Physiology: An Integrated Approach, 8th ed. (Pearson, 2019), Ch. 26.
Guyton, A. C. & Hall, J. E., Textbook of Medical Physiology, 14th ed. (Elsevier, 2021), Ch. 80--83.
Palermo, G. D., Joris, H., Devroey, P. & Van Steirteghem, A. C., "Pregnancies after intracytoplasmic injection of single spermatozoon into an oocyte", Lancet 340 (1992), 17--18.
Seminara, S. B., Messager, S., Chatzidaki, E. E. et al., "The GPR54 gene as a regulator of puberty", N. Engl. J. Med. 349 (2003), 1614--1627.
Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group, "Revised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome", Hum. Reprod. 19 (2004), 41--47.
World Health Organization, WHO Laboratory Manual for the Examination and Processing of Human Semen, 6th ed. (WHO, 2021).
Johnson, M. H., Essential Reproduction, 8th ed. (Wiley-Blackwell, 2018), Ch. 1--8.
Maynard Smith, J., The Evolution of Sex (Cambridge University Press, 1978).
The Practice Committee of the American Society for Reproductive Medicine, "Diagnostic evaluation of the infertile male: a committee opinion", Fertil. Steril. 103 (2015), e18--e25.

Prerequisites

18.09.01 pending

Used in

18.09.03

Tier anchors

beginner: Sherwood, Human Physiology, 9th ed. (2016), Ch. 20
intermediate: Silverthorn, Human Physiology, 8th ed. (2019), Ch. 26
master: Guyton, A. C. & Hall, J. E. — Textbook of Medical Physiology, 14th ed. (2021), Ch. 80-83

References

Sherwood, L. — Human Physiology, 9th ed. (Cengage, 2016) · Ch. 20 Reproductive Physiology
Silverthorn, D. U. — Human Physiology: An Integrated Approach, 8th ed. (Pearson, 2019) · Ch. 26 Reproduction
Guyton, A. C. & Hall, J. E. — Textbook of Medical Physiology, 14th ed. (Elsevier, 2021) · Ch. 80-83 Reproductive and pregnancy physiology
Knobil, E. & Neill, J. D. — The Physiology of Reproduction, 3rd ed. (2006) · Ch. 1-5 Gametogenesis, hypothalamic-pituitary-gonadal axis

Estimated time

beginner: 15m
intermediate: 30m
master: 45m