Difficulty: Intermediate | Prerequisites: Sex determination and differentiation notes, basic understanding of meiosis and mitosis
Once the gonads have differentiated in foetal life, the next question is: when and how do they start producing gametes and hormones? This topic bridges embryonic development and adult reproductive function. Gametogenesis (the production of sperm and eggs) begins in utero but completes on very different timelines in males and females. Sexual maturation at puberty is the hormonal event that activates the reproductive axis, and the hypothalamic-pituitary-gonadal (HPG) axis is the feedback system that keeps the whole machine running. If you understand this material, the clinical topics (puberty disorders, infertility, hormonal therapies) will make sense.
Germ cells arise in the embryonic gonad and undergo mitosis and meiosis to produce haploid gametes. In females, meiosis begins before birth and pauses until puberty; in males, meiosis does not begin until puberty. Puberty is triggered by reactivation of the hypothalamic GnRH pulse generator, which drives pituitary gonadotrophin (LH and FSH) secretion, which in turn stimulates gonadal sex steroid and gamete production. The whole system is regulated by long-loop and short-loop feedback.
Gametogenesis
The process by which diploid germ cells undergo mitosis and meiosis to produce haploid gametes (sperm or eggs).
Think of it as the production line that halves your chromosome number so that fertilisation restores the full set.
Oogenesis
Gametogenesis in females, producing oocytes (eggs).
Spermatogenesis
Gametogenesis in males, producing spermatozoa (sperm).
Primary gamete
The cell produced after the first stage of meiosis (meiosis I). It is still diploid in DNA content at the start of the process.
Secondary gamete
The cell produced after meiosis I, containing a haploid set of chromosomes (though with duplicated chromatids until meiosis II).
Haploid
A cell containing a single set of chromosomes (23 in humans), as found in mature gametes.
GnRH (gonadotrophin-releasing hormone)
A peptide hormone released in pulses by the hypothalamus. It is the master signal that drives the reproductive hormonal axis.
In simple terms, GnRH is the pacemaker for the entire reproductive system.
LH (luteinising hormone)
A gonadotrophin released by the anterior pituitary in response to GnRH. In males, it stimulates Leydig cells to produce testosterone. In females, it triggers ovulation and supports the corpus luteum.
FSH (follicle-stimulating hormone)
A gonadotrophin released by the anterior pituitary in response to GnRH. In males, it acts on Sertoli cells to support spermatogenesis. In females, it stimulates follicle growth.
Inhibin
A peptide hormone produced by the gonads (Sertoli cells in males, granulosa cells in females) that selectively inhibits FSH secretion via negative feedback.
Activin
A peptide hormone that stimulates FSH secretion via positive feedback. Works in opposition to inhibin.
In simple terms, inhibin is the brake on FSH; activin is the accelerator.
Steroid hormones
Lipid-derived hormones synthesised from cholesterol. In the reproductive system, these include testosterone, DHT, oestrogen and progesterone.
Hypothalamic-pituitary-gonadal (HPG) axis
The three-tier hormonal feedback loop controlling reproduction: hypothalamus (GnRH), anterior pituitary (LH and FSH) and gonads (sex steroids and peptide hormones).
Puberty
The developmental period from the onset of secondary sex characteristics to the end of somatic growth (approximately 20 years of age), during which endocrine and gametogenic functions mature to make reproduction possible.
Menarche
The first menstrual period, a notable landmark of female puberty.
Germ cells exist within the embryonic gonad from early in foetal life
Mitotic divisions increase the number of germ cells
Meiosis (reduction division) produces haploid gametes:
Primary gamete (after DNA replication, before meiosis I)
Secondary gamete (after meiosis I, haploid chromosome number)
Mature haploid gamete (after meiosis II)
The critical difference between the sexes is timing
Mitosis of germ cells (oogonia) occurs during foetal life and ceases before birth
All the eggs a woman will ever have are present (as primary oocytes arrested in prophase I) at birth
Meiosis resumes only after puberty, one oocyte at a time, triggered by hormonal signals each menstrual cycle
Meiosis I completes at ovulation, producing a secondary oocyte and a polar body
Meiosis II completes only if fertilisation occurs
One primary oocyte yields one mature egg (the polar bodies disintegrate)
Mitosis of germ cells (spermatogonia) continues throughout the male's reproductive life, beginning at puberty
Spermatogonia are maintained by ongoing mitotic divisions, so the supply is not fixed (unlike eggs)
Meiosis begins at puberty and continues throughout life
One primary spermatocyte yields four spermatozoa
The full process takes approximately 65 to 70 days
A new cycle begins every two to three weeks, so multiple stages are always in progress simultaneously
Before puberty, the gonads produce gamete precursors but almost no hormones
The hypothalamic GnRH pulse generator is largely suppressed
Two mechanisms keep it quiet:
High sex steroid feedback sensitivity (very small amounts of circulating steroid are enough to suppress GnRH)
Intrinsic CNS inhibition of GnRH release from the hypothalamus
Puberty requires maturation and reactivation of the hypothalamic-pituitary-gonadal axis
Several factors are involved:
Body weight and leptin: fat tissue secretes leptin; increased body fat raises leptin levels, which may contribute to earlier onset of puberty
Genetic factors
Nutritional status
Climate and geography
Melatonin: a possible modulating role (still under investigation)
The key physiological events at puberty onset:
Increased frequency and amplitude of GnRH pulses
GnRH stimulates the anterior pituitary
Upregulation of GnRH receptors on pituitary cells
Increased synthesis and release of gonadotrophins (LH and FSH)
Gonadotrophins stimulate the gonads to produce sex steroids and begin gamete production
In males:
Hair pattern changes (pubic hair from around age 12, axillary and facial hair)
Increased libido
Deeper voice
Changed fat distribution pattern
Increased muscle mass
Increased erythrocyte production
Altered fat content
Sperm production begins
Seminal vesicle maturation
Penile enlargement (growth typically ages 13 to 15)
First ejaculation typically ages 12 to 13
In females:
Pubic and axillary hair
Breast development
Menarche (first menstrual cycle)
Growth spurt (typically peaks earlier than in males)
GnRH pulse frequency and amplitude increase
LH and FSH rise, stimulating the gonads
Gonads begin producing significant quantities of sex steroids (testosterone in males, oestrogen and progesterone in females)
The role of leptin in puberty onset is still being studied
All steroid hormones share a common precursor: cholesterol
Cholesterol is the starting block for all sex steroids
Production of specific sex steroids differs between males and females:
Ovary: primarily oestrogen and progesterone
Testis: primarily testosterone and DHT
Adrenal gland: secretes small amounts of sex steroids (approximately 5% of total), relevant in both sexes
Key conversion pathways:
Cholesterol is converted to progesterone
Progesterone can be converted to testosterone
Testosterone can be converted to DHT (via 5-alpha reductase) or to oestradiol (via aromatase)
Three levels:
Hypothalamus: contains the GnRH pulse generator; neurons release GnRH in periodic pulses (every one to three hours)
Anterior pituitary: responds to GnRH by releasing LH and FSH
Gonads: respond to LH and FSH by producing sex steroids and peptide hormones, and by supporting gamete production
Feedback loops:
Long-loop feedback: gonadal steroids and peptide hormones feed back to the hypothalamus and pituitary
Generally negative (suppresses GnRH, LH, FSH)
One important exception: sustained high oestrogen (more than 36 hours) exerts positive feedback on LH in females, triggering the LH surge and ovulation
Short-loop feedback: pituitary hormones feed back on the hypothalamus (generally negative)
Feedback summary table:
Steroid hormone level | Effect | Gonadotrophin response |
|---|---|---|
Low oestrogen or androgen | Absence of negative feedback | LH and FSH increase |
Moderate oestrogen or androgen | Negative feedback | LH and FSH decrease |
High androgen | Negative feedback | LH and FSH decrease |
Sustained high oestrogen (more than 36 hours) | Positive feedback | LH increases (females only) |
Peptide hormone feedback:
Inhibin (from gonads) selectively suppresses FSH (negative feedback)
Activin (from gonads) selectively stimulates FSH (positive feedback)
No mathematical formulas. Key diagrams to review:
The HPG axis feedback loop: hypothalamus (GnRH) to anterior pituitary (LH, FSH) to gonads (steroids, inhibin, activin), with long-loop and short-loop arrows
Steroid synthesis pathway from cholesterol through progesterone, testosterone, DHT and oestradiol
Side-by-side comparison of oogenesis and spermatogenesis timelines
The HPG axis is the target of most hormonal contraceptives. Combined oral contraceptive pills supply exogenous oestrogen and progesterone, which exert negative feedback on GnRH and gonadotrophins, suppressing ovulation. The same axis is manipulated in fertility treatments, where exogenous FSH is given to stimulate multiple follicles, and GnRH analogues are used to control the timing of ovulation.
Students often think puberty is triggered by rising sex steroids. It is the other way round: puberty begins with increased GnRH pulsatility, which then drives gonadotrophin release, which then stimulates steroid production.
A frequent error is stating that females "make new eggs." Oogenesis (mitosis of oogonia) is complete before birth. After birth, only the meiotic maturation of existing oocytes continues.
Students sometimes treat inhibin and activin as interchangeable. They have opposite effects on FSH: inhibin suppresses it, activin stimulates it.
The adrenal gland's contribution to sex steroids is often forgotten. It produces a small but clinically relevant amount (roughly 5%).
⚠️ Be clear on the timing difference: oogenesis (mitosis) finishes before birth; spermatogenesis (mitosis) continues throughout reproductive life.
⚠️ The GnRH pulse generator concept is heavily tested. Know that it is the pulsatile nature of GnRH release that is critical; continuous GnRH actually downregulates receptors and suppresses the axis (the basis for GnRH agonist therapy).
⚠️ Know the long-loop feedback rules, especially the exception: sustained high oestrogen triggers positive feedback on LH in females (the ovulatory surge).
⚠️ Inhibin acts specifically on FSH (not LH). This is a common exam trap.
⚠️ Leptin's role in puberty is a favourite short-answer topic. Know the link: increased body fat leads to increased leptin leads to earlier puberty onset.
True or false: In males, mitosis of germ cells ceases before birth.
Fill in the blank: The hypothalamic hormone that drives the reproductive axis is _______, released in a _______ pattern.
True or false: Inhibin selectively suppresses LH secretion.
Fill in the blank: One primary oocyte yields _______ mature egg(s), while one primary spermatocyte yields _______ spermatozoa.
True or false: High sustained oestrogen levels exert positive feedback on LH release in females.
Answers: 1. False (mitosis of spermatogonia continues throughout life from puberty). 2. GnRH; pulsatile. 3. False (inhibin suppresses FSH, not LH). 4. One; four. 5. True.
Q: Compare and contrast the timing of gametogenesis in males and females.
A: In females, mitosis of oogonia is complete before birth, and meiosis begins in foetal life but arrests in prophase I until puberty. One oocyte completes meiosis per cycle. In males, mitosis of spermatogonia does not begin until puberty and continues throughout life. Meiosis also begins at puberty and is a continuous process, with a new cycle every two to three weeks.
Q: A patient has a tumour that destroys the anterior pituitary. What happens to their gonadal function, and why?
A: LH and FSH secretion would cease because the anterior pituitary is the source of these gonadotrophins. Without LH, Leydig cells (in males) would not be stimulated to produce testosterone. Without FSH, Sertoli cells would not support spermatogenesis (in males), and follicular development would not occur (in females). The result is gonadal atrophy, loss of sex steroid production and infertility.
Q: Explain how leptin links body composition to puberty onset.
A: Fat tissue secretes leptin in proportion to fat mass. Rising leptin levels signal to the hypothalamus that energy stores are sufficient to support reproduction. This is thought to contribute to the reactivation of the GnRH pulse generator. Higher body fat in childhood is associated with earlier puberty, and conditions of severe malnutrition or very low body fat are associated with delayed puberty.
Q: Why does continuous (non-pulsatile) GnRH administration suppress rather than stimulate the reproductive axis?
A: GnRH receptors on the anterior pituitary require pulsatile stimulation to remain upregulated. Continuous exposure leads to receptor downregulation and desensitisation, which paradoxically decreases LH and FSH secretion. This principle is exploited clinically with GnRH agonists to suppress gonadal function (for example, in prostate cancer treatment or precocious puberty).
Q: Distinguish between long-loop and short-loop feedback in the HPG axis.
A: Long-loop feedback refers to gonadal hormones (steroids, inhibin, activin) feeding back to the hypothalamus and anterior pituitary to modulate GnRH, LH and FSH release. Short-loop feedback refers to pituitary hormones (LH, FSH) feeding back directly on the hypothalamus to modulate GnRH release. Both are predominantly negative, with the notable exception of sustained high oestrogen exerting positive feedback on LH.
This material connects backwards to sex determination and differentiation (the gonads built during foetal life are the same ones activated at puberty) and forward to male reproductive system physiology (spermatogenesis in detail, testicular anatomy, Sertoli and Leydig cell functions). It also underpins clinical topics such as puberty disorders (precocious and delayed puberty), infertility and hormonal contraception.
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