Reproductive System &
Hormonal Control
Master the HPG axis, gametogenesis, and the hormonal cascades that regulate male and female reproduction for the MCAT.
MCAT Biology
Foundational Concept 3B
Why Hormonal Control Matters
Reproduction depends on a tightly regulated hormonal feedback system connecting the brain to the gonads. Understanding this axis is essential because it explains fertility, puberty, contraception, and numerous disease states tested on the MCAT.
The Key Hormones of Reproduction
GnRH
Released in pulses from the hypothalamus: stimulates FSH and LH release from the anterior pituitary.
FSH
Follicle-Stimulating Hormone: promotes follicle maturation in females and spermatogenesis in males.
LH
Luteinizing Hormone: triggers ovulation in females and stimulates Leydig cells to produce testosterone in males.
Estrogen
Primary female steroid: drives endometrial proliferation, secondary sex characteristics, and feedback regulation.
Progesterone
Secreted by the corpus luteum: maintains the endometrium and supports early pregnancy.
Testosterone
Primary male androgen: drives spermatogenesis, secondary sex characteristics, and negative feedback on GnRH.
The HPG Axis: A Feedback Loop
Gametogenesis: Making Gametes
Gametogenesis is the process of forming haploid gametes from diploid precursor cells via meiosis. The two major pathways are spermatogenesis (males) and oogenesis (females). Both involve meiosis I and II but differ in timing, output, and regulation.
Spermatogenesis
Continuous from puberty. One spermatogonium produces 4 functional spermatids. Takes approximately 64 days. Occurs in the seminiferous tubules.
Oogenesis
Begins in fetal life, pauses at prophase I. One oogonium produces 1 functional ovum and 2-3 polar bodies. Resumes at puberty, one oocyte per cycle.
Spermatogenesis vs Oogenesis
| Spermatogenesis | Oogenesis | |
|---|---|---|
| Start Time | Puberty | Fetal development |
| Duration | Continuous (~64 days/cycle) | Arrested: resumes cyclically |
| Functional Gametes | 4 sperm per precursor | 1 ovum per precursor |
| Meiosis I Arrest | None | Prophase I (until ovulation) |
| Meiosis II Arrest | None | Metaphase II (until fertilization) |
| Location | Seminiferous tubules | Ovary (follicles) |
| Key Hormone | Testosterone + FSH | Estrogen + FSH/LH |
The Menstrual Cycle: Four Phases
1
Menstrual Phase (Days 1-5): Endometrial lining sheds. Low estrogen and progesterone. FSH begins to rise.
2
Follicular Phase (Days 1-13): FSH stimulates follicle growth. Estrogen rises as follicles mature. Dominant follicle selected.
3
Ovulation (Day 14): Estrogen surge triggers positive feedback on LH. LH surge causes release of the secondary oocyte.
4
Luteal Phase (Days 15-28): Corpus luteum secretes progesterone and estrogen. Maintains endometrium. If no implantation, corpus luteum degenerates.
Hormone Levels Across the 28-Day Cycle
Male Reproductive Hormonal Control
Negative vs Positive Feedback
Negative Feedback
The default mode. High sex steroid levels suppress GnRH, FSH, and LH release. This keeps hormone levels stable and prevents over-stimulation of the gonads.
Positive Feedback
Occurs only once per cycle: a sustained high level of estrogen (from the dominant follicle) switches the pituitary response to amplify LH secretion, causing the LH surge and ovulation.
Fertilization and Early Events
1
Capacitation: Sperm undergo biochemical changes in the female tract, enabling them to penetrate the ovum.
2
Acrosome Reaction: Enzymes released from the sperm head digest the zona pellucida of the oocyte.
3
Cortical Reaction: Upon sperm entry, cortical granules release enzymes that harden the zona pellucida, blocking polyspermy.
4
Meiosis II Completion: The secondary oocyte completes meiosis II only after sperm entry, forming the mature ovum.
5
Zygote Formation: Male and female pronuclei fuse, restoring the diploid chromosome number \(2n = 46\).
Hormones of Pregnancy
hCG
Human Chorionic Gonadotropin: secreted by the embryo/trophoblast. Maintains the corpus luteum during early pregnancy so progesterone stays high.
Progesterone
Initially from the corpus luteum, later from the placenta. Maintains the endometrium, suppresses uterine contractions, and prevents immune rejection.
Estrogen
Promotes uterine growth, increases blood flow to the uterus, and stimulates breast development for lactation.
Relaxin
Softens the cervix and relaxes pelvic ligaments in preparation for delivery. Secreted by the corpus luteum and placenta.
How to Apply: Solving HPG Axis Problems
1
Identify the level: Is the problem at the hypothalamus, pituitary, or gonad level?
2
Trace the cascade: GnRH stimulates FSH/LH, which stimulate the gonads. Disruption at any level affects downstream targets.
3
Apply feedback rules: If sex steroids are high, expect low GnRH/FSH/LH (negative feedback). If steroids are low, expect high FSH/LH (loss of inhibition).
4
Check for exceptions: Positive feedback (mid-cycle estrogen surge), exogenous hormones (oral contraceptives suppress GnRH), or tumors that secrete hormones autonomously.
5
Predict the outcome: Use the feedback logic to predict whether a patient will be fertile or infertile, and which hormones will be elevated or suppressed.
Worked Example: Primary Gonadal Failure
A patient with primary ovarian failure has non-functional ovaries. What happens to hormone levels?
1
Identify the level: The problem is at the gonad level: ovaries cannot produce estrogen or progesterone.
2
Apply feedback: Low estrogen and progesterone means loss of negative feedback on the hypothalamus and pituitary.
3
Predict outcome: GnRH, FSH, and LH will all be elevated because there is no signal to shut them down.
Worked Example: Oral Contraceptives
Combined oral contraceptives (COCs) contain synthetic estrogen and progesterone. How do they prevent pregnancy?
1
Exogenous steroids are perceived by the hypothalamus and pituitary as endogenous sex steroids.
2
Negative feedback activates: Constant synthetic estrogen + progesterone suppress GnRH pulses, reducing FSH and LH.
3
No LH surge: Without the LH surge, ovulation does not occur. No follicle matures, no egg is released.
Common Mistakes on the MCAT
✕ Confusing Primary vs Secondary
Primary failure (gonad): high FSH/LH. Secondary failure (pituitary): low FSH/LH. Check which organ is dysfunctional before predicting hormone levels.
✕ Forgetting the Estrogen Switch
Low estrogen causes negative feedback. Only sustained high estrogen switches to positive feedback. Many students forget the threshold effect.
✕ Mixing Up Sertoli and Leydig
Sertoli cells respond to FSH and secrete inhibin/ABP. Leydig cells respond to LH and produce testosterone. Do not confuse their targets.
Question 1 of 5
A woman with a pituitary tumor that destroys gonadotroph cells would most likely show which hormone profile?
Destruction of gonadotroph cells means the anterior pituitary cannot produce FSH or LH. Without these signals, the ovaries are not stimulated, so estrogen is also low. This is a secondary (pituitary) failure pattern: all three are low.
Question 2 of 5
Which of the following directly triggers ovulation?
The LH surge is the direct trigger of ovulation. It is caused by the positive feedback loop: sustained high estrogen from the dominant follicle switches the pituitary response from inhibition to amplification of LH. FSH helps follicle growth but does not trigger ovulation. Progesterone rises after ovulation.
Question 3 of 5
In the male, Sertoli cells are stimulated by which hormone, and they secrete which substance that provides negative feedback?
Sertoli cells are stimulated by FSH from the anterior pituitary. They secrete inhibin, which specifically feeds back to inhibit FSH release. They also produce androgen-binding protein (ABP). Leydig cells (not Sertoli) are the ones stimulated by LH and producing testosterone.
Question 4 of 5
A secondary oocyte is arrested at which stage of cell division, and what event triggers completion?
The secondary oocyte is arrested at metaphase II. It only completes meiosis II when a sperm penetrates (fertilization). Before that, the primary oocyte was arrested at prophase I (from fetal life), and the LH surge triggered completion of meiosis I at ovulation.
Question 5 of 5
What is the primary role of hCG in early pregnancy, and what happens if hCG levels are insufficient?
hCG (human chorionic gonadotropin) is secreted by the trophoblast of the embryo. Its critical role is to maintain the corpus luteum so it continues to secrete progesterone. Without sufficient hCG, the corpus luteum degenerates, progesterone drops, the endometrium sheds, and the pregnancy is lost.