3.1 Reproductive Systems & Hormonal Regulation

Why This Section Matters

Reproduction and development is about 20% of the Biology 30 diploma exam, and almost every question in it depends on knowing which hormone does what. Examiners love context-dependent sets: a graph of hormone levels across a 28-day cycle, then several multiple-choice and numerical-response items that ask you to read events off it.

The single most tested idea is hormonal feedback — how the hypothalamus, pituitary, and gonads talk to each other. If you can trace those loops and label the male and female structures, you will handle most of Unit B's marks.

The Male Reproductive System

Sperm are produced in the seminiferous tubules of the testes by spermatogenesis. Two key cell types sit in these tubules:

• Sertoli (sustentacular) cells nourish developing sperm and respond to FSH.
• Leydig (interstitial) cells lie between the tubules and secrete testosterone in response to LH.

Sperm mature and are stored in the epididymis, travel through the vas deferens, and mix with fluids from the seminal vesicles, prostate gland, and bulbourethral (Cowper's) glands to form semen. These fluids supply fructose for energy and buffer the acidity of the vagina.

The scrotum keeps the testes about 2 to 3 degrees C below body temperature, which is required for viable sperm production. A common diploma question links a labelled structure to its function, so be able to trace the path seminiferous tubule to epididymis to vas deferens to urethra.

Male Hormonal Control

The hypothalamus releases GnRH (gonadotropin-releasing hormone), which makes the anterior pituitary secrete FSH and LH.

• LH stimulates Leydig cells to make testosterone.
• FSH plus testosterone stimulates Sertoli cells to support sperm production.
• Sertoli cells also secrete inhibin, which selectively suppresses FSH.

Testosterone drives male secondary sexual characteristics (facial and body hair, deeper voice, increased muscle and bone mass) and sustains the sex drive. High testosterone feeds back negatively on the hypothalamus (reducing GnRH) and on the pituitary, holding levels relatively steady — there is no monthly cycle in males. This steady-state negative feedback is a frequent contrast with the female cycle's brief positive-feedback LH surge, so make sure you can describe both.

The Female Reproductive System

The ovaries produce eggs (ova) and secrete estrogen and progesterone. At ovulation the egg is released into the oviduct (Fallopian tube), where fertilization normally occurs in the upper third. A fertilized egg travels to the uterus and implants in the endometrium, the inner lining that thickens each cycle. The lower neck of the uterus is the cervix, which opens into the vagina.

Unlike the continuous male pattern, the female system runs on a roughly 28-day menstrual cycle that couples ovarian events to changes in the uterine lining. The same two hormones, estrogen and progesterone, also produce female secondary sexual characteristics such as breast development and the widening of the pelvis.

The Menstrual Cycle

The cycle has two synchronized parts. The table summarizes the dominant hormone and event in each phase (Day 1 = first day of menstruation).

If no pregnancy occurs, the corpus luteum degenerates near day 28, progesterone falls, and the lining sheds to begin a new cycle.

Hormonal Feedback in the Cycle

Think of the cycle as a relay of four hormones. In the follicular phase:

• FSH stimulates a follicle to mature; the growing follicle secretes estrogen.
• Rising estrogen rebuilds the endometrium and, late in the follicular phase, briefly switches to positive feedback, causing a sharp LH surge.
• The LH surge triggers ovulation (~day 14) and converts the ruptured follicle into the corpus luteum.

In the luteal phase:

• The corpus luteum secretes progesterone (and estrogen), which maintains the secretory endometrium and, by negative feedback, suppresses FSH and LH so no new follicle matures.
• Without fertilization, the corpus luteum dies, progesterone drops, menstruation begins, and FSH rises again.

A common diploma trap is confusing the two feedback modes: estrogen is usually negative, but it becomes positive only briefly at high levels to fire the LH surge.

Reproductive Technologies

These applications manipulate the same loops you just learned, so the exam tests them as feedback problems.

• Hormonal contraceptives (the pill) supply estrogen and/or progesterone. Through negative feedback they suppress FSH and LH, so no follicle matures and ovulation is prevented. The progestin component also thickens cervical mucus, adding a second barrier to sperm.
• In vitro fertilization (IVF) fertilizes an egg outside the body. Ovaries are hormonally stimulated to mature several follicles, mature eggs are retrieved, combined with sperm in a lab dish, and a resulting embryo is transferred into the uterus to implant.
• Fertility hormone treatments can stimulate ovulation in people who do not ovulate naturally, and hCG in fertility or pregnancy contexts mimics LH activity.

Expect a question that asks how a method works in feedback terms, not just what it is. For example, the pill works by lowering FSH and LH, which is the opposite of how a fertility drug deliberately raises follicle stimulation.