Difficulty: Intermediate | Prerequisites: Introductory biology (cell structure, basic genetics, meiosis vs mitosis, endocrine signalling basics)
This unit covers how organisms reproduce and how hormones coordinate the entire process, from gamete production through pregnancy and lactation. It sits at the intersection of endocrinology and developmental biology, connecting earlier material on cell division (meiosis) and hormone signalling (receptor types, feedback loops) to the specific organ systems that make reproduction possible. If you are comfortable with the hypothalamic-pituitary axis and the basics of meiosis, you are ready for this material. If not, revisit those topics first, because the HPG axis is the backbone of nearly every mechanism discussed here.
Reproduction comes in two flavours: asexual (one parent, genetically identical offspring) and sexual (two parents, genetically diverse offspring). In vertebrates, the hypothalamic-pituitary-gonad axis orchestrates gametogenesis, the reproductive cycles, pregnancy and lactation through a cascade of hormones, chiefly GnRH, FSH, LH, testosterone, estradiol and progesterone. Understanding the feedback loops in this axis is the single most important thing for exam success in this unit.
Asexual reproduction
A single parent produces offspring that are genetically identical (or nearly so) to itself. Think of it as cloning without the lab.
Sexual reproduction
Two parents each contribute genetic material, producing offspring with unique genotype combinations. In simple terms, this is the engine behind genetic diversity in a population.
Parthenogenesis
Asexual reproduction in which an egg develops without fertilisation. No male gamete is required. Think of it as a female producing offspring solo, as in whiptail lizards.
Gametogenesis
The production of gametes (sex cells) through meiosis. In males it is called spermatogenesis; in females, oogenesis.
Sry gene
The gene on the Y chromosome that triggers male development. If present, the bipotential gonad differentiates into a testis. If absent, female development proceeds by default.
Hypothalamic-Pituitary-Gonad (HPG) axis
The hormonal signalling chain from the hypothalamus (GnRH) to the anterior pituitary (FSH and LH) to the gonads (steroid hormones). In simple terms, it is the command chain for reproduction.
Gonadotropin-releasing hormone (GnRH)
A peptide hormone synthesised in the hypothalamus that triggers FSH and LH release from the anterior pituitary.
Follicle-stimulating hormone (FSH)
A gonadotropin from the anterior pituitary. In males it acts on Sertoli cells to support spermatogenesis. In females it stimulates follicle growth in the ovaries.
Luteinising hormone (LH)
A gonadotropin from the anterior pituitary. In males it stimulates Leydig cells to produce testosterone. In females a surge of LH triggers ovulation.
Androgens (e.g. testosterone)
Steroid hormones derived from cholesterol, produced mainly in the testes (Leydig cells). They drive secondary sex characteristics and support spermatogenesis.
Estrogens (e.g. estradiol)
Steroid hormones produced in the ovaries (and placenta during pregnancy). They regulate the menstrual cycle and secondary sex characteristics in females.
Progestins (e.g. progesterone)
Steroid hormones critical for maintaining the endometrium. Produced by the corpus luteum (and later the placenta). Think of progesterone as the hormone that keeps the uterine lining in place.
Sertoli cells
Cells in the seminiferous tubules that support and nourish developing sperm. They also produce inhibin, which feeds back to inhibit FSH.
Leydig cells
Cells in the testes that produce testosterone in response to LH stimulation.
Corpus luteum
The structure that forms from the remnants of the ovarian follicle after ovulation. It secretes progesterone (and some estrogen) to maintain the endometrium.
Human chorionic gonadotropin (hCG)
A hormone produced by the chorion (part of the developing embryo/placenta) that maintains the corpus luteum during early pregnancy. This is the hormone detected by pregnancy tests.
Prolactin
A peptide hormone from the anterior pituitary that stimulates milk production. Found in all vertebrates, even those without mammary glands.
Oxytocin
A hormone released from the posterior pituitary. It induces uterine contractions during labour, stimulates milk ejection (let-down reflex) and promotes maternal recognition of offspring.
Reproduction is necessary for maintenance of a species and falls into two broad categories.
Asexual reproduction: a single individual produces offspring that are genetically identical (or very similar) to the parent
Genomic variation is not necessary in relatively constant environments
Examples: budding in hydra, fragmentation in starfish (arms split off from somatic tissue to generate progeny)
Sexual reproduction: two parents contribute roughly equal amounts of genetic material, generating offspring with unique genotype combinations
Creates the genetic diversity on which natural selection acts
Key mechanisms: independent assortment (random reorganisation of maternal and paternal chromosomes), recombination/cross-overs producing hybrid chromosomes, and the production of haploid gametes from a diploid parent
Parthenogenesis is a special case of asexual reproduction in which an egg develops without fertilisation. Offspring can be haploid or diploid. No male gamete is required.
Example: whiptail lizards, where two females simulate copulation. Female behaviour is hormonally driven: high estradiol produces female-like behaviour (gets mounted, oogenesis stimulated), while progesterone drives male-like behaviour.
Overview of sexual reproduction (sequence):
Sex determination
Gametogenesis
Mating
Fertilisation
Development and growth
Sex determination establishes whether an individual develops ovaries (female) or testes (male). There are two stages:
Primary sex determination: occurs during embryonic development. Bipotential gonads differentiate into either ovary or testis.
Secondary sex differentiation: occurs during growth and later development. Produces sexually dimorphic characteristics: reproductive structures (other than gonads), coat colour/patterns, behaviours.
Genetic sex determination systems:
Mammals (XX/XY): the Sry gene on the Y chromosome is the critical switch. If Sry is present, the individual develops male. If absent, female development proceeds by default.
Heterogametic = XY (two different sex chromosomes)
Homogametic = XX (two identical sex chromosomes)
Birds (ZW/ZZ): females are heterogametic (ZW), males are homogametic (ZZ). Less of a difference in the amount of genetic material between sexes compared to mammals.
Honeybee (haplodiploidy): diploid individuals (2 sets of chromosomes) are female; haploid individuals (1 set) are male.
Fruit fly (Drosophila): sex depends on the ratio of X chromosomes to autosomes (X:A). Female = X:A of 1.0, Male = X:A of 0.5.
C. elegans: also depends on X:A ratio. A high X:A ratio produces a hermaphrodite.
Environmental sex determination:
Temperature-dependent: determined by environmental temperature during early development
Lizards: low temperature = female, high temperature = male
Sea turtles: low temperature = male, high temperature = female (opposite pattern)
Other environmental factors include social cues and pH.
Hermaphrodites:
Simultaneous hermaphrodites: produce both egg and sperm at the same time. Some can self-fertilise.
Serial hermaphrodites: change sex in response to environmental cues
Protogynous: female to male
Protandrous: male to female
Gametogenesis is the production of haploid gametes (sperm and ova) from diploid precursors via meiosis. The nuclear events of meiosis are the same for spermatogenesis and oogenesis, but the cytoplasmic division differs. The whole process is driven by the HPG axis.
HPG axis signalling chain:
Hypothalamus synthesises and releases GnRH
GnRH is delivered to the anterior pituitary, where it regulates synthesis and release of the gonadotropins FSH and LH
FSH and LH (peptide hormones) travel to the gonads and control steroid hormone synthesis
Gonads produce the reproductive steroid hormones: androgens (e.g. testosterone), estrogens (e.g. estradiol) and progestins (e.g. progesterone)
Reproductive steroid hormones:
All derived from cholesterol
Bind to nuclear hormone receptors in target cells and regulate gene expression
Produced in the gonads in vertebrates
The three main classes are androgens, estrogens and progestins
In most species males produce sperm continuously throughout life. The testes produce spermatozoa within the seminiferous tubules.
Cell types in the seminiferous tubules:
Spermatogenic cells progress through stages: spermatogonium, spermatocyte, spermatid, spermatozoon. Development proceeds from the outer wall of the tubule towards the centre. Mature spermatozoa are released into the lumen.
Sertoli cells fill the gaps between developing sperm cells and aid their development. They respond to FSH and produce inhibin.
Leydig cells sit outside the tubules and produce testosterone in response to LH.
Spermatogenesis: spermatids lose cytoplasm, develop an axoneme (the core of the flagellum) and condense their DNA to become spermatozoa. Nearly mature spermatozoa that cannot yet swim are released into the tubule lumen, then stored in the epididymis to mature further and develop swimming ability.
Sperm release during ejaculation:
Sperm travel along the vas deferens (propelled by cilia or smooth muscle contractions) to the urethra
Seminal fluid is added by exocrine glands:
Seminal vesicles: nutrients and alkaline fluids
Prostate: nutrients, mainly citrate
Bulbourethral gland: mucus as lubricant
Male hormonal regulation:
Hypothalamus produces GnRH, which acts on the anterior pituitary
FSH acts on Sertoli cells, stimulating spermatogenesis
LH acts on Leydig cells, stimulating androgen (testosterone) synthesis and release
Negative feedback: testosterone inhibits both anterior pituitary (FSH/LH release) and hypothalamus (GnRH release). Inhibin from Sertoli cells also feeds back to inhibit FSH at the anterior pituitary.
Prostaglandins from the seminal vesicles induce changes in the mate's uterus that affect sperm motility
Estrous cycle (most mammals, except some primates):
Sexual receptivity coincides with a specific phase of the cycle ("estrus" or "in heat")
Minimal to moderate uterine tissue loss
Females exhibit behavioural cues and pheromones when fertile
Timing varies hugely: rodents cycle every ~4 days, deer ~28 days with seasonal breeding, dogs every 3 to 4 months
Menstrual cycle (some primates and a few exceptions):
Sexual receptivity occurs at many phases of the cycle
Substantial uterine tissue loss during menstruation
Female anatomy (key structures): ovaries, oviducts (fallopian tubes), uterus (with myometrium, the muscle layer important in childbirth), cervix, vagina.
The three phases of the menstrual cycle:
1. Follicular phase:
Estrogen starts relatively low and gradually increases
Initially estrogen exerts negative feedback on gonadotropins (FSH and LH)
As estrogen rises past a critical threshold, the feedback switches to positive (stimulatory)
FSH and LH surge, with the FSH surge slightly preceding LH
Hormonal chain: hypothalamus (GnRH), anterior pituitary (FSH + LH), ovaries/follicles (estradiol), uterus (endometrium thickens)
A dominant follicle is selected from several growing follicles; the number of follicles a female has is set at birth, and the follicle that matures is selected randomly
2. Ovulation:
Triggered by the LH surge specifically (FSH surge precedes it slightly)
The mature follicle releases the oocyte
3. Luteal phase:
The remnants of the follicle form the corpus luteum, which secretes progesterone
Progesterone maintains the thickened endometrium
If fertilisation does not occur, progesterone levels fall below a threshold and menstruation begins (shedding of the uterine lining)
Simultaneously, a new follicle is selected for maturation in the next cycle
Pregnancy and the corpus luteum:
When the egg is fertilised, the corpus luteum is maintained by hCG from the developing chorion. Progesterone continues to be secreted, keeping the endometrium thick.
Fate of the egg (three reproductive strategies):
Ovipary: eggs laid externally; all development external. Fertilisation can be external or internal. Examples: fish, reptiles, birds.
Vivipary: young develop within the female body; fertilisation is internal. Mammals and a few other taxa.
Ovo-vivipary: eggs "laid" within the mother's body, develop and hatch internally. Some reptiles and fish.
The placenta:
Interface between mother and fetus, composed of cells from both
The trophoblast (outermost cells of the blastula) invades the endometrium and forms the placenta
First trimester: the placenta has a vital endocrine function. The chorion secretes hCG, which maintains the corpus luteum so it continues producing estrogen and progesterone.
Later in pregnancy: the placenta itself takes over production of estrogen and progesterone
Parturition (birth):
Induced by contraction of smooth muscle (myometrium) of the uterus
Progesterone levels decrease, allowing uterine muscle to contract
Prostaglandins and oxytocin induce uterine contractions
Fetal cells produce oxytocin that causes the placenta to release prostaglandins
The hypothalamic-pituitary axis also releases oxytocin (positive feedback loop)
The placenta is expelled soon after birth ("afterbirth")
Prolactin (PRL):
Peptide hormone released from the anterior pituitary gland
Increases mammary gland mass and ensures biosynthetic machinery is in place for milk production
Released in response to increased estrogen levels during pregnancy
High levels of progesterone and estrogen suppress milk production during pregnancy itself (milk production begins after birth when progesterone drops)
Also promotes maternal behaviour through effects on the brain
Found in all vertebrates, even those without mammary glands
Oxytocin:
Produced in hypothalamic magnocellular neurosecretory cells and released from the posterior pituitary via exocytosis
Stimuli for release: suckling, visual cues, psychological stimuli. These travel via sensory/visual afferent nerve fibres, through the spinal cord, to the hypothalamus.
Oxytocin acts on smooth muscles surrounding the gland ducts, causing contraction and milk secretion (let-down reflex)
Oxytocin receptors are found on glandular cells (milk synthesis) and myoepithelial cells (surround clusters of milk-producing cells)
Oxytocin also allows for maternal recognition of offspring
No mathematical formulas are central to this unit. The key diagrams to know are:
HPG axis flowchart: Hypothalamus (GnRH) > Anterior pituitary (FSH + LH) > Gonads (steroid hormones), with negative feedback loops from the gonads back to both the pituitary and hypothalamus.
Menstrual cycle hormone graph: track estradiol, progesterone, FSH and LH levels across the follicular phase, ovulation and luteal phase. Pay attention to the estrogen threshold that switches feedback from negative to positive.
Spermatogenesis progression: spermatogonium > spermatocyte > spermatid > spermatozoon, moving from the outer wall of the seminiferous tubule towards the lumen.
Oxytocin positive feedback loop for parturition: fetal oxytocin > placental prostaglandins > uterine contractions > more oxytocin release.
Hormonal contraceptives work by manipulating the same HPG axis covered here: synthetic estrogen and progestin suppress GnRH, FSH and LH, preventing follicle maturation and ovulation. IVF protocols use exogenous FSH to stimulate multiple follicles simultaneously, then trigger ovulation with an hCG injection that mimics the LH surge. Pregnancy tests detect hCG in urine, which is the same chorionic gonadotropin discussed in the placenta section.
Students often think estrogen is always inhibitory on gonadotropins. It is not. During the follicular phase, once estrogen crosses a threshold concentration, it switches to positive feedback and drives the LH surge that triggers ovulation.
Students often confuse which cells respond to which gonadotropin. FSH acts on Sertoli cells (males) and follicle cells (females). LH acts on Leydig cells (males) and triggers ovulation (females). Do not swap them.
Students sometimes think the corpus luteum forms before ovulation. It forms after. The corpus luteum is the remnant of the follicle once the oocyte has been released.
Students often assume all mammals have menstrual cycles. Most mammals have estrous cycles. Menstrual cycles with substantial uterine tissue loss are found only in some primates and a few exceptions.
⚠️ The HPG axis feedback loops (negative and the switch to positive for estrogen) are heavily tested. Be able to trace the full chain from hypothalamus to gonad and back.
⚠️ Know which cell type responds to which hormone: Leydig cells to LH, Sertoli cells to FSH.
⚠️ The corpus luteum secretes progesterone to thicken/maintain the endometrium. This appeared as an iClicker question in lecture.
⚠️ Be able to distinguish estrous from menstrual cycles and give examples of species for each.
⚠️ Understand the three reproductive strategies (ovipary, vivipary, ovo-vivipary) and which taxa use each.
⚠️ Know the role of hCG in maintaining pregnancy and why it is the basis for pregnancy tests.
True or false: The Sry gene, when present, causes development of female characteristics. (False. Sry drives male development.)
Fill in the blank: The anterior pituitary releases two gonadotropins called ______ and ______. (FSH and LH)
True or false: In the menstrual cycle, progesterone is primarily responsible for maintaining the endometrium during the luteal phase. (True.)
Fill in the blank: The structure that secretes hCG during early pregnancy is the ______. (Chorion)
True or false: Prolactin is found only in mammals with mammary glands. (False. It is found in all vertebrates.)
Q: Which structure secretes progesterone to maintain the endometrium during the luteal phase?
A: The corpus luteum. It forms from the remnants of the ovarian follicle after ovulation and produces progesterone (and some estrogen) to keep the endometrium thick and receptive.
Q: Describe the hormonal chain that leads to ovulation.
A: The hypothalamus releases GnRH, which stimulates the anterior pituitary to release FSH and LH. FSH promotes follicle growth and estradiol production. Rising estradiol initially inhibits gonadotropins (negative feedback), but once it exceeds a threshold it switches to positive feedback, causing an LH surge. The LH surge triggers ovulation.
Q: A female dog shows behavioural signs of fertility every 3 to 4 months. Is this a menstrual or estrous cycle, and how do you know?
A: Estrous cycle. The dog's sexual receptivity is limited to a specific phase (estrus/"in heat"), and dogs are not primates. Menstrual cycles with substantial uterine tissue loss occur only in some primates.
Q: What maintains the corpus luteum during early pregnancy, and what happens later?
A: During the first trimester, hCG from the chorion maintains the corpus luteum, which continues to secrete progesterone and estrogen. Later in pregnancy, the placenta takes over production of these hormones directly.
Q: In the male HPG axis, what is the function of inhibin?
A: Inhibin is produced by Sertoli cells and feeds back to the anterior pituitary to inhibit FSH release. This is part of the negative feedback that regulates spermatogenesis.
Q: Name three differences between estrous and menstrual cycles.
A: (1) Estrous cycles confine sexual receptivity to the fertile phase; menstrual cycles allow receptivity at many phases. (2) Estrous cycles involve minimal to moderate uterine tissue loss; menstrual cycles involve substantial loss. (3) Estrous cycles are found in most mammals; menstrual cycles occur only in some primates and a few other species.
This material connects directly to the endocrine system unit: the HPG axis is one specific instance of the hypothalamic-pituitary control model you will see repeated for the thyroid (HPT axis) and adrenal glands (HPA axis). The feedback principles are identical.
Gametogenesis ties back to your meiosis material. If the stages of meiosis I and II are fuzzy, revisit them, because exam questions may ask you to place spermatocytes or oocytes at specific meiotic stages.
Oxytocin and prolactin reappear in the neuroscience portion of the course as examples of neuroendocrine signalling from the hypothalamus/posterior pituitary.
Reproduction, asexual reproduction, sexual reproduction, parthenogenesis, gametogenesis, spermatogenesis, oogenesis, meiosis, HPG axis, hypothalamic-pituitary-gonad axis, GnRH, gonadotropin-releasing hormone, FSH, follicle-stimulating hormone, LH, luteinising hormone, testosterone, estradiol, progesterone, androgens, estrogens, progestins, Sry gene, sex determination, XX/XY, ZW/ZZ, temperature-dependent sex determination, hermaphrodite, protogynous, protandrous, seminiferous tubules, Leydig cells, Sertoli cells, inhibin, epididymis, vas deferens, seminal vesicles, prostate, estrous cycle, menstrual cycle, follicular phase, ovulation, luteal phase, corpus luteum, endometrium, myometrium, hCG, human chorionic gonadotropin, trophoblast, placenta, vivipary, ovipary, ovo-vivipary, parturition, prolactin, oxytocin, let-down reflex, milk ejection, BIO 225, animal physiology, reproductive physiology