HPG Axis: Hormones, Regulation, and Puberty Mechanisms

What does HPG stand for?

Hypothalamic-pituitary-gonadal.

Which hypothalamic hormone initiates the HPG-axis sequence?

Gonadotropin-releasing hormone, GnRH.

How is GnRH secreted?

In a pulsatile manner.

Where is GnRH released from hypothalamic neurons?

At the median eminence.

Into which vessels is GnRH released?

The hypophyseal portal blood vessels.

Which pituitary region responds to GnRH?

The anterior pituitary gland.

Which two gonadotrophins are released from the anterior pituitary?

Luteinising hormone, LH, and follicle-stimulating hormone, FSH.

Which organs respond to LH and FSH?

The ovaries and testes.

Name the gonadal products shown in the lecture diagram.

Gametes and hormones including oestradiol, progesterone, testosterone and inhibin.

State the HPG-axis sequence from hypothalamus to gonads.

Hypothalamic GnRH pulses → anterior pituitary LH and FSH → ovarian or testicular response → sex steroids, inhibin and gamete-related function.

Why is the portal circulation important in the HPG axis?

It carries GnRH directly from the median eminence to the anterior pituitary, allowing the hypothalamic signal to control pituitary gonadotrophin release.

How do gonadal hormones regulate the HPG axis?

They provide feedback to the hypothalamic-pituitary system and influence further hormone secretion.

According to the lecturer, do sex steroids directly provide negative feedback to GnRH neurons?

No. The lecture states that an intermediate neuronal population is involved.

Why must a description of the HPG axis include pulsatility?

GnRH pulses drive pulsatile LH and FSH release. The change in pulse activity is central to sexual maturation and puberty.

A student writes "the hypothalamus releases LH." What is the correction?

The hypothalamus releases GnRH. GnRH stimulates the anterior pituitary to release LH and FSH.

A student writes "FSH is released by the ovary." What is the correction?

FSH is released by the anterior pituitary. It acts on the ovary.

What is the difference between a releasing hormone and a gonadotrophin in this pathway?

GnRH is a hypothalamic releasing hormone that controls the pituitary. LH and FSH are pituitary gonadotrophins that act on the gonads.

How does the HPG axis connect molecular signalling to reproductive function?

A hormonal signal from the hypothalamus changes pituitary secretion, which alters gonadal hormone production and gamete-related processes, producing organism-level reproductive capacity.

How is puberty defined in the lecture?

The physiological and behavioural process of change through which an individual becomes capable of sexual reproduction.

What happens to testosterone at male puberty?

It increases markedly.

What happens to testosterone later in male life?

It gradually declines, although the decline is less dramatic than female reproductive ageing.

What happens to oestrogen at female puberty?

It increases and begins to cycle.

How does prepubertal LH secretion appear?

LH concentrations are low and show only small changes across the day.

How does postpubertal LH secretion differ?

LH pulses are larger and overall LH concentrations are higher.

What endocrine change enables reproductive capacity at puberty?

Increased pulsatile activity of the HPG axis produces stronger LH and FSH stimulation of the gonads, leading to sex-steroid production and mature reproductive function.

What happens to the ovarian reserve over time?

Oocyte numbers and the number of functioning follicles decline.

Link ovarian-reserve depletion to menopause.

Fewer follicles → lower oestrogen production → insufficient positive feedback for an LH surge → ovulation ceases → menopause.

Why is an LH surge required in the menopause explanation used in the lecture?

The LH surge is needed to stimulate ovulation. When oestrogen can no longer generate sufficient positive feedback, the surge and ovulation do not occur.

What reproductive-ageing difference between humans and rodents is emphasised?

Humans are among the few species that undergo menopause. Rodents do not typically reproduce this human life-history pattern.

A graph shows much larger LH fluctuations after puberty. What does this indicate?

The GnRH-LH pulse system has become more active, consistent with postpubertal HPG-axis function.

Why is puberty more than a rise in one hormone?

It involves coordinated physiological and behavioural changes across the HPG axis, gonadal hormones, gamete production and reproductive capability.

How was puberty viewed before the scientific revolution, according to the slide?

As part of nature, with its causes considered unknowable.

What was predicted in the 1940s?

The presence of a hypothalamic releasing signal later identified as GnRH.

Which hormones were isolated in mammals in the 1970s?

GnRH, FSH and LH.

What was demonstrated later in the 1970s?

The importance of pulsatile GnRH release for sexual maturation.

What unanswered question remained after GnRH, LH and FSH were identified?

What triggers the increase in pulsatile GnRH release at puberty?

Why can human pathologies reveal normal biological control mechanisms?

An abnormal phenotype can identify a disrupted gene or pathway. Recreating the disruption experimentally can show what the pathway normally contributes.

Which human condition is used to investigate the control of puberty?

Functional hypogonadotrophic hypogonadism, FHH.

What is the major puberty phenotype of FHH?

Affected individuals do not undergo normal puberty.

What sex-steroid pattern is associated with FHH in the lecture?

Very low oestrogen or testosterone.

What sensory feature was mentioned in affected people?

An absent sense of smell.

Which gene/receptor was linked to the condition?

GPR54, also called the KISS1 receptor, KISS1R.

What ligand binds GPR54/KISS1R?

Kisspeptin.

What class of receptor is GPR54/KISS1R?

A G-protein-coupled receptor.

Describe the signalling sequence after kisspeptin binds its receptor.

Kisspeptin binds KISS1R → G protein is activated → intracellular signalling cascade is triggered → cell function changes.

What happens when KISS1R is non-functional?

Normal kisspeptin signalling cannot occur, preventing the pathway from supporting normal puberty activation.

What conclusion does the lecture draw about kisspeptin?

Kisspeptin signalling is integral to puberty timing.

A receptor is present but cannot activate a G protein. What is the predicted effect on the lecture pathway?

Kisspeptin binding would fail to generate the normal intracellular signal, so normal puberty activation would be disrupted.

What did researchers alter in the transgenic mouse model?

They disrupted the receptor gene so the mouse could not produce a functional GPR54/KISS1R product.

Why is this described as recreating the human condition in mice?

The mouse was engineered to have the same essential signalling failure identified in affected humans.

Did the mutant mice undergo normal puberty?

No.

How did mutant reproductive organs compare with wild-type organs?

The testes, ovaries and uterus were much smaller.

What happened to seminiferous-tubule organisation in mutant males?

The tubules were smaller and disorganised.

Was sperm production present in the mutant testes shown?

No normal sperm production was observed.

What ovarian structure was absent in mutant females?

Corpora lutea.

Why does absence of corpora lutea indicate failed ovulation?

A corpus luteum forms from the follicular cells remaining after ovulation. If no corpora lutea are present, ovulation has not occurred.

What sex-steroid pattern occurred in the mutants?

Very low oestrogen and testosterone.

What was the fertility outcome of receptor disruption?

Infertility.

Link the genetic change to the reproductive phenotype.

Receptor-gene disruption → failed kisspeptin signalling → inadequate puberty activation → small gonads and low sex steroids → absent sperm production or ovulation → infertility.

How does the transgenic model provide evidence for normal receptor function?

Loss of receptor function removes puberty and fertility. This loss-of-function phenotype shows that the intact receptor is required for normal HPG-axis maturation.

What is the experimental comparison in the transgenic-mouse example?

Wild-type mice with functional signalling vs genetically altered mice lacking functional GPR54/KISS1R signalling.

A mutant ovary contains follicles but no corpora lutea. What conclusion is supported?

Follicular structures may be present, but ovulation and normal cyclic reproductive function are not occurring.

Why is a mouse model useful for this genetic question?

Researchers can deliberately disrupt a gene, examine organs and histology, and connect the molecular defect to puberty and fertility outcomes in a whole organism.

At approximately what age does puberty occur in rats?

Around 5 to 7 weeks.

When does daily puberty assessment begin in the lecture example?

From about the start of week 5.

What external sign is used to assess female rat puberty?

Vaginal opening.

What external sign is used to assess male rat puberty?

Preputial separation.

What is preputial separation?

The stage when the foreskin can retract from the glans.

How often are puberty markers checked?

Daily until the marker is observed.

On an age-at-puberty graph, what does a higher bar mean?

Puberty occurred later, so the treatment delayed puberty.

On an age-at-puberty graph, what does a lower bar mean?

Puberty occurred earlier.

Why must the marker be checked repeatedly rather than only once?

The exact age of onset is the outcome. Daily assessment narrows the timing and allows groups to be compared.

Why are synthetic glucocorticoids given when preterm birth is expected?

They help mature fetal organs before birth.

Why might prenatal glucocorticoid treatment raise a later-life research question?

Changing the fetal endocrine environment may have unintended developmental effects that appear later, including altered puberty timing.

How can maternal stress alter fetal glucocorticoid exposure?

High stress can increase natural maternal glucocorticoids, which may increase exposure of the developing fetus.

What was the main outcome measured after prenatal glucocorticoid manipulation?

Age at puberty.

What did the control group represent?

Normal glucocorticoid exposure during pregnancy.

What does metyrapone do in the study design?

It blocks glucocorticoid synthesis, creating low exposure.

What does dexamethasone provide?

A synthetic glucocorticoid, creating high glucocorticoid exposure.

What does carbenoxolone do?

It inactivates or inhibits 11β-HSD2, increasing exposure to active natural glucocorticoids.

What does adrenalectomy do in the study design?

It removes the site of glucocorticoid synthesis, creating low exposure.

What is the normal placental role of 11β-HSD2 in this lecture?

It inactivates glucocorticoids crossing toward the fetus and limits fetal exposure.

How does carbenoxolone increase fetal glucocorticoid exposure?

Carbenoxolone inhibits 11β-HSD2 → less glucocorticoid inactivation in the placenta → more active maternal glucocorticoid reaches the fetus.

How do dexamethasone and carbenoxolone both create high exposure by different mechanisms?

Dexamethasone supplies a synthetic glucocorticoid. Carbenoxolone prevents normal inactivation of natural glucocorticoids.

What happened to puberty timing after low- and high-dose dexamethasone?

Puberty occurred later.

What happened to puberty timing after carbenoxolone exposure?

Puberty occurred later.

Which groups showed relatively normal puberty timing in the comparison presented?

Control, metyrapone and adrenalectomised groups.

What is the main conclusion of the rat glucocorticoid study?

Increased fetal glucocorticoid exposure delayed puberty in the rat model.

Why is "higher age at puberty" a delay rather than an improvement?

The y-axis records age. A higher value means the puberty marker appeared on a later day.

Which treatment would you choose to increase natural, rather than synthetic, fetal glucocorticoid exposure?

Carbenoxolone, because it inhibits placental 11β-HSD2 and increases active natural glucocorticoid transfer.

Which treatment directly supplies a synthetic glucocorticoid?

Dexamethasone.

Which two manipulations were intended to lower glucocorticoid exposure?

Metyrapone and adrenalectomy.

Why is age at puberty measured after birth rather than during the prenatal treatment?

The study tests developmental programming. Prenatal exposure is the experimental cause, while puberty timing is a later postnatal outcome.

Which rat strain was mentioned in the litter-size study?

Wistar rats.

When were pups reassigned to litter-size groups?

At birth.

What feeding condition does a small litter model?

Overfeeding.

Why does a small litter produce overfeeding?

There are fewer pups sharing the dam's milk and resources, so each pup receives more.

What feeding condition does a large litter model?

Underfeeding.

Why does a large litter produce underfeeding?

More pups share one dam's milk and resources, so each pup receives less.

What does the control litter represent?

Standard feeding and an intermediate resource level.