Puberty

Question 1

Puberty

Answer 1

Is a complex developmental event
It is a continuum of changes leading to somatic and sexual maturation
There are profound physiological, psychological and physical changes

The main reproductive goal is to produce mature gametes:
Testes 🡪 spermatozoa
Ovaries 🡪 oocyte

From a clinical perspective puberty is defined as: Breast development in females, and increased testicular volume in males (Above 4ml)

Question 2

Two Endocrine Events of Puberty

Answer 2

1st = Adrenarche
The switching on of adrenal androgens
This is responsible for the growth of pubic hair, axillary hair and height.

2nd = Gonadarche
Switching on of the HPG axis
Results in the secretion of gonadotrophins: FSH and LH
LH - steroid synthesis –> secondary sex characteristics
FSH - growth of testis (male)/steroid synthesis/folliculogenesis (female)

These two endocrine events combinate in puberty. The two events are independently regulated, so you don’t need one for the other to occur.

Question 3

Adrenarche

Answer 3

Is the first endocrine event of puberty
Occurs at 6-8 years

Is characterised by (re-)instigation of adrenal androgen secretion:
Dehydro-epiandrosterone (DHEA)
Dehydro-epiandrosterone sulphate (DHEA-S)
Studies have shown that there is no change in cortisol/other adrenal hormones during adrenarche
It is not a global activation of HPA axis

Question 4

Adrenarche- DHEA/DHEAS

Answer 4

DHEA/DHEAS levels start to rise at 6y
It continues to rise, there is a peak in the 20s.
After there is a gradual decline

DHEA/DHEAS secretion is from the zona reticularis
The zone reticularis is the innermost layer of the adrenal cortex
This is the result of inherent maturation of cellular compartments of the adrenal cortex. So… there is a remodelling of the adrenal cortex in order for it to be able to secrete DHEA/DHEAS.

Question 5

How is DHEA/S made?

Answer 5

DHEA/S is made via a series of steroidogenic conversions.
Cholesterol is the original precursor, there is then a series of conversions that occur leading to the production of DHEA/S. This is done with the aid of steroidogenic enzymes.
Eg: CYP11A which is responsible for the conversion of progesterone to pregnenolone.
Eg; CYP17 20 Lyase which is responsible for DHEA production
SULT2A1 which is responsible for making DHEAS from DHEA

Answer 6

In an individual undergoing puberty there is an increased expression of CYP11A in the zona reticularis (progesterone -> pregnenolone)
In an individual undergoing puberty there is an increased expression of CYP17 20 Lyase in the ZR (DHEA production)
Also in an individual undergoing puberty there is increased expression of SULT2A1

Note: There is also a sideways pathway (green). This is responsible for the synthesis of glucocorticoids and mineralocorticoids.
Occurs after the initial conversion of cholesterol to pregnenolone
It is modulated by 3bHSD which converts pregnenolone and hydroxypregnenolone into the corticosteroids.
BUT in a pubertal individual, there is a significantly reduced expression of 3bHSD compared to an infant. This is because in adrenarche, this sideways pathway is shutdown to prioritise the pregnenolone to go into the production of DHEA/S.

Question 7

Adrenal Remodelling

Answer 7

During adrenarche there is increased expression of SULT2A1 in the zona reticularis of an individual undergoing puberty. As this is responsible for DHEAS production.
During adrenarche there is a reduced expression of 3bHSD to prevent the loss of pregnanolone to the sideways pathway.

Question 8

Function of the DHEA/S

Answer 8

DHEA is synthesised by the adrenal gland in the zona reticularis of the adrenal cortex.
It is synthesised and makes it way through the blood system where it is synthesised by peripheral tissue where it is converted into DHT.
DHT is responsible for the growth of auxiliary hair, change in skin glands and prostate secretions.

Question 9

What instigates adrenarche?

Answer 9

ACTH
POMC
POMC-related peptides
Other factors ruled out include prolactin, IGF-1 and insulin

Question 10

How does ACTH instigate adrenarche?

Answer 10

Dexamethasone (a synthetic corticosteroid) suppresses adrenal androgen production.
Children with ACTH receptor mutations fail to undergo adrenarche.
But, no change in ACTH/cortisol levels during adrenarche
Are there Divergent mechanisms for cortisol and androgen production at adrenarche?

Question 11

How does POMC instigate adrenarche?

Answer 11

POMC = Pro-opiomelanocortin - 241AA sequence that undergoes cleavage into multiple peptides.
Proximal 18 AA region that positively regulated adrenal androgen production.
BUT In vitro studies did not substantiate this

Question 12

How do POMC- relayed peptides instigate adrenarche?

Answer 12

b-lipotrophin and b-endorphin plasma levels correlate with increased DHEA/S at adrenarche

No real evidence

Question 13

Gonadarche

Answer 13

Gonadarche is characterised by a (Re-)Activation of HPG axis.
Occurs several years after adrenarche (typically ~11 yrs of age).
Driven by hypothalamic GnRH & pituitary gonadotrophins.
Gonadarche is modulated via the HPG axis:

Hypothalamus (GnRH)–> Pituitary (Gonadotrophins: LH/FSH)–> Gonads (Steroidogenesis)

Question 14

Puberty depends on..

Answer 14

Puberty depends on reactivation of GnRH release

Question 15

HPG Axis and Puberty

Answer 15

During the 16th gestational week of development, there is activation of HPG axis for the first time.
There is pulsatile GnRH secretion in foetus
This continues until just before birth, where it is switched off.
It is believed to be switched off to make way for the feedback from the placental hormones.
Then after birth the HPG axis is switched on again for 1-2 years postnatal increased.
Believe it is switched on again after birth as a priming of the HPG axis.
It is then switched off again

In the week 12 pregnancy scan, you cannot determine the sex of the baby, but in the week 20 scan you can.
This is because of the 16th week activation of the HPG axis which is required for completion of sexual differentiation.

After the axis is switched off after the postnatal period, we believe that the Neurones ‘restrained’ during postnatal period 🡪 10 years or more

At puberty there is a gradual rise in pulsatile release- around 1 year before breast budding observed
The reactivation usually begins with a nocturnal rise in GnRH levels;
A you progress through puberty you start to have more consistent levels throughout the day.

Question 16

HPG axis summary

Answer 16

GnRH is synthesised & secreted
↓
Synthesis and secretion of pituitary gonadotrophins (LH & FSH)
↓
Gonadal steroid production
↓
Negatively/positively feedback onto hypothalamus-pituitary to regulate GnRH and LH/FSH production

Question 17

Consonance and Tanner Stages of Puberty

Answer 17

Everybody progresses from stage 1->5. This movement through the stages is called consonance.
BUT what will differ is the time spent in each stage and the time it takes to move from one stage to another.

The tanner stages are a useful tool to detect when puberty is not in consonance.

Question 18

Onset of puberty (current theories)- Potential adverse risks from early puberty:

Answer 18

cardiovascular disease
metabolic disease
obesity
diabetes
disordered behaviour
In early puberty you have a case where a person undergoes sexual maturation before mental maturation.
decreased adult height
Bone fusion occurs earlier than it should
decreased life expectancy

Question 19

What controls the onset of puberty?

Answer 19

It’s a dialogue between our individual genetics and environmental factors
They all impinge at different points of the HPG axis

A number of theories have been put forward:
Inherent maturation of CNS
Body fat/nutrition – Leptin and Ghrelin?
Hypothalamic hormones - Kisspeptin, other factors?
Latest theories - Epigenetics?
Epigenetics is changes in the expression of a gene, changes in the phenotype without any changes in the structure. These changes in expression can be passed to offspring.
Over the past 100 years there is a uniform decrease in the age of onset of puberty (17 -> 13)
Now we are better fed, more lethargic.
This suggests a link between energy homeostasis and reproduction.

Question 20

Role of nutrition and body fat in puberty

Answer 20

Extremes of energy excess (body fat mass) impact the timing of puberty in both sexes - particularly females

Under- and over-nutrition in foetal and/or neonates alters the timing of puberty in rodents and humans

Morbid obesity (females) can cause precocious puberty

Frisch et al.: “critical fat mass” hypothesis
Threshold % fat/body weight is required to attain (17%) and maintain female reproductive ability (22%)
As pregnancy is an energy intensive process.

There is cross talk between metabolic status and puberty onset/maintenance of reproduction

Question 21

Nutritional Gating and Puberty

Answer 21

Based on evidence available we believe that secretions from the gut and secretions from adipose tissue act on the hypothalamus and in turn modulate gonadal function via the HPG axis.

Question 22

Adipose tissue: leptin (mouse model)

Answer 22

In 1950s a mouse was isolated for its erratic behaviour, after tests they found the mouse was:
Hyperphagic
Hyperglycaemic
Insulin resistant
Infertile
Leptin deficient – Mutation in gene coding for leptin

Question 23

Leptin Role

Answer 23

1994: leptin gene first cloned
Expressed in adipocytes- WAT (white adipose tissue)
Is a sensor of energy sufficiency
It is a satiety factor - tells brain you’re full
It stimulates energy expenditure
The circulating levels of leptin are directly proportional to amount of body fat

Question 24

Effect of Leptin on Repro system

Answer 24

In most common forms of obesity, the leptin receptor does not respond properly to the leptin signal.

Influence on reproductive system
ob/ob mice & humans present with hypogonadotrophic hypogonadism
Delayed/absent puberty
BUT Can be reversed with leptin injection
Some leptin-deficient patients have normal menses/LH/oestradial levels- and it is unknown why.

Question 25

Leptin and Obesity

Answer 25

Boy was morbidly obese, and leptin deficient.

Therefore, brain was not getting the satiety signal

Question 26

Does leptin trigger puberty?

Answer 26

There are difference in males in females in the expression of leptin: sexual dimorphism:
Females have rise in leptin levels ~ 2 years prior to puberty (accompanied increased GnRH pulsatility)
Males- no rise in leptin

Obesity increases leptin and earlier puberty occurs

Knockout leptin in rodents and humans = delayed/absent puberty

BUT, leptin administration can not stimulate early puberty

There are NO leptin receptors on GnRH neurons

There is a threshold of leptin required to be reached for puberty but not a driver of puberty itself.
So it plays a permissive role in puberty.

Question 27

Gut peptides: Ghrelin effect on puberty

Answer 27

Ghrelin senses the fasted state (when you are hungry), to stimulate feeding and fat deposition.
OPPOSITE of Leptin

A bolus of ghrelin stimulates the GH/IGF axis Via GHSR.
Growth hormone secretagogue receptor

In ‘starvation’ (high ghrelin) there is decreased activity of the HPG axis.
Eg: low energy levels cannot reproduce

Ghrelin levels decreases as puberty proceeds.

Ghrelin can decrease the level of hypothalamic kiss1 (kisspeptin) expression in rats.
There is a subset of kiss1 neurons in selective hypothalamic nuclei that express GHSR and respond to Ghrelin.

Oestradiol can also increase GHSR expression and response to Ghrelin in kiss1 neurons
It can modulate the ghrelin receptor activity.

Low levels of leptin and high levels ghrelin = decreased LH levels.

Question 28

Current theory on how kisspeptin might lead to onset of puberty

Answer 28

There are kisspeptin receptors expressed on GnRH neurons
¾ of the GnRH neurons co-express Kiss1R mRNA .

There is kisspeptin expression in the Anteroventral periventricular nucleus (AVPV) and the Arcuate nucleus (ARC)

Kisspeptin and Puberty

In pubertal individuals there is an increase in kisspeptin mRNA expression, but there is no change in the expression of the receptor.
This shows that the triggering of puberty is more to do with kisspeptin rather than its receptor.

When there is continuous kisspeptin
There is an initial spike in LH
You then see a drop in LH levels to the point where it is nearly 0.

When there is pulsatile kisspeptin
There is pulsatile LH
SO… Pulsatiltiy is crucial for the activity of kisspeptin.

Question 29

New Role of Kisspeptin?

Answer 29

A lot of the information that we have was done on knockout mice for the kisspeptin receptor.

Original observations in GPR54 null mice / humans (mutations):
Abnormal development of GnRH neurones -> hypogonadism
Failure to enter puberty
KO mice for GPR54 or kisspeptin -> hypothalamic hypogonadism
Mutations in humans -> hypothalamic hypogonadism
Activating mutations of GPR54 (kisspeptin receptor) -> precocious puberty

Phenotype (mice):
Male: small testes and epididymis, delayed spermatogenesis = infertility
Female: small oviducts, folliculogenesis-no progression to ovulation, no oestrous cycles = infertility

Question 30

So we know that kisspeptin is the gate keeper of puberty, but what drives kisspeptin?

Answer 30

Experiment compared levels of kisspeptin in the starved state between pubertal individual and adult:
In the starved state of a puberal individual, there is a decreased expression of kisspeptin when compared to the adult.
Only in pubertal mice is the level of expression markedly reduced; not in the adult mice

Integration of Kisspeptin-GnRH system with metabolic cues

Reduced leptin in starvation = decreased GnRH secretion

Leptin directly excites Kiss1 neurones in the ARC

Leptin deficiency »↓Kiss 1 mRNA in ARC

But only 10-40% of Kiss1 neurones express the LepR

There is a lot still unknown, but there is indirect and direct mechanisms of leptin action in the hypothalamus on the HPG axis.

Question 31

What switches on puberty?

Answer 31

A: We don’t know

Its an integration of central and peripheral inputs determined both by genetics and environment/nutrition.