Lecture 6 - Puberty & HPG Axis

Question 1

What is puberty & which sex typically starts/finishes earlier?
What occurs during puberty in both sexes?

Answer 1

• When sexual maturation and growth are completed resulting in the ability to reproduce. Girls begin and end puberty before boys (male puberty lasts longer)
• Accelerated somatic growth, maturation of primary sexual characteristics (gonads & genitals), appearance of secondary sexual characteristics & menstruation/spermatogenesis begins.

Question 2

Define:

1) Adrenarche
2) Gonadarche
3) Thelarche
4) Pubarche
5) Menarche

Answer 2

1) maturation and increased activity of adrenal glands prior to puberty
2) activation of reproductive glands by FSH & LH
3) onset of breast development occuring near beginning of puberty
4) onset of the growth of pubic hair
5) onset of menstruation at puberty

Question 3

What may affect the timing of onset of puberty?

Answer 3

• Body weight key factor in girls, as well as nutrition
• Leptin (from adipose tissue)
• Low body weight can cause cessation of menstruation
• Economic factors

Whilst onset of puberty can change, the sequence of events that occur after are consistent.

Question 4

What scale is used to measure how far along puberty an individual is?

Answer 4

The Tanner Scale

• based on pubic hair & breasts in female
• based on pubic hair & genetalia in males

Question 5

How is accelerated somatic growth in puberty achieved?

How is this accelerated growth ended?

Answer 5

• Due to growth hormone, IGF1 & sex steroids
• Earlier + shorter in girls
• Ended when epiphyseal fusion (fusion of growth plates at end of long bones) occur.

Question 6

How is accelerated somatic growth in puberty achieved?

How is this accelerated growth ended?

Answer 6

• Due to growth hormone, IGF1 & sex steroids
• Earlier + shorter in girls
• Ended when epiphyseal fusion (fusion of growth plates at end of long bones) occur.

Question 7

Describe the full HPG axis in relation to puberty

Answer 7

1) Hypothalamus release GnRH in pulsatile manner (several years before phenotypic changes)
2) Causes LH & FSH release from anterior pituitary
3) Causes release of testosterone/oestrogen from the gonads

NB: GnRH binds to receptors on pituitary to cause release of FSH & LH, therefore release of GnRH in sufficient amounts triggers puberty

Question 8

Why is the release of GnRH pulsatile (every 1-3 hours) and not constant?

Answer 8

Because otherwise receptor desensitisation would occur which would stop the signalling cascade

Question 9

What initiates gonadal function in children between the age of 9-12?

Answer 9

• Increased amplitude of LH & FSH pulses, before then LH & FSH is insufficient to initiate gonadal function

Question 10

What negative feedback is there in the HPG axis?

Answer 10

• Testosterone & oestrogen from gonads prevent release of GnRH from hypothalamus

Question 11

What cells do FSH & LH stimulates in males?

Answer 11

1) FSH stimulates Sertoli cells to begin spermatogenesis and secrete inhibin which provides neg feedback of FSH on the anterior pituitary
2) LH stimulate Leydig cells. Leydig cells secrete testosterone which have neg feedback on hypothalamus and ant pit.

Question 12

Describe the histological appearance of the testes and the role of the cells within.

Answer 12

• Each lobule contains 1-4 coiled seminiferous tubules. In between there is loose connective tissue and blood vessels.
• Interstitial tissue contains leydig cells which produce testosterone
• Seminiferous tubules lined by epithelium containing sertoli cells which support developing sperm and secrete inhibin.

Question 13

What cells does LH & FSH stimulate in women?

What are the role of these cells?

Answer 13

1) FSH stimulates granulosa cells which release inhibin and convert androgens into oestrogen.
2) LH stimulates theca interna cells which release androgens (converted to oestrogen by granulosa cells).

Question 14

What is the effect of oestrogen, progesterone and inhibin in the female?

Answer 14

1) Oestrogen in low amounts inhibit GnRH, in high amounts promote GnRH (above a threshold). Reduces GnRH per pulse.
2) Progesterone increases inhibitory effect of low oestrogen, prevents positive effect of high oestrogen. Reduces frequency of GnRH pulses.
3) Inhibits secretion of FSH, allowing LH to be dominant hormone in luteal phase

Question 15

What is central precocious puberty, its causes and diagnosis?

Answer 15

• Early onset puberty with elevated GnRH levels
• Mostly idiopathic
• Causes premature activation of HPG axis, early initiation of pubertal changes
• See increased LH & FSH with GnRH, use GnRH stimulation test

Question 16

What is peripheral precocious puberty, its causes and diagnosis?

Answer 16

• Early onset puberty without elevated GnRH levels
• Due to increased androgen/oestrogen production, i.e.: due to tumours
• In lab tests see increased testosterone/oestrogen with decreased LH & FSH (due to GnRH suppression from high neg feedback from high gonadal hormones)

Question 17

What is delayed puberty?

What are the 3 main causes?

Answer 17

• Absent or incomplete development of secondary sex characteristics by age of 14 in boys or 13 in girls
  1) Constitutional growth delay (most common) - not caused by any pathological process, just delayed, no treatment needed
  2) Malnutrition/other chronic diseases - e.g.: hypothyroidism
  3) Hypogonadism