L6: puberty and the hypothalamic pituitary gonadal axis

Question 1

Define puberty

Answer 1

Preparation for sexual maturity, to enable us to reproduce

Creates sexual dimorphism (distinct difference between male and female sexes)

Question 2

Define primary and secondary sexual characteristics

Answer 2

Primary sexual characteristics – sexual characteristics at birth – includes the anatomy of the internal and external genitalia
Secondary sexual characteristics – characteristics that develop after puberty

Question 3

Describe the sequence of events in girls at puberty

Answer 3

Start puberty between the ages 9-13
Thelarche – breast bud development
Pubic hair – develops under the influence of testosterone
Growth spurt
Menarche – initiation of the menstrual cycle
Pubic hair – becomes coarser
Breast development – further growth until adult breast enlargement is reached

Question 4

Describe the sequence of events in boys at puberty

Answer 4

Start puberty between the ages 10-14
Genital development
Pubic hair growth – under the influence of testosterone
Spermatogenesis
Growth spurt – lower levels of oestrogen, therefore boys typically grow a lot taller than girls
Genital enlargement
Pubic hair – coarseness and pattern of pubic hair will change

Question 5

Describe early onset puberty

Answer 5

Can lead to levels of oestrogen rising earlier

Therefore, can result in short stature

Question 6

Describe central precocious puberty

Answer 6

Precocious puberty with elevated GnRH levels
Idiopathic in most cases
Premature activation of the hypothalamo-hypophyseal axis -> abnormally early initiation of pubertal changes -> early development of secondary sexual characteristics & gonadarche

Question 7

Describe peripheral precocious puberty

Answer 7

Precocious puberty without elevated GnRH levels
Increased androgen production eg. ovarian cyst (most common cause)
Lab tests – increased oestrogen/testosterone, decreased LH & FSH

Question 8

Describe delayed onset of puberty

Answer 8

Absent/incomplete development of secondary sex characteristics by the age of 14 in boys or 13 in girls
Causes:
1) Constitutional growth delay
2) Malnutrition and other chronic diseases
3) Hypogonadism

Question 9

Describe the HPG axis in males

Answer 9

Gonadotrophin releasing hormone is produced in the hypothalamus (pulsatile release & stimulates hormone release from the anterior pituitary)
GnRH causes release of LH and FSH from gonadotrophs in the anterior pituitary
FSH and LH stimulate the testes to release testosterone
-sertoli cells produce sperm under the influence of FSH
-leydig cells produce testosterone under the influence of LH

Question 10

Describe the HPG axis in females

Answer 10

Gonadotrophin releasing hormone is produced in the hypothalamus (pulsatile release & stimulates hormone release from the anterior pituitary)
GnRH causes release of LH and FSH from gonadotrophs in the anterior pituitary
FSH and LH stimulate the ovaries to produce oestrogen and progesterone
-granulosa cells respond to FSH
-theca cells respond to LH

Question 11

Describe the feedback on the HPG axis

Answer 11

Subject to negative feedback
Raising levels of androgen/oestrogen causes negative feedback on the hypothalamus, which then stops the release of GnRH
When gametes are stimulated, they release inhibin, which specifically causes negative feedback on FSH to prevent further sperm production/maturation of any more follicles

Question 12

Describe the action of oestrogen and progesterone on GnRH

Answer 12

Moderate levels of oestrogen reduce GnRH secretion
High levels of oestrogen alone promote GnRH secretion – positive feedback (LH surge)
Progesterone increases inhibitory effects of moderate oestrogen
Progesterone prevents positive feedback of high oestrogen
Oestrogen reduces GnRH per pulse, progesterone reduces frequency of pulses