Puberty and the HPG axis Flashcards
When are the primary sexual characteristics established?
Before birth
When is the reprouductive system active?
From puberty (to menopause in women)
What are primary sexual characteristics?
Those that develop at birth like anatomy of internal and external genitalia
What are secondary sexual characteristics?
Those that develop after puberty e.f. pubic hear, genital growth and menstruation
What influences the onset of puberty?
Pineal gland - disorders of this can cause precocious puberty
Body weight - females must be above 47kg - nutrition and leptin released from adipose tissue is also a factor
What is used to assess stages of puberty in both sexes?
The Tanner Scale
What age does female puberty begin?
From ages 9-13
What are the stages in female puberty?
- Thelarche
- Pubic hair
- Growth Spurt
- Menarche
- Pubic hair growth (coarseness s and adult distribution)
- Breast development
What is thelarche?
bread bud development
How does pubic hair develop?
Under the influence of testosterone
What causes growth spurts in both boys and girls and what are the significant differences?
Oestrogen causes the closure of the epiphyseal growth plates causing cessation of growth. Women have higher oestrogen levels so they fuse quicker hence why males are usually taller than females but they grow taller.
How is the oestrogen made?
Converting androgens to oestrogen via the aromatase enzyme (occurs in peripheral tissues)
What other hormone has a key role in growth?
GH
What is the role of leptin in menarche?
important signaller for the developmental process
Why is menarche occurring younger and younger in European girls?
Reaching the 47kg critical wight younger due to increased nutrition
When does Male puberty occur?
Between ages 10-14
What is the sequence of events in male puberty?
- Genital developemt
- Pubic hair growth
- Spermatogenesis
- Growth spurt
- Genital enlargement
- Pubic hair
What occurs in first stage of male genital development?
testicular volume increases - the testes and external male genitalia increase in size and volume until adult genitalia has developed
What is the purpose of the tanner scale?
Marks pre-puberty to adult formation
Also used to deduce how far someone is through puberty
What is the HPG axis?
They hypothalamus Pituitary Gonad axis
What happens hormonally when puberty starts?
Puberty is initiated in the brain. GNrH is produced in the hypothalamus and nocturnal GnRH pulsatiltiy stimulated the anterior pituitary which release FSH and LH which communicate with the gonads
What do FSH and LH stimulate?
FSH and LH stimulate the gonads (ovaries and testes) to release androgens and oestrogen which drive the physical changes in secondary sexual characteristics that are visible at puberty
Why is GnRH secretion in pulses?
ties to internal biological clock - synchronised with external signals like light
What does GnRH do immediately as it leaves the hypothalamus?
Acts on specific membrane receptors and transducer signals via secondary messengers to stimulate release of stored pituitary hormones and synthesis of them too. It also stimulates hyperplasia and hypertrophy of target cells
How is the anterior pituitary connected to the hypothalamus?
By the superior hypophyseal artery
Why doesn’t puberty occur in young children?
The levels of FSH and LH in the blood are insufficient to initiate gonadal function
Where does FSH and LH work on in Males?
FSH stimulates the sertoli cells to produce sperm and LH stimulates leydig cells to produce testosterone
What is significant about testosterone in males?
Once production in the testis starts, it tends to be maintained throughout their life hence when male productive life is much longer than female
When is testosterone in males the highest?
in the morning
Where are leydig cells found?
In the interstitial tissue between the seminiferous tubules
What are seminiferous tubules lined with?
complex epithelium made of sertoli cells and germ cells
What do sertoli cells do?
provide nutrition and hormonal support to the germ cells allowing sperm formation
How does FSH and LH work on gonads?
via the GaS to stimulate adenylate cyclase
Where does FSH act in women?
On the granulosa cells to produce oestrogen and inhibin. It also stimulates developing follicels
Where does the LH act in women?
On the theca cells to produce androgens and inhibin.
What happens to the androgens produced by the theca cells?
Theca cells lack the aromatase enzyme to convert the androgens into oestrogen so diffuse through the basement membrane into granulosa cells where aromatase enzyme is found and are converted to oestrogen here
What is the feedback with low levels of oestrogen?
negative feedback - feeds back to the hypothalamus to reduce GnRH secretion
What is the feedback with high levels of oestrogen?
positive feedback - promotes GnRH secretion and causes the LH surge
Why does the FSH not surge?
Due to inhibin released from the granulosa cells which dampens down the FSH surge (inhibits it)
What does a surge in LH do?
triggers ovulation
What does progesterone do
prevents the positive feedback of high oestrogen so you don’t get the LH surge hence why progesterone is given as contraceptive
Explain how GH secretion from the pituitary causes a growth spurt?
- increases TSH
- increases metabolic rate
- this promotes tissue growth
- increased androgens cause retention of minerals in the body to support bone and muscle growth
What is leptin?
an adipocyte-derived protein hormone
why is leptin imporatnt?
- regulates neuroendocrine function
2. signals information about energy stores to CNS
What can occur with leptin deficiency?
reproductive dysfunction
