4. Puberty And The HPG Axis Flashcards
What generally happens during puberty?
Accelerated somatic growth
Maturation of primary sexual characteristics
Appearance of secondary sexual characteristics
Menstruation and spermatogenesis begin
What is thelarche?
The onset of breast development occurring near the beginning of puberty in girls
What is adrenarche?
Maturation and increased activity of the adrenal glands prior to puberty
What is gonadarche?
Activation of reproductive glands by the pituitary hormones FSH and LH
What is menarche?
The onset of menstruation in puberty
What can impact on timing of puberty?
Disorders of pineal gland can cause early onset puberty
Body weight in girls (released from adipose tissue)
Low body weight can cause cessation of menstruation
Varies in different areas of globe
What is the order of female secondary sexual characteristics development?
9-13 years
Starts with breast bus (thelarche) and pubic hair growth
Leads to adenarche - growth spurt, onset of menstrual
Menstrual cycle begin (menarche) - pubic hair adult, breasts adult
What is the order of male secondary sexual characteristics development?
10-14 years
Starts with genital development
Progresses to pubic hair growth and spermatogenesis
Completed by growth spurt, genitalia adult, pubic hair adult
What does accelerate somatic growth depend on?
Growth hormone and IGF-1 and sex steroids
What does the hypothalamus and pituitary control in terms of reproduction?
Reproductive glands (hypothalamic-pituitary-gonadal axis HPG) Also control somatic growth, lactation, milk secretion and water metabolism
What does the HPG axis control?
Puberty initatied by the brain
Nocturnal GnRH pulsatility (LH secretion) precedes phenotypic changes by several years
Phenotypic changes
Onset of puberty associated with steady rise in FSH and LH secretion
What does GnRH regulate?
Puberty onset, sexual development and ovulatory cycles
What does GnRH secretion stimulate?
Anterior pituitary gland gonadotrophs to secrete FSH and LH
What is the negative feedback in puberty?
Hypothalamus releases GnRH
Stimulates pituitary to release LH and FSH
Stimulates gonads to produce androgens and oestrogen
Which negatively feedback to hypothalamus to stop produces GnRH
What does LH stimulate in males?
Stimulates leydig cells in testis
Produce steroid hormone testosterone
Once production starts in the medium long-term testosterone levels remain constant
What does FSH stimulate in males?
Sertoli cells for spermatogenesis
Where are leydig cells located?
In interstitial tissue of testis
What are seminiferous tubule cells lined by?
Complex epithelium made of 2 cell types - supporting cells (Sertoli cells) and spermatogenic cells/germ cells
What does FSH stimulate in females?
Granulosa cells
What are granulosa cells function?
Follicular development
- releases inhibin, specifically inhibits FSH only
- granulosa cells convert androgens to oestrogen
What does LH stimulate in females?
Theca interna cells
What do theca interna cells do?
Releases androgens, converted to oestrogen by granulosa cells
What does LH maintain?
Corpus luteum after ovulation
Releases progesterone and oestrogen
Lasts 12-14 days unless further stimulated by pregnancy hormone beta-hCG
What do moderate amounts of oestrogen do to GnRH secretion?
Reduce GnRH secretion - negative feedback
What do high amounts of oestrogen do to GnRH secretion?
Promotes GnRH secretion - positive feedback, LH surge
What effects does progesterone have on oestrogen?
Increases inhibitory effects of moderate oestrogen
Prevents positive feedback of high oestrogen
No LH surge
What is the role of inhibin on FSH in females?
From granulosa cells of corpus luteum
Inhibits secretion of FSH
Has a small inhibitory effect on LH
What does growth hormone secretion from pituitary do?
Increases TSH
Increases metabolic rate
Promotes tissue growth
Increase androgens - retention of minerals in body to support bone and muscle growth
What is the role of leptin?
Signal information about energy stores to CNS
Can accelerate onset of reproductive function
What is central precocious puberty?
Precocious puberty with elevated GnRH levels
What are the causes of central precocious puberty?
Idiopathic or constitutional CNS lesions Pituitary gonadotropin secreting tumours System conditions - tuberous sclerosis, neurofibromatosis Obesity related
What is the pathophysiology of central precocious puberty?
Premature activation of the hypothalamo-hypophyseal axis
Leads to abnormally early initiation of pubertal changes
Leads to early development of secondary sexual characteristics and gonadarche
What is peripheral precocious puberty?
Precocious puberty without elevated GnRH levels
What causes peripheral precocious puberty?
Increased androgen production (ovarian cyst, adrenal hyperplasia, leydig-cell tumour)
Increased oestrogen production (HCG secreting germ cell tumours)
Increased beta-HCG production
Primary hypothyroidism
Obesity
What is delayed onset of puberty?
Absent or incomplete development of secondary sex characteristics by the age of 14 in boys or 13 in girls
What can cause delayed onset of puberty?
Constitutional growth delay
Malnutrition and other chronic diseases
Hypogonadism
