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Reproductive systems > Puberty and the HPG axis > Flashcards

Puberty and the HPG axis Flashcards

1
Q

What is puberty?

A
  • A stage of human development when sexual maturation and growth are completed
  • Results in ability to reproduce due to activation of reproductive system
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2
Q

What influences the timing of puberty?

A
  • Pineal gland
  • Body weight
  • Nutrition - need leptin
  • Environment - onset age differs across globe
  • Genetics
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3
Q

What is the critical weight for puberty onset?

A
  • 47 kg
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4
Q

At what age does puberty begin in boys?

A
  • 9-14 years
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5
Q

Outline how puberty progresses in boys

A
  • Starts with genital development
  • Pubic hair growth
  • Spermatogenesis
  • Growth spurt
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6
Q

What are male secondary sexual characteristics?

A
  • Increased and thickened hair on trunk, pubis, axillae, and face
  • Increased laryngeal size
  • Deepening of voice
  • Increased bone mass
  • Increased muscle mass and strength
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7
Q

When does puberty begin in girls?

A
  • 8-13
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8
Q

Outline how puberty progresses in girls

A
  • Thelarche - breast bud enlargement (8-11)
  • Adrenarche - pubic hair growth (11-12)
  • Growth spurt (10-14)
  • Menarche (11-15)
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9
Q

What are the female secondary sexual characteristics?

A
  • Pubic and axillary hair
  • Enlargement of labia minora and majora
  • Keratinisation of vaginal mucosa
  • Uterine enlargement
  • Increased fat in hips and thighs
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10
Q

Compare accelerated somatic growth in boys vs in girls

A
  • Depends on growth hormones in both sexes
  • Earlier and shorter in girls
  • Boys have longer and faster growth spurt
  • Ended in both sexes by epiphyseal fusion
  • Oestrogen closes epiphyses earlier in girls
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11
Q

What is the average age for onset of puberty?

A
  • Boys = 12.5
  • Girls = 11.5
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12
Q

What is the first sign of puberty?

A
  • Increased testicular volume
  • Breast bud development
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13
Q

How does early/late maturation affect boys and girls?

A
  • Early maturation is a positive experience for boys but may be negative for girls
  • Late maturation is usually more difficult for boys than for girls
  • Adolescents compare themselves to their peers
  • Affected by reaction of others to their changing bodies
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14
Q

What is switched on at puberty?

A
  • HPG axis
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15
Q

Which part of the brain regulates the function of the reproductive glands?

A
  • Hypothalamus-pituitary unit
  • FSH, LH and prolactin most significant hormones
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16
Q

How do we know the HPG axis is about to be switched on?

A
  • Increased frequency and amplitude of pulsatile GnRH
  • Occurs first at night and then progressively throughout day
  • Onset of puberty associated with steady rise in LH and FSH
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17
Q

What is GnRH?

A
  • Gonadotrophin releasing hormone
  • Produced in neurons of hypothalamus
  • Regulates puberty onset, sexual development and ovulatory cycle in females
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18
Q

How does GnRH release differ in men and women?

A
  • In males, pulses 2 hourly
  • In females at changes according to phases of menstrual cycle
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19
Q

Why does GnRH have to be released in pulses?

A
  • Continuous release would stop production of LH and FSH because receptors would get desensitised
  • Can use high levels of GnRH to delay onset of puberty or treat endometriosis
20
Q

What affect does GnRH have on the anterior pituitary?

A
  • Stimulates anterior pituitary gland gonadotrophs to secrete LH and FSH
  • GnRH reaches anterior pituitary via hypophysial portal system
  • Intensity of GnRH stimulus is affected by frequency and intensity of release
21
Q

What is the important function of LH and FSH in puberty?

A
  • High levels of LH and FSH initiate gonadal development
  • Between 9-12 years, blood levels of LH and FSH increase
22
Q

What is sleep dependent rise in nocturnal LH?

A
  • In adolescent boys there is sleep related LH increase
  • Stimulates nocturnal testosterone rise
  • Androgen increase could account for some early pubertal changes in males
  • Females have similar increase in oestrogen
23
Q

Outline the HPG axis

A
  • Hypothalamus secretes GnRH
  • GnRH causes anterior pituitary to secrete LH and FSH
  • FSH stimulates gonads to produce androgens in males or oestrogen in females
    -Increased levels of oestrogen/androgens have a negative-feedback effect on hypothalamus
24
Q

What does FSH stimulate in males?

A
  • Stimulates Sertoli cells
  • Spermatogenesis
  • Inhibin released - negative feedback on anterior pituitary and possibly hypothalamus
25
Q

What does LH stimulate in males?

A
  • Leydig cells
  • Testosterone released
  • Negative feedback on hypothalamus and anterior pituitary
26
Q

Describe the structure of the seminiferous tubules

A
  • Testes divided into lobules
  • Each lobule contains 1-4 tightly coiled seminiferous tubules (~60 cm long)
  • Sertoli cells found in seminiferous tubules
  • Location for spermatogenesis
  • Loose connective tissue and blood vessels found between seminiferous tubules
27
Q

Where are Leydig cells found in the testes?

A
  • Found in interstitial tissue
  • Produce testosterone
  • Cells function independently of seminiferous tubules
28
Q

Which cells line the seminiferous tubules?

A
  • Sertoli cells
  • Spermatogenic cells/germ cells
  • Makes a complex epithelium
29
Q

What is the function of the seroli cells?

A
  • Provide nutrition and hormonal support to germ cells allowing sperm formation
  • Sensitive to FSH (increase sperm production)
  • Secrete inhibin - negative feedback on anterior pituitary and FSH
30
Q

What is the function of FSH in females?

A
  • Stimulates granulosa cells
  • Follicular development
  • Release of inhibin - specifically inhibits FSH only
  • Granulosa cells convert androgens to oestrogen
31
Q

What is the function of LH in females?

A
  • Stimulates theca interna cells to release androgens
  • These androgens are then converted to oestrogen by granulosa cells
  • LH surge required for ovulation
  • LH maintains corpus luteum after ovulation
32
Q

How does oestrogen affect GnRH?

A
  • Moderate titres of oestrogen reduce GnRH secretion
  • Negative feedback
  • High titres of oestrogen alone promote GnRH secretion
  • Positive feedback
  • LH surge stimulates ovulation
33
Q

How does progesterone affect oestrogen and LH/FSH?

A
  • Increases inhibitory effects of moderate oestrogen
  • Prevents positive feedback of high oestrogen
  • No LH surge
  • Increases frequency of GnRH pulses
34
Q

Which cells release inhibin in the female?

A
  • Granulosa cells
35
Q

Which hormones lead to a growth spurt?

A
  • Growth hormone
  • Increased androgens
  • Leptin
36
Q

What are the effects of growth hormone?

A
  • Secreted by pituitary gland
  • Increases TSH
  • Increases metabolic growth
  • Promotes tissue growth
37
Q

What are the effects of increased androgens on a growth spurt?

A
  • Retention of minerals in body to support bone and muscle growth
38
Q

What is leptin?

A
  • Adipocyte-derived protein hormone
  • Has a pulsatile release pattern
39
Q

What is the function of leptin?

A
  • Signals information about energy stores to CNS
  • Important role in regulating neuroendocrine function
  • Reproductive dysfunction associated with leptin deficiency
  • Can accelerate onset of reproductive function
  • Can regulate GnRH levels
40
Q

What is central precocious puberty?

A
  • Precocious puberty with elevated GnRH levels
  • Causes premature activation of hypothalamic- pituitary axis
41
Q

What are the causes of central precocious puberty?

A
  • Idiopathic or constitutional (most cases)
  • CNS lesions
  • Pituitary gonadotropin secreting tumours (rare)
  • Systemic conditions
  • Obesity
42
Q

How is central precocious puberty diagnosed?

A
  • Laboratory tests
  • Basal LH and FSH increased
  • GnRH stimulation tests
  • Serum testosterone/oestrogen increased
43
Q

What is peripheral precocious puberty?

A
  • Precocious puberty without elevated GnRH levels
44
Q

What are the causes of peripheral precocious puberty?

A
  • Increased androgen production
  • E.g. due to ovarian cyst or congenital adrenal hyperplasia
  • Increased oestrogen production
  • E.g. due to HCG secreting germ cell tumours
  • Increased beta-HCG production e.g. hepatoblastoma
  • Primary hypothyroidism
  • Obesity-related
45
Q

What can cause delayed onset of puberty?

A
  • Constitutional growth delay
  • Malnutrition
  • Chronic diseases e.g. IBD, hypothyroidism
  • Hypogonadism

Reproductive systems (20 decks)

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