Reproductive Physiology Flashcards Preview

• Repro system quiescent during early childhood. 
• Increased activity of repro system during adolescence. 
• When inihibition is withdrawn or overcome at puberty, phenotypic and behavioural changes occur. 
• Increases in GnRH (gonadotrophin-releasing hormone) lead to increases in LH and FSH secretion (both females and males). 
• Age of onset varies (according to various factors e.g. sex, nutritional state, race. 

• 6-8 years of age (adrenal gland secretes androgens (e.g. dehydroepiandrosterone, DEHA) - similar to male puberty). 
  • Trigger not known - "pre-programmed"?
  • Androgens eventually cause growth spurt
  • Pubic hair growth starts
  • Breast development starts

• Onset of menstrual cycle at 10-16 years old.
  • Ability to produce mature ova and an endometrium that could support a zygote. 
  • Increased FSH and LH from pituitary (triggered by GnRH).
  • Ovaries respond to FSH and LH by producing steroids. 
  • Oestrogen induces ovulation. 
  • Onset related to critical level of body fat - triggers GnRH release. 

• Secondary sexual characteristics induced by ovarian oestrogens.
  • Pubic hair
  • Growth / maturation of reproductive tract (including uterus) and external genitalia.
  • Fat deposition - breasts, buttocks, thighs.
  • Closure of epiphyseal plates (stops growing) (at end of puberty).
• Somatic growth
  • ​Begins ~2 years earlier in girls compared with boys. 
  • Growth induced by gonadal sex steroids, growth hormone and insulin-like growth factor (as for males). 

• Bursts of GnRH release begin at 8-12 years (initially at night) to initiate puberty.
  • ​Exact trigger for GnRH production is not clear. 
  • "Pre-programmed"? Critical body weight? Inhibition of melatonin secretion?
• GnRH triggers bursts of FSH and LH release (as for females).
• This triggers testes to produce androgens and sperm. 
• Frequency of bursts increases until levels of GnRH, LH, FSH and testosterone are the same as in the adult. 
• Rising levels of testosterone produce secondary sexual features. 

• Secondary sexual characteristics appear (caused by testosterone and metabolites).
  • ​Testicular enlargement
  • Pubic hair growth
  • Growth of larynx
  • Deepening of voice
  • Increased bone mass
  • Increased mass and strength of skeletal muscle
  • Thickened skin
  • Increased and thickened har on trunk, arms, legs, face
• Somatic growth
  • ​Induced by gonadal sex steroids, growth hormone and insulin-like growth factor. 
• Puberty lasts many years - facial hair pattern may not mature until 20-25 years of age. 

• Ovarian cycle - production and release of ova. 
• Cycle lasts for 28 days (average).
• Has 2 phases:
  • Follicular phase
    • ​1st half of cycle.
    • Maturation of egg, ready for ovulation at midcycle - ovulation signals end of follicular phase. 
  • Luteal phase
    • ​2nd half of cycle. 
    • Development of corpus luteum (yellow body).
    • Induces preparation of reproductive tract for pregnancy (if fertilisation occurs). 
• Menstrual cycle - signified by blood loss via vagina due to sloughing of uterine endometrial lining - if it is not required to maintain a pregnancy. 

Triggers ovulation at ~ day 14. 

It falls. 

Inhibitory

• Menopause - determined retrospectively: begins at 12 months after the end of last menstrual bleed. 
• Cessation of menstruation - commonly occurs between approximately 45 and 55 years. 
• Menopause manifests physically but isn't a disease - it is a normal part of ageing. 

• Precise trigger still not known. 
• Oocyte depletion? Post-menopause there may still be some healthy oocytes so this can't be the full picture. 
• Remaining follicles might not be as sensitive to LH and FSH? In the reproductive cycle, these gonadotrophins trigger ovaries to produce a follicle (containing the oocyte).
• Age-related changes in CNS alter GnRH secretion?

• Before menopause (menopause transition):
  • ​Menstrual cycle may be irregular- may be shorter due to lack of complete follicular development, sometimes no ovulation occurs. 
• Progressively:
  • ​Ovaries atrophy - there are few or no healthy follicles.
  • Decrease in oestrogen secretion with concomitant increase in LH and FSH.
  • Increased LH stimulates ovaries to produce androstane-dione. 
  • Androstene-dione is androgen precursor → hirsuteness and precursor for estrone.
    • Estrone becomes the dominant oestrogen post-menopause, beta-oestradiol was dominant pre-menopause.
  • Overall decrease in oestrogens → breast and reproductive tract atrophy, vaginal dryness.
  • LH pulses coincide with hot flushes but are not responsible for flushes. More likely to be due to temporary disturbances of hypothalamus thermoregulatory centres (linked somehow to GnRH pulse generator). 
  • Increased bone mineral loss → decreased density.
  • Increased CV disease risk due to lack of protection due to oestrogen concentration falling. 

• There is no distinct andropause in males. 
• However, as men age:
  • Gonadal sensitivity to LH decreases.
  • Androgen production decreases. 
  • Serum LH and FSH increase.
  • Sperm production typically declines after age ~50.
  • Many men maintain reproductive function and spermatogenesis throughout life. 

• Production of ova
• Reception of sperm
• Capacitation
• Transport of sperm and ova to site of fertilisation
• Gestation 
• Parturition
• Nourishment of the infant by lactation

• Identical meiotic and mitotic divisions to male sperm production - but:
  • Oogenesis takes many years to complete (spermatogenesis takes 60-68 days). 
  • Begins in utero.
  • Suspended for many years.
  • Begins again at puberty.
  • Completed at fertilisation.
  • Oogenesis ceases at menopause.