lecture 3: endocrinology of the ovary

Question 1

What is oestrous vs oestrus?

Answer 1

• oestrous is an adjective e.g. an oestrous cycle • oestrus is a noun e.g. that cow is in oestrus (heat), very specific stage

Question 2

Is there a difference between progestagen and progestogen?

Answer 2

Both are correct - collective noun for progesterones

Question 3

What is progestin?

Answer 3

synthetic progestagens

Question 4

What happens to the ‘contenders’ along the path of follicular development?

Answer 4

• don’t know the fine details of how this process occurs
• initial recruitment: blocked from antimalarian hormone, blocked from some of the other factors made by larger follicles, certain cytokines (e.g. CKIT, LIF, Stem Cell Factor) involved in the promotion of primordial follicles (don’t know how they interplay but do know they are important)
• cohort of follicles recruited every few days - conveyer belt
• lose a few along the way → atresia
• these cohorts have to get to a certain point by a certain time and if they don’t they make it they get left behind → atresia
• secondary follicles onwards under the control of FSH and LH, start to move a lot faster
• dominant follicle selected to go on to ovulation: potentially first follicle that also has LH receptors on its granulosa cells, thereby LH and FSH can work together to pump up the production of oestrogen that can be produced by that follicle
• inhibin secreted by some of these larger follicles to suppress the smaller ones thereby increasing its advantage over its cohort → shoots away in terms of its diameter while others become atretic
• IGF factors (cytokines) can be important in follicular fluid by mediated stimulation of LH and FSH action and can be controlled by the binding proteins
• different ratios of binding proteins can mediate IGF thereby mediating FSH → dominant follicle more binding proteins cleaved therefore more IGF available
• i.e. sort of know how it works but not entirely sure
• very different mechanisms in polyovular animals

Question 5

When does atrestia occur in follicular development?

Answer 5

primordial follices

• quiescent - can sit for 40 or 50 years
• virtually no atresia (percentage-wise) but lots recruited so actually high losses recruited follicles
• no turning back
• low rate of atresia gonadotrophin responsive
• FSH and LH affect development • some atresia gonadotrophin dependent
• atresia if FSH too low
• high rate of atresia (may be less atresia numbers wise than primordial follicle but percentage-wise is high ratio of those follicles undergoing atresia) dominant follicle(s)
• ovulates with LH surge
• without LH → atresia

Question 6

What are the two main phases of the ovarian cycle?

Answer 6

• follicular phase: oestrogens dominant
• point of ovulation marks switch
• luteal phase: progestagens dominant, formation of CL, luteolysis (luteal regression)
• progesterone tells uterus to expect fertilised egg

Question 7

Where does cholesterol come from in the body?

Answer 7

• mostly from diet
• small proportion acetate

Question 8

What are the progestagens?

Answer 8

• pregnenolone
• progesterone
• 17α-hydroxypregnenolone
• 17α-hydroxyprogesterone

Question 9

What are the androgens?

Answer 9

• DHEA
• androstenedione
• testosterone
• DHT

Question 10

What are the oestrogens?

Answer 10

• oestrone
• oestradiol

Question 11

What is the action of cholesterol side-chain cleavage enzyme?

Answer 11

• cleaves side chain of cholesterol
• converts cholesterol to pregnenolone

Question 12

What is the action of 17α-hydroxylase?

Answer 12

• converts pregnenolone to 17α-hydroxypregnenolone
• converts progesterone to 17α-hydroxyprogesterone

Question 13

What is the action of 3Beta-Hydroxysteroid dehydrogenase?

Answer 13

• converts pregnenolone to progesterone
• converts 17α-hydroxypregnenolone to 17α-hydroxyprogesterone
• converts DHEA to androstenedione

Question 14

What is the action of 17-20-lyase?

Answer 14

• converts 17α-hydroxypregnenolone to DHEA
• converts 17α-hydroxyprogesterone to androstenedione

Question 15

What is the role of 17beta-HSD?

Answer 15

• converts androstenedione to testosterone
• converts oestrone to oestradiol

Question 16

What is the action of aromatase?

Answer 16

• converts androstenedione to oestrone
• converts testosterone to oestradiol

Question 17

What is the action of 5alpha-reductase?

Answer 17

• converts testosterone to DHT

Question 18

what is this molecule?

Answer 18

• cholesterol
• obtained from diet
• minority obtained by converting acetate
• acted upon by cholesterol side-chain cleavage enzyme to produce pregnenolone

Question 19

what is this molecule?

Answer 19

• pregnenolone
• created by conversion of cholesterol via CSCC enzyme
• 3beta-HSD converts to progesterone
• 17α-hydroxylase converts to 17α-hydroxypregnenolone
• progestagen

Question 20

What is this molecule?

Answer 20

• progesterone
• created by conversion of pregnenolone by 3B-HSD
• converted to 17α-hydroxyprogesterone by 17α-hydroxylase
• progestagen

Question 21

What is this molecule?

Answer 21

• 17α-hydroxypregenolone
• created by conversion of pregnenolone via 17α-hydroxylase
• converts to 17α-hydroxyprogesterone via 3B-HSD
• converts to DHEA via 17-20-lyase
• progestagen

Question 22

What is this molecule?

Answer 22

• 17α-hydroxyprogesterone
• created by conversion of 17α-hydroxypregenolone via 3B-HSD
• created by conversion of progesterone via 17α-hydroxylase
• converts to androstenedione via 17-20-lyase
• progestagen

Question 23

What is this molecule?

Answer 23

• DHEA
• created by conversion of 17α-hydroxypregenolone via 17-20-lyase
• converts to androstenedione via 3B-HSD
• androgen

Question 24

What is this molecule?

Answer 24

• androstenedione
• created by conversion of 17α-hydroxyprogesterone via 17-20-lyase
• created by conversion of DHEA via 3B-HSD
• converts to oestrone via aromatase
• converts to testosterone via 17B-HSD
• androgen

Question 25

What is this molecule?

Answer 25

• testosterone
• created by conversion of androstenedione via 17B-HSD
• converts to DHT via 5a-reductase
• converts to oestradiol via aromatase
• androgen

Question 26

What is this molecule?

Answer 26

• DHT
• created by conversion of testosterone via 5a-reductase
• androgen

Question 27

what is this molecule?

Answer 27

• oestrone
• created by conversion of androstenedione via aromatase
• converts to oestradiol via 17B-HSD
• oestrogen

Question 28

What is this molecule?

Answer 28

• oestradiol
• created by conversion of testosterone via aromatase
• created by conversion of oestrone via 17B-HSD
• oestrogen

Question 29

Are these pathways the same or different between males and females?

Answer 29

same

Question 30

What is the endocrinology of the antral follicle?

Answer 30

• stimulated by gonadotrophins (FSH & LH)
• produce high amounts of oestrogens (oestradiol-17B)
• inhibin produced in the granulosa cells (-ve feedback)

Question 31

What is the endocrionology of the corpus luteum?

Answer 31

• stimulated by luteotrophins (LH, prolactin (rodents), hCG)
• produces progestagens, mainly progesterone (oestrogens in some species e.g. humans, pigs, rodents)
• oxytocin: own weapon of mass destruction, high concentration inside its cells
• luteolysis, destruction of CL (triggered by Prostaglandin F2alpha)

Question 32

What is the two cell theory of oestrogen production?

Answer 32

• requires both theca and granulosa cells to be produced
• LH binds to receptor on theca cell
• cholesterol from blood in theca cell → pregnenolone → 17-OH pregnenolone → DHEA → androstenedione
• LH activates cAMP → PKA → acts on pathway
• androgen (androstenedione moreso than testosterone) transferred across the basement membrane into the granulosa cell
• granulosa cells have the ability to use this androgen to convert to oestradiol that can be used locally or excreted into the blood, this is aided by the binding of FSH (acts through cAMP → PKA)
• theca cells can independently make a v. small amount of oestrogen
• granulosa cells cannot make oestradiol without the androgen supplied by the thecal cells
• work together to produce oestrogen in the follicle
• whole process has negative feedback loop regulated by inhibin produced in granulosa cells
• theca: cholesterol → androgens
• granulosa: androgens → oestrogens

Question 33

What is the process of luteinisation?

Answer 33

• LH surge → ovulation
• ruptured follicle wall
• angiogenesis
• theca transforms and enters centre → small luteal cells (progesterone, responsible for about 20% of the progesterone put out by the corpus luteum)
• granulosa transforms → large luteal cells (progesterone, oxytocin [in store])
• i.e. the process of turning into a corpus luteum from a follicle

Question 34

What are the phases of cycle?

Answer 34

generally:

• follicular phase
• luteal phase

otherwise:

• pro-oestrus
  
  • growing follicles
  • oestrogen dominant
• oestrus (in heat)
  
  • large follicle
  • high oestradiol
  • receptive to mating
  • ovulation (spontaneous or induced)
• met-oestrus
  
  • CL forming
  • Decreasing oestradiol; increasing progesterone
• Di-oestrus
  
  • Active CL
  • progesterone dominant
  • luteal phase/pseudopregnancy
• Anoestrus
  
  • cessation of ovarian cycles
  • season, pregnancy, lactation, old age, illness or stress
  • i.e. shut down of the process

Question 35

What are the varying lengths of ovarian cycles across species?

Answer 35

• Human: 24-32 days
  
  • Follicular phase: 10-14
  • Luteal phase: 12-15
• Cow: 20-21
  
  • follicular: 2-3
  • luteal: 18-19
• pig: 19-21
  
  • follicular: 5-6
  • luteal: 15-17
• sheep: 15-17
  
  • follicular: 1-2
  • luteal: 14-15
• horse: 20-22
  
  • follicular: 5-6
  • luteal: 15-16
• mouse/rat: 4-5
  
  • follicular: 2
  • luteal: 2-3
• rabbit: 1-2
  
  • follicular: 1-2
  • luteal: 0
• mouse/rat + *infertile male: *12-14
  
  • follicular: 2
  • luteal: 11-12
• rabbit + *infertile male: *14-15
  
  • follicular: 1-2
  • luteal: 13

Question 36

what is the main hormone in the follicular phase?

Answer 36

oestradiol

Question 37

what is the main hormone in the luteal phase?

Answer 37

progesterone

Question 38

What event occurs between the follicular and luteal phase?

Answer 38

ovulation

Question 39

What is pseudopregnancy?

Answer 39

• in animals with the short cycles
• infertile mating, or even manual stimulation of the tract
• mouse/rat/rabbit
• doesn’t alter follicular phase
• only luteal phase
• doesn’t take it out to full length of pregnancy
• by a certain point in time realises it isn’t actually pregnant

Question 40

What is the human female reproductive cycle?

Answer 40

• follicular and luteal phase approximately equal
• humans don’t have ‘oestrus’
• 14 days follicular phase → ovulation → luteal phase
• menses from day 0 to about day 4
• very distinct phases - never cross over
• body temp moves slightly, by part of a degree, during the luteal phase
• oestradiol ramps up during follicular phase with growing follicle
• LH surge causes ovulation
• progesterone taking over in luteal phase
• in the uterus:
  
  • menstruation occurs for those first few days
  • not much happening during oestrogen phase
  • progesterone really looks at ramping up endometrial tissue, spiral artery formation, preparation for implantation of fertilised embryo (it hopes)

Question 41

What is the oestrous cycle in cattle?

Answer 41

• 21 days between oestrus events
• as soon as Oestrus occurs, we’ve had ovulation so you get CL forming and progesterone is taking over , dominant for a long time
• dominant follicle effect
• rather than having seperate follicular and luteal phases, in which the follicular is all about the growth of the follicle, and luteal is all about growth of CL, they actually put the two on top of each other
• 2 wave growth of follicle

Question 42

What are the features of the short cycles in rat, mouse and hamster?

Answer 42

• 4-5 day cycles
• ovulate spontaneously
• CL regresses rapidly
• If mated,
  
  • cervical stimulation → surge of prolactin
  • PRL → prolongs life of CL
  • neural connections at the tract will send information to the brain → PRL
  • not the only signal required for pregnancy, so if no second signal mother quickly realises and tries to get back into 4-5 day cycle
• cycle strongly influenced by diurnal lighting pattern
  *

Question 43

What are induced ovulators?

Answer 43

• e.g. Rabbit, Cat
• follicles mature and persist, constantly
• potentially extended period of oestrus
• so actually no luteal phase normally unless…
• mating or cervix stimulation induces LH release → ovulation → CL formed
• LH surge that causes ovulation responsible for pseudopregnancy if not mated
• not one follicle for prolonged periods of time
• continual topping up of dominant follicle without ovulation

Question 44

What are the follicular waves is sheep and cow?

Answer 44

• follicular waves grow 2-3 per cycle (~4-5 days sheep/cow, 7-9 days cow)
• by late Antral stage
  
  • CL present → atresia
  • No CL or atretic CL → maturation and ovulation
• When CL regresses → the most mature wave of follicles can go on to ovulate 3 - 5 days later
• humans??? not substantial evidence
• why bother wasting time getting another wave going?
  
  • 21 days, seasonal breeder
  • finite time in which receptive to male
  • unsuccessful mating etc etc
  • Have a follicle waiting in the wings in case CL demises quickly
  • really an evolutionary trait for e.g. cows and sheep/seasonal breeders, to have one sort of sitting in the wings, just in case, 21 days is a long time for these types of breeders

Question 45

What are the different kinds of oestrous cycles?

Answer 45

• spontaneous ovulators
  
  • human, cow, sheep, mouse (most usual)
  • i.e. will ovulate even without the presence of a guy
• induced ovulators
  
  • rabbit, cat
  • ovulate when mated, physical stimulation to tract
• follicular waves within a cycle
  
  • cow, sheep
  • humans maybe?

Question 46

What is the ovarian (oestrous) cycle?

Answer 46

• the ovarian cycle is the time between ovulations
• ovarian output is cyclic
• as the follicle becomes antral, it develops FSH and LH receptors
• FSH → oogenesis
• LH and FSH → steroidogenesis - oestrogens
• LH surge → ovulation, luteinisation (loss of aromatase)
• LH (& Prolactin or E2 in some species) support CL and → progesterone
• luteolysis occurs if there is no pregnancy and allows the cycle to start again

Question 47

How can the cycle end?

Answer 47

• actively
  
  • action of luteolytic agent (PGF_2α)
  • overcomes the anti-luteolytic benefits of LH
  • most mammals – cow/sheep etc
  • common, more complicated
• passively
  
  • withdrawal/inadequate luteotrophic support (LH)
  • some also hCG, PRL, and progesterone
  • higher primates and humans

Question 48

How did researchers figure out how luteolysis occurs in sheep?

Answer 48

• normally have uterus, venous flow from uterus runs by arterial flow to ovary
• hysterectomy: continuation of progesterone, no luteolysis
• this indicates that there must be something being produced in the tract in the uterus that is actually mediating the CL’s demise
• contralateral hysterectomy vs ipsilateral hysterectomy
  
  • contralateral still resulted in cyclic event , therefore luteolytic agent is still come and having an action
  • ipsilateral: maintained CL and no luteolysis occurs
  • therefore it is the local environment that is important

Question 49

How does luteolysis occur in sheep?

Answer 49

Prostaglanding F_2α (PGF_2α):

• Produced by the endometrium
• rapidly metabolised/inactivated in peripheral circulation (lungs)
• Local transfer
  
  • (PGF_2α) in uterine venous drainage
  • transfer in vascular plexus to ovarian artery (countercurrent transfer)
  • transfer directly to CL (lost if ovaries graphed elsewhere in body)
  • release of Oxytocin (OT)
  • OT stimulate PGF_2α release from uterus (positive feedback loop)
  • luteolysis
• Effects of PGF2α two-fold
  
  • structural luteolysis
    
    • breakdown of CL structure
    • decrease in blood flow in CL (e.g. TNF-α)
  • functional luteolysis
    
    • stops progesterone production pathway
    • stimulates cytokine expression (e.g. Endothelin-1)

Question 50

How does passive luteolysis occur in humans?

Answer 50

• after LH surge (ovulation) low levels of LH maintains the CL for 10-12 days
• a second exponential rise in LH required (from embryo) otherwise luteolysis is induced spontaneously
• PG has no effect (only very high concentrations)