WK 2 ( Oestrous cycles, mating behaviour and embryo signalling) Flashcards
Oestrous
Regular period of mating behaviour
Dioestrus
The period between oestrous cycles
Anoestrus
Absence of oestrous cycles
Monoestrus
A single oestrous period per season
E.g. Bitch
Polyestrus
Several oestrous periods per season
E.g. Mare + lots of other animals
Most domestic animals are seasonally polyestrus
Induced ovulators
Continuous oestrous until mating
E.g. Rabbit, ferret
Reasons for anoestrous
Pre puberty: Animal not yet cycling
Seasonal: Animal out of breeding season
Pregnancy: High P4 levels override cyclicity
Lactational: Sensory input suppress GnRH release
Stress: High cortisol levels suppress cyclicty
Pathology: Anatomical or physiological abnormality
Follicular phase (time-frame)
From regression of CL to ovulation (~20% of cycle)
Luteal phase (time-frame)
From ovulation to regression of CL (~80% of cycle) involves preparation for pregnancy
Follicular phase
o PhasebetweenregressionofCLandovulationwhenfolliclesgrowandmature
Incorporatesprooestrus andoestrus
Dominatedbyoestrogen which:
- TriggersLHsurge
- Stimulatesoestrousbehaviour
- Preparesfemaletractforspermandovumtransportandfertilization
- Increases blood flow and secretions lubricates repro tract
- Causes myometrium contractions
Lengthoffollicularphasevariablewithinspecies
Thelongerthefollicularphase,themorevariabilitythereisbetween cyclesandindividuals makes AI harder
If fertilization occurs, the egg should reach the uterus around the beginning of Diestrus, when progesterone levels are at their highest
Hormonal roles during the follicular phase
Early in the follicular phase (recruitment phase):
- Tonic center of hypothalamus produces fairly low levels of GnRH GnRH causes the anterior pituitary to release Gonadotropins (preferentially FSH at this stage of the cycle) stimulates follicular growth
- Oestrogen should be feeding-back to tonic entre at this stage (not surge center)
- Negative feedback loop keeps hormone levels relatively stable but other factors acting on the ovary still causes the follicles to grow
Medium follicle phase (selection phase):
- Towards the end of the proestrus stage, starts to move into selection phase
- As follicles grow oestrogen levels increase continue to feedback to tonic center causing release of GnRH shifts from preferential release of FSH to equal release of FSH and LH (due to medium sized follicles starting to produce another hormone called PGF which inhibits FSH production)
- Other factors on the ovary still allow follicles to keep growing in response to low level FSH
Large follicle phase (Dominance phase):
- Occurs during the Estrus phase
- Huge amounts of oestrogen being produced which feeds back to the surge center of the hypothalamus causing a surge release of GnRH = positive feedback
- Dominant follicle is releasing high levels of inhibin causing a shift to preferential release of LH causes breakdown of the follicle and ovulation
The 2 cell, 2 gonadotropin model
Follicular steroidogenesis = production of oestrogen by follicles
Requires both the granulosa and the theca cells and both FSH and LH = The 2 cell, 2 gonadotropin model
Process:
- Theca interna cells have got receptors for LH LH binds to those receptors then through a cAMP – protein kinase pathway, it transforms cholesterol into testosterone
- The testosterone is then passed over to the granulosa cells (close relationship of theca and granulosa cells makes it an efficient transfer)
- Granulosa cells have receptors for FSH FSH binds to those receptors then through another cAMP – protein kinase pathway it transforms testosterone into Estradiol (potent form of oestrogen)
- Estradiol is then released by granulosa cells into circulatory blood travels to the brain where it has effects on behaviour and also travels to the reproductive tract where it causes increased blood flow and secretions and increased smooth muscle activity
- This is why both gonadotropins must be released throughout follicular phase to allow the production of oestrogen
Follicular development
Follicular development occurs at random locations on the ovary (except in the mare)
As follicle grows it has more theca and granulosa cells able to produce more oestrogen so by the time it reaches antral phase it is has lots of theca and granulosa cells and high levels of gonadotropins so it can produce high levels of oestrogen
Antral follicles are only present when the animal is in her estrus phase
True corpus luteum only present during luteal phase – otherwise female would have high levels of progesterone all the time
By the time a follicle becomes dominant it is usually pushed to the edge to allow it access to the outside of the ovary for ovulation
The mare has ovulation bursa and only ever ovulate on one specific site of the ovary
Follicular waves
Even during the luteal phase, recruitment, selection and early dominant phases occurring on the ovary but instead becoming fully dominant and ovulation occurring, these cells are becoming atretic and being resorbed
High levels of progesterone are suppressing follicular growth so that instead of becoming dominant they become atretic and are resorbed
After Luetolysis, another follicular wave will occur and this time the follicles will be allowed to become fully dominant and ovulation will occur as progesterone levels are low
Different species will go through different numbers of follicular waves (some species don’t have follicular waves but most domestic species do)
Follicular dominance in regards to superovulation
Superovulation is more effective if the female is in the luteal or early follicular phase
Presence of dominant follicle can override the effect of FSH
Effect of GnRH agonist on FSH induced superovulation
GnRH agonist minimises the amount of GnRH being produced –> suppresses follicle development
Two weeks pre-treatment with GnRH agonist suppresses follicle development so that no dominant follicle emerges
Subsequent FSH administrations is more effective
A larger follicle pool is recruited
Characteristics of oestrus (high levels of oestrogen)
Anatomical/histological changes:
- Reddened vulva
- Open cervix (allows sperm entry)
- Cornification of vaginal epithelium (Theory - layer of dead cells protect vagina during copulation = less damage to healthy vaginal tissue)
Mucous changes:
- Cervical mucus thins and pH falls
- Vaginal lubrication
- Sperm transport
Fertility:
- Ovulation normally associated with oestrus – usually during but occasionally after
- Mating Behaviour
Mating behaviour in mice & rats:
Pre-copulatory:
- Genital sniffing
- Display of “ear quiver response”
Copulatory:
- Assumes lordosis position
Post-copulatory:
- Genital licking
Mating behaviour in cats:
Pre-copulatory:
- Calling
- Restlessness
- Poor appetite
- Head rubbing
- Frequent urination
Copulatory:
- Assumes lordosis position
Post-copulatory:
- Genital licking
Mating behaviour in rabbits:
Don’t have oestrous cycles- they have ovarian cycles - are induced ovulators
Pre-copulatory:
- Mate seeking
- Permits make to sniff genitals
Copulatory:
- Assumes lordosis position
Post copulatory:
- Male falls off
- Male may thump
Mating behaviour in dogs:
Pre-copulatory:
- Genital sniffing
- Tail deviation
Copulatory:
- Accepts male
Post copulatory:
- Genital lock
- Genital licking
Mating behaviour in sheep:
Subtle signs
Pre-copulatory:
- Mate seeking
- May tail wag
- More frequent urination
Copulatory:
- Accepts male
Post-copulatory:
- Minimal
Mating behaviour in cows:
Pre-copulatory:
- Genital sniffing
- Bellowing
- Mounting (males of females)
Copulatory:
- Accepts male
Post-copulatory:
- Minimal
Mating behaviour in Pigs:
Pre-copulatory:
- Grunting
- Poor appetite
- Restlessness
- Nose-nose contact with mate
Copulatory:
- Assumes standing position
Post copulatory:
- Minimal
Pre-ovulatory LH surge
Dominant follicles are producing high levels of oestrogen
Circulatory oestrogen level reaches threshold level
Switch to positive feedback
Triggers surge center of hypothalamus
Surge of GnRH is released
Surge of gonadotropins are released – preferentially LH because of the amount of inhibin being produced by the dominant follicle
Preovulatory LH surge triggers ovulation (through a cascade of events)
Ovulation
Preovulatory LH surge triggers:
- Increase in blood flow to ovary and dominant follicle – this is also triggered by the hormone Prostaglandin E2 (PGE2) –> Edema
PGE2 is released from the ovary and increases blood flow to dominant follicle and ovary by increasing diameter of capillaries –> Edema
Increase in the production of the hormone (PGF2alpha) which increases the contraction of ovarian smooth muscle – putting pressure on the follicle wall & causes the release of lysosomal enzymes which weakens the follicle wall.
Shift from oestrogen to progesterone as follicular cells begin to transform into luteal cells which causes an increase in progesterone concentration, leading to an increase in Collagenase which breaks down the wall of the follicle
This cascade of events works together to weaken the follicular wall and cause ovulation
Act of ovulation/ Preovulatory LH surge also causes a wave of oocyte maturation which is important before fertilization can occur:
- Removal of meiotic inhibition (un-pause)
- First polar body is released haploid oocyte
- Fertilization
Luteal phase
Stage between ovulation (formation of CL) and regression of CL i.e. when the corpus lueteum is present
Incorporates metoestrus and dioestrus
CL secretes P4 which:
- Is essential for maintenance of pregnancy
- Exerts negative feedback on pituitary to restrict gonadotropin secretion
- Length of luteal phase consistent within species
- At end of luteal phase, corpus luteum regresses and P4 levels fall, leading to new follicular phase
Formation of a corpus luteum
Preovulatory follicle - granulosa and theca interna cells merge as follicle starts to break down
Corpus haemorrhagicum - small blood vessels rupture. Follicle implodes and folds inwards on itself
Functional corpus luteum - Large luteal cells were originally granulosa and small Luteal cells were theca
Occurs after LH surge – breakdown of follicular basement membrane
Theca and granulosa cells transform into luteal cells
Progesterone production follows the same pathway as oestrogen production
Progesterone produced by the CL
Progesterone produced by the CL has negative feedback to pituitary and hypothalamus - GnRH, FSH and LH are suppressed - little oestrogen is produced -ovaries will stay relatively quiescent
Progesterone has positive feedback effect on mammary glands and uterus preparing for lactation and implantation of the fetus (only has a few days to prepare the uterus before attachment)
Under the influence of P4 the uterine glands secrete materials into the uterine lumen. Progesterone inhibits the myometrium and reduces its contractility
Luteolysis
Brings about the demise of corpus luteum
If fertilsation doesn’t occur:
In a monoestrous animal corpus luteum will remain for a period until the next season
In a polyestrous animal corpus luteum will regress so she has another opportunity to mate
The corpus luteum has a natural life longer than the normal length of the luteal phase and production of luteolysin terminates it
The uterus and lueteolysis
Signaling pathway between Corpus Luteum and the uterus
Urartian vein comes out of uterus and winds around ovarian artery = close relationship with blood
When you have a fully intact uterus or contralateral uterectomy release of luteolysin from the that side of the uterus going to the ovary behaves just like normal (close relationship with blood)
When you have a ipsilateral uterectomy, the uterine veins have been removed which is the source of luteolysin the close relationship with blood is lost and the breakdown of corpus luteum relies on luteolytic activity coming out of the other side and then spreading through the circulation around the whole body until eventually getting to this side this is why there is a delay in CL breakdown
Signalling for luteolysis
The CL produces P4, which primes the uterus to produce PGF2alpha
PGF2alpha stimulates formation of oxytocin receptors (OtR) on the uterine endometrium
OtR respond to pulses of ovarian Ot, causing the release of more PGF2alpha
Positive feedback loop:
More PGF2alpha causes the formation of more OtRs - which causes more PGF2alpha to be released - huge surge in PGF2alpha –> signals breakdown of CL
Why PGF2alpha acts locally?
PGF2alpha leaves the uterus via the uterine veins which then drains into the ovarian arteries
PGF2alpha in ovarian vein destroys CL
Embryonic signalling
The embryo signals its presence via release of an antiluteolysin (in in a species dependent way)
May cause inhibition or diversion of PGF2:
- Oestradiol sulphate (in pigs)
- Prolactin (in rodents)
Embryo signaling MUST occur in time to prevent luteolysis
The embryo MUST be in place (and release sufficient antiluteolyic factor) at the right time and must be healthy enough to release signal
The embryo takes a few days to reach the uterus during which time the Corpus Luteum has produced P4 to prepare the uterus for pregnancy
Too early and the uterus is not ready and implantation cannot occur
Too late and the uterus is ‘past its prime; and implantation cannot occur
This is why donors and recipients must be synchronous to within 1 day (embryo transfer)
Embryo signalling - ruminants
Blastocyst is producing interferon tal (IFN-T) and it is interacting with the receptors on the endometrium of the uterus blocking oxytocin receptors positive feedback loop is blocked so PGF2 levels remain low and cant signal breakdown of Coprus Luteum
Embryo signalling - sows
Blastocyst is producing estradiol sulphate which changes the flow of PGF2alpha and instead of being directed to the veins it is being directed to the lumen of the uterus.
So it doesn’t get to the ovaries in a high enough concentration to cause breakdown of CL
One embryo is not sufficient to make this change happen (cant produce enough estrodiol) need at least 4 embryos to produce enough estrodial to change PGF2 flow to go into the lumen
Important that embryos are spread out so that the flow of PGF2is diverted along the whole length of the uterine horns
Embryo signalling - mares
Don’t know what the signal is that causes this change may have something to do with a particular protein (not enough evidence yet)
Blastocyst in mares moves around a lot before it implants thought that it does that to spread the message around the uterus to stop breakdown of corpus luteum (to make up for the fact that they only have 1 embryo)
Embryo transfer in sheep (timing of transfer)
EmbryoremovalbeforeD13leadstoCLregressionandreturntooestrusatthe normal time (because embryo hasn’t been able to signal that it is there)
TransferofaD13embryotoaD13non‐pregnanteweseestheCLpersist
TransferofaD13embryotoa>D13nonpregnanteweseesCLlysis (signal hasn’t happened in time)
Oestrous cycle in the mare
Seasonal polyestrous (long-day)
Oestrous cycle is ~21 days
FSH action early in diestrus may produce a mid-cycle follicle that will sometimes ovulate, but more usually regresses
Unique increase in FSH early in luteal phase (instead of the beginning of the follicular phase), this is because horses need a boost of progesterone at the beginning of the pregnancy so surge in FSH is due to the horses body assuming she is going to get pregnant and is preparing for pregnancy
Surge of FSH causes growth of follicles – these follicles usually don’t ovulate but do lutealyse which gives the mare a boost of progesterone at the beginning of pregnancy
Oestrous cycle in the bitch
Cycle stretched out over weeks
Luteal phase is stretched out a lot monoestrous animal only goes through one oestrous cycle a year
Pseudo-pregnancy – allows corpus luteum to maintain for the same amount of time regardless of whether she is pregnant or not don’t need to worry about embryo signaling
Lactation may occur regardless of whether pregnant or not
Pseudo-pregnancy in dog
False pregnancy, or pseudopregnancy, is a term used to denote a common condition in a non-pregnant female dog that is showing symptoms of pregnancy, lactation, or nursing, without producing puppies. The affected female dog shows these symptoms about a month or two after her estrus (heat) is over.
Oestrous cycle in the queen
Induced ovulatory hormone cycle is different
Non-mated queen will go through regular phases of oestrous but instead of the luteal phase they go through postoestrus (No CL formation) follicles grow but wont ovulate unless induced
If ovulation occurs progesterone levels are high and maintained until birth
Oestrous is suppressed during lactation after lactation oestrous cycles will begin again
Tom cat has spines on penis which stimulate cervix to induce ovulation however not very successful (50%) multiple mating attempts are often required (e.g. lions >100x per oestrous)
