Parturition (in sheep) Flashcards
What is parturition?
- process by which a conceptus is expelled from the uterus
What is the conceptus?
- its the foetus and the placenta
What is the gestation period in the sheep?
*143-147 days
What stimulates parturition?
- a signal from the foetus (foetal stress)
- cervical softening
- co-ordinated myometrial contractions
How many stages are there to labour/parturition?
- 3 stages
What happens prior to the 1st stage of parturition (in the days/weeks leading to it)?
- udder development
- relaxation of pelvic ligaments
- passing of mucous plug from cervix
- vulva swelling
How long can the 1st stage of parturition be?
- 2-6 hrs in sheep
- up to 24hrs in cattle
What happens in the 1st stage?
- regular uterine contractions
- cervical shortening and dilation occurs
- foetus enters birth canal
In the 1st stage there is a cervical dilation - what are the two phases involved?
- latent phase = cervix slowly dilates
- active phase = rapid dilation of cervix
How long is the 2nd stage of parturition?
- 0.5-2 hrs in sheep
- up to 4 hrs in cattle
What happens in the 2nd stage of parturition?
- foetus present at vagina
- abdominal contractions start
- cervix is completely dilated
- rupture of membranes
- complete delivery of foetus
How long is the 3rd stage?
- 5-8 hrs in sheep
- 2-4 hrs in cattle
What happens in the 3rd stage?
- uterine contractions continue and involution begins
- delivery of placenta
What may be signs a lamb was dead before stage 2?
- congenital malformation
- infection
- placental insufficiency
What signs suggest a lamb was dead during stage 2?
- anorexia or trauma
- bradytocia = sloe birthing, dystocia, malpresentation, twins, arthrogryposis
What signs suggest a lamb died after stage 2?
- infection
- hypothermia
- exposure
- trauma
- blood clot in foetal artery
What can loss of progesterone cause?
- induced abortion or initiates parturition
What hormone is essential to maintain pregnancy?
- progesterone
Progesterone is a steroid hormone and therefore is a derivative of what?
- cholesterol
What is the main cause of onset of parturition?
- achieved by foetal stress response
Once the foetus becomes stressed what is released and what increases?
- ACTH is released
- cortisol increases
What axis is involved in foetal stress?
- activation of foetal hypothalamic-pituitary-adrenal axis
What does a rise in foetal cortisol lead to?
- leads to decrease in maternal progesterone
What are the 3 layers of the uterus?
- endometrium
- perimetrium
- myometrium
What does the myometrium (muscle layer) consist of?
- smooth muscle fibres
In pregnancy oestrogen stimulates what in the myometrium?
- muscle cell hypertrophy
What do muscle cells in the myometrium form?
- muscle cell syncytium electrically coupled via gap junctions (nexuses)
Why do we need well coupled syncytium muscle cells in the myometrium?
- because co-ordinated contraction requires simultaneous activation of all muscle cells
What type of control are myometrial contractions under?
- under endocrine control
As well as smooth muscles what else does myometrial contraction require?
- alterations in the membrane potential
For myometrial cells to contract they must be depolarised by what?
- depolarised by 50mV
What two things can be used to measure myometrial activity?
- a tocodynamometer (TOCO)
- electromyography (EMG)
What does a TOCO measure?
- measures the pressure during contractions
What does an EMG measure?
- measures electrical activity in the muscle
Uterine contractions progressively increase in what?
- increase in frequency and amplitude
How does spontaneous depolarisation occur before labour?
- low frequency and low amplitude
What do prostaglandins stimulate and in response to what?
- stimulates liberation of intracellular calcium for stronger contraction
- in response to increased oestrogen (and decreased progesterone)
What does oxytocin lower the excitation threshold of and in response to what?
- lowers the excitation threshold of the myometrial cells = more likely to trigger
- released in response to stimulation of the cervix by the foetus known as the Fergusson reflex (positive feedback loop)
Where is oxytocin made and secreted from?
- made in the hypothalamus
- secreted from the pituitary gland
What is oxytocin produced by in the hypothalamus?
- produced by neurones of the paraventricular nucleus of the hypothalamus
What does oxytocin induce?
- induces contractions of target smooth muscle fibres of the mammary gland and uterus
What does oxytocin cause?
- cause contraction of the myoepithelial cells and milk ejection (milk-let-down)
- cause contraction of myometrium (uterine contractions
What happens in brachystasis?
- uterine muscles retract
- uterine wall thickness
- uterine volume reduces
The uterus is divided into two segments - what are these?
- upper segment = contractile
- lower segment = passive
What is the junction between the contractile and passive uterus called?
- the retraction ring
Palpation of the retraction ring can be used how?
- as an indicator of progression through labour
What is the cervix job in relation to the foetus?
- retains the foetus in the uterus
What is the cervix composed of?
- composed of lots of connective tissue
- multiple collagen fibre bundles
- proteoglycan matrix
What does the structure of the cervix allow for during pregnancy?
- during pregnancy this means that it can resist stretch so as the foetus grows and the uterus distends, the cervix remains closed
The cervix needs to soften to allow the foetus out during parturition - how is this done?
- reducing collagen fibres
- increasing proteoglycan matrix
- under hormonal control
What hormones does the cervix produce?
- prostaglandin E2 (PGE2)
- prostacyclin (PGI2)
- prostaglandin F2a (PGE2a)
What softens the cervix?
- prostaglandins
Prostaglandins act to do what?
- breakdown collagen
- stimulate uterine contraction
During pregnancy how does progesterone inhibit PG production?
- by blocking phospholipase A2, required in prostaglandin synthesis
What does oestrogen liberate (switch on)?
- phospholipase A2
What is ring womb?
- failure of the cervix to soften and dilate with normal uterine contractions leading to inability to birth foetus naturally
What is ring worm caused by?
- lack of prostaglandin production
- lack of prostaglandin receptors
- linked to selenium deficiency
- genetic component
What is the treatment for ring womb?
- prostaglandin gel to cervix (not sheep)
- caesarean section
- in farm animals cull (potential genetic)
What is false ring womb?
- cervix dilation slow/partial with normal uterine contractions leading to inability to birth foetus naturally
What is false ring womb caused by?
- contraction of muscles around cervix
- can palpate a tight ring at cervix
What is false ring womb associated with?
- premature intervention by shepherd
- malpresentation of lamb (not pressing on cervix)
What is the treatment for false ring womb?
- gentle stretching/manipulation with fingers
- can take up to 45 mins
When do vaginal (and cervical) prolapse usually occur?
- last month of pregnancy
What are vaginal (and cervical) prolapse linked to?
- obesity
- multiple/large foetus
- high fibre diet
- limited exercise
What complications can a vaginal and cervical prolapse cause?
- abortion
- ring womb at parturition
- prolapse again at parturition
- uterine prolapse at parturition
What methods can be used for vaginal and cervical prolapse?
- replace the prolapse
- suture vulva
- retention spoon
- prolapse harness
- cull after parturition and don’t use offspring for breeding
