Download Effects of Oestrogens and Progesterone

Hypothalamus and Pituitary
synthesis
TRH
CRH
GnRH
PRH PIH GHRH SS
+
Blood
oxytocin
ADH
TSH
ACTH
FSH
-
+
-
vessels
LH prolactin
GH
storage and secretion
breast
kidney
adrenal
thyroid
uterus blood vessels
cortex
gonads
breast
many
tissues
Effects of Oestrogens and Progesterone
1
Parturition
• Labour, delivery, birth
• Requires
– Dilation of cervical canal to accommodate passage of
fetus from uterus
– Sufficiently strong contractions of uterine myometrium to
expel fetus
• Exact factors triggering increase in uterine contractility and
initiating parturition not fully established
• Last weeks of pregnancy – weak contractions that
strengthen over time (Braxton-Hicks)
• Cervix softens under influence of relaxin
2
Trigger for Parturition
– Unknown, but current theory suggests signal is CRH
– Placenta: CRH released, acts on fetal pituitary, ACTH
• DHEA synthesis
• DHEA  Oestrogens
• Oestrogens enter maternal blood stream
– Oestrogen increases number of gap junctions between
myometrial cells; increases oxytocin receptors in uterus;
increases prostaglandins
– Prostaglandins increase uterine contractions
– High oestrogen: progesterone ratio  parturition
– Oxytocin increases uterine contractions
• Acts directly on smooth muscle
• Increases prostaglandins
– Role of inflammation: NFκB
Parturition
Contractions begin at labour onset, positive-feedback cycle
progressively increases force
Pressure of fetus against cervix reflexly increases oxytocin
secretion
Oxytocin causes stronger contractions
Positive-feedback cycle progressively increases until cervical
dilation and delivery are complete
Oxytocin secretion from
posterior pituitary
uterine
contractions
strengthen
Pressure of fetus
against cervix
Parturition
Initially,
weak uterine
contractions
3
Stages of labour
Cervical dilation (longest stage)
Lasts from several hours to 24 hours+ in a first pregnancy
Delivery of baby (usually lasts 30 to 90 min)
Begins when cervical dilation is complete
Delivery of placenta
Second series of uterine contractions separates placenta from uterine wall
Shortest stage – usually completed within 15 - 30 min after baby is born
After delivery, uterus
shrinks to pregestational
size (involution)
4
Lactation
• During gestation
– Elevated placental estrogen and progesterone promote
development of ducts and alveoli in mammary glands
• Prolactin
– Stimulates synthesis of enzymes essential for milk
production by alveolar epithelial cells
– Decrease in placental steroids at parturition initiates
lactation
• Sustained by suckling
– Triggers release of oxytocin and prolactin
– Oxytocin
• Causes milk ejection by stimulating cells
surrounding alveoli to squeeze secreted milk out
through ducts
– Prolactin
• Stimulates secretion of more milk to replace milk
ejected as baby nurses
Lactation
– Much of breast development occurs during first months
of pregnancy - mammary glands capable of milk
production by mid-pregnancy
– Inhibition of lactation during pregnancy
• Oestrogen and progesterone during last half of
pregnancy block effect of prolactin
– Physiological amenorrhoea: possibly due to inhibition
of GnRH by prolactin
5
Mammary Gland
– Milk produced in alveoli
of breasts
– Contraction of
myoepithelial cells
causes milk ejection
– Prolactin –
milk synthesis
– Oxytocin –
milk ejection
Suckling
Reflexes
6