Parturition Endocrine Control - Anatomy & Physiology

Regulation of Prostaglandins

The uterus is an important site of prostaglandin synthesis. During pregnancy, the presence of the foetus inhibits the production of prostaglandins in order to prevent luteolysis. At parturition, an increase in prostaglandin synthesis is required.

• Prostaglandins are synthesized from Arachadonic Acid, which is derived from Glycerophospholipids by the enzyme Phospholipase A2 (PLA2).
  • Oestrogen liberates Phospholipase A2 from lysosomes.
  • Progesterone stabilises lysosomes, so Phospholipase A2 cannot be liberated from lysosomes.
• An increase in Oestrogen:Progesterone ratio increases prostaglandin production.
• The increase in Oestradiol (bioactive oestrogen) also increases expression of endometrial Oxytocin receptors.

Regulation of Oxytocin

Details of Oxytocin Synthesis and Secretion may be found here

• Oxytocin acts to:
  • Increase myometrial contraction (positive feedback)
  • Increase Prostaglandin release

Furgesson Reflex

• Neuroendocrine reflex
• Oxytocin is released in response to cervical stimulation by the foetus.
• Oxytocin release causes further synthesis and secretion of oxytocin, thus exibiting positive feedback.
• Oxytocin concentrations continue to rise until the foetus is expelled, thus the cervical stimulation ceases.

Onset of Parturition

Species	Ewe	Horse	Queen	Goat	Sow	Cow
% of pregnancy when the Corpus Luteum is the principle source of Progesterone	37	61	79	100	100	100
% of pregnancy when the Placenta is the principle source of Progesterone	63	39	21	0	0	0