Parturition Endocrine Control - Anatomy & Physiology

Introduction

The foetus triggers the onset of parturition by initiating a cascade of complex endocrine/biochemical events. The foetal hypothalamo-pituitary-adrenal axis is obligatory for the initiation of parturition.

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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.

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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

Species that depend on the Corpus Luteum to produce progesterone throughout pregnancy (Goat, Cow, Sow, Bitch)

• The foetus initiates parturition.
• For parturition to occur, progesterone production by the Corpus Luteum must be terminated.
• As the foetus grows in a restrictive environment it becomes stressed. Stress leades to a rise in foetal ACTH. The rise in ACTH causes production of Cortisol from the foetal Adrenal Glands.
• The rise in foetal cortisol leased to an increased Oestrogen:Progesterone ratio by upregulating Aromatase enzymes which convert Androgens to Oestrogens. Cortisol also reduces Progesterone production.

Species that depend on Placental Progesterone in late pregnancy (Ewe,Guinea Pig,Queen,Mare)

• As previously explained, parturition is dependent on a rise in foetal Cortisol. However, in this case, the rise in foetal cortisol leads to a decrease in maternal Progesterone.
• Foetal Cortisol causes upregulation of the enzymes 17α Hydroxylase, C17-C20-lyase and Aromatase in order to increase the Oestrogen:Progesterone ratio.

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Role of Relaxin

• Glycoprotein hormone
• Produced by either the corpus luteum or placenta depending on species.
• Synthesis is stimulated by PGF2α
• Causes softening of the connective tissue in the cervix.
• Promotes elasticity of pelvic ligaments.
• Prepares the birth canal by loosening supportive tissues so that the passage of the foetus can occur with relative ease.