Parturition- Lecture 12

Question 1

Ferguson reflex: oxytocin and cervical stretch

Answer 1

Uterine contraction–> cervical stretch–> afferent impulses from cervical stretch receptors–> reflex oxytocin secretion. (Positive feedback)

Question 2

Liggins contribution

Answer 2

The fetus controls the timing of parturition. Hypophysectomy or adrenalectomy prevent parturition. Infusion of ACTH or cortisol induces premature parturition and visceral maturation.

Question 3

Glucocorticoids

Answer 3

Stimulate preterm labor in sheep but NOT in humans.

Question 4

Liggins legacy

Answer 4

Women threatening to preterm labor are treated with glucocorticoid to accelerate lung development and reduce the need for ventilator suppport after birth.

Question 5

Steroid biosynthesis

Answer 5

In placenta, cortisol increases estrogen biosynthesis at the expense of progesterone biosynthesis.

Question 6

Overview of the HPA axis in Partuition: sheep

Answer 6

Question 7

Thorburn hypothesis.
Observation: Prostaglandin E2 injections into fetuses intravenously increase fetal ACTH secretion.

Answer 7

Hypothesis: PGE2 secreted from the ovine placenta is a hormonal signal which stimulates fetal ACTH secretion at the end of gestation.
However… PGE2 does not circulate in plasma at concentrations large enough to stimulate fetal ACTH secretion.

Question 8

Do estrogens and androgens influence fetal ACTH secretion?

Answer 8

Estradiol augments fetal ACTH secretion. Androstenedione inhibits cortisol negative feedback inhibition of fetal ACTH secretion. Cortisol negative feedback sensitivity is reduced before birth.

Question 9

Positive feedback hypothesis

Answer 9

Question 10

Fetal HPA

Answer 10

The Fetal HPA ais is also involved in fetal homeostasis and stress reponsiveness. The HPA axis is the “stress” axis of the endocrine system. Impairments in fetal HPA axis function impair fetal survival of stress

Question 11

Human parturition vs. sheep parturition

Answer 11

Humans have feto-placental unit. May still depend upon fetal HPA axis activity.

Question 12

The human feto-placental unit

Answer 12

The fetal zone of the fetal adrenal lacks 3beta-HSD. The placenta lacks P450c17.

Question 13

Overview of HPA axis in parturition: primates

Answer 13

Question 14

Anencephaly

Answer 14

Evidence that fetal brain does not control parturition.

Anencephaly- Fetises without functional brains, no functional control of pituitary. The timing of parturition in anencephalics is not normal, but not consistently delayed.
But… anencephalic fetuses often have polyhydramnios, and labor might be induced by direct uterine stretch.