Control of parturition in ruminants.

Parturition is a process which, when set into motion, occurs to completion. This review concerns the control of parturition in ruminants. Parturition is an endocrine event, dependent upon the activation of the fetal hypothalamus-pituitary-adrenal (HPA) axis. In sheep and other ruminants, increases in plasma concentrations of cortisol induce the activity of 17-hydroxylase and 17,20 lyase in the placenta, increasing the biosynthesis of oestrogen relative to progesterone. The increase in the so-called E:P ratio increases myometrial activity and culminates in labour and delivery. Much work has been done to identify the mechanism of the endogenous activation of the fetal HPA axis. Recent work suggests that production of prostanoids within the fetal brain influences fetal ACTH secretion, and that induction of prostanoid biosynthesis at the end of gestation might be important in the process of parturition. Oestrogen and androgens, secreted by the placenta at the end of gestation, augment activity of the fetal HPA axis by increasing fetal ACTH secretion and by decreasing negative feedback sensitivity to cortisol. Although significant progress has been made concerning the neuroendocrinology of parturition, many significant questions remain. Is parturition regulated or simply programmed? Is parturition the ultimate result of neuronal maturation within the fetal hypothalamus, or is there a complex interplay between the placenta and fetal hypothalamus? Answers to these and other important questions await further research, but may provide key information which will prove useful in understanding general principles of parturition in many mammalian species.