Oestrogens are a group of steroid compounds that function as the primary female sex hormone. Developing follicles in the ovaries, the corpus luteum and the placenta primarily produces them and some secondary sources in smaller amounts can be found in the liver, adrenal glands and breasts. Although oestrogen is present in both male and females, it is found to be significantly higher in women and is involved in controlling the menstrual cycle. As does progesterone, oestrogen also affects all aspects of the pregnancy and is particularly important in maintaining the health of the genital track, the reproductive organs and the breasts.
Throughout pregnancy gradually the levels of progesterone in the blood fall and the levels of oestrogen rise. It is this process that brings about parturition as the oestrogen helps the uterine to contract & progesterone inhibits it. It is thought that miscarriage or premature births can be a result of insufficient levels of progesterone.
As in most biological control systems the release and control mechanism for managing these hormones is known as negative feedback. The feedback loop causes the detector (pituitary gland) to either inhibit or stimulate the production of oestrogen or progesterone when required and keep the levels from deviating from the ‘norm’.
However, there is a more direct form of labour, which is caused from the release of the hormone oxytocin, a nine amino acid peptide hormone, synthesised in the neurosecretory cells in the hypothalamus which instructs the posterior lobe of the pituitary gland to secrete. The oestrogen encourages the uterine muscle to be more sensitive to oxytocin and therefore the uterine contracts further.
This is known as positive feedback as when the mechanism causes the detector to increase, not decrease its message to the controller, the loop is positive. In this case, the hormone oxytocin is released during childbirth and stimulates the contractions of the uterus, which in turn stimulates more oxytocin production leading to further rapid contractions until the baby is born (parturition). A synthetic form of this hormone oxytocin can be given to induce labour or speed up contractions.
Progesterone & oestrogen are also responsible during pregnancy for growth of the mammary glands in readiness for milk production. After the birth the message (release factor) sent to the pituitary gland from the hypothalamus is to secrete from the anterior lobe the hormone prolactin. Prolactin has many effects and increased serum during pregnancy helps cause enlargement of the mammary glands in the breast and more importantly increases production of milk. Before the birth the hormones progesterone and oestrogen inhibit the prolactin secretion but after parturition the sudden drop in levels of these two hormones together with the loss of the placenta enables the onset of lactation.
The breasts are mainly composed of millions of tiny milk glands together with their small ducts and are joined to come out at the nipple. Although there is certainly an overlap in the effect of hormones in this process it is fair to say that oestrogen stimulates the growth of the ducts whilst progesterone stimulates the enlargement of the glands themselves together with increased serum prolactin.
After childbirth, prolactin levels fall as the internal stimulus for them is removed and therefore suckling from the baby on the nipple will promote further prolactin release and maintain the ability to continue to lactate. When the demand for breastfeeding is no more the lactation will usually cease with one to two weeks. High prolactin levels also tend to suppress the ovulatory cycle by inhibiting FSH.
Oxytocin also plays a role in lactating, as it is this hormone that stimulates the myoepithelial cells causing the milk to be ejected into the ducts of the mammary glands. Suckling by the baby on the nipple again will stimulate oxytocin release. The release of oxytocin in this instance, and prolactin are controlled by negative feedback unlike the release of oxytocin during childbirth.
Based on information taken from:
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Biology a Functional Approach by MBV Roberts, 4th Edition
- Pregnancy and Birth by Miriam Stoppard Published 1986
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