Which of the following inhibits milk secretion before birth?
a) Low levels of prolactin and estrogen
b) High levels of oxytocin and prolactin
c) Low levels of estrogen and progesterone
d) High levels of estrogen and progesterone
D is correct. High levels of estrogen and progesterone. Before birth, high levels of estrogen and progesterone inhibit milk secretion from the breasts. As childbirth approaches, however, the levels of both progesterone and estrogen begin to fall. After birth, there is an even more significant decrease in these hormone levels, releasing the inhibition on milk production and secretion.
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During pregnancy, females undergo various hormonal changes (Figure 1). The release of human chorionic gonadotropin (hCG) helps maintain the corpus luteum, which secretes progesterone until the placenta takes over. Simultaneously, along with the rise in progesterone levels, estrogen levels will rise until the approach of parturition when they begin to fall quickly.
Parturition refers to childbirth. While the fetus is developing, increasing progesterone levels inhibit uterine contractions, so parturition does not occur too soon. However, as childbirth approaches, progesterone levels drop, which allows for the contraction of the uterus. Similarly, estrogen levels rise during pregnancy, as well. Estrogen functions to promote the growth of the myometrium, which has a vital role in uterine contractions. Also, estrogen induces the expression of oxytocin receptors in the myometrium. Oxytocin is the primary hormone for driving uterine contractions, so with more receptors, the myometrium will become more responsive to oxytocin, which will drive more muscular contractions.
The release of oxytocin is also stimulated by cervical dilation and uterine distention. As the fetus grows bigger, its movements will cause the uterus to distend, which will eventually lead to cervical dilation. This distension and dilation process will lead to oxytocin, which will help drive uterine contractions and produce even more distention and dilation, causing the further release of oxytocin. This cyclical process will continue, creating more forceful contractions, until the child is born.
Lactation refers to the process of milk secretion. This process is vital for nurturing the newborn baby, and it helps provide nutrition and immune protection from the mother to the child. The milk duct system in the breasts, known as the lactiferous ducts, begin to develop at around week 18 of pregnancy. Various hormones are involved in this process. For example, progesterone, estrogen, and prolactin influence the alveoli and lobes of the breast to develop and grow in size. Before birth, high levels of estrogen and progesterone will also inhibit lactation in the newly developed milk duct system. This inhibition is important because lactation is unnecessary before the child is born. However, as Figure 2 shows, the levels of estrogen and progesterone begin to fall before birth and reach very low levels after birth. The loss of these hormones during and after birth indicates that lactation is no longer inhibited, and milk production can increase.
Unlike estrogen and progesterone, however, rising prolactin levels do not fall after childbirth. Prolactin contributes to galactopoiesis, which is the regulation and maintenance of milk production. In a condition known as hyperprolactinemia, high levels of prolactin can cause spontaneous milk flow from the breast in non-pregnant women and even men. A common cause of this condition is a tumor in the pituitary gland known as a prolactinoma.
Another important hormone for lactation is oxytocin. Recall that oxytocin is important for smooth muscle contraction during childbirth. Similarly, it can induce the contraction of smooth muscle cells surrounding alveoli, which helps to squeeze milk into the duct system. Like prolactin, oxytocin is maintained at a high level after birth. When the nipples of the mother are stimulated, oxytocin is released and allows for milk letdown from the mammary glands and milk secretion for the baby. In other words, while prolactin is essential for milk production, oxytocin is essential for milk secretion.
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