After childbirth, the breast begins to produce colostrum, a substance rich in antimicrobial and anti-inflammatory proteins. Without the presence of placental progesterone, regular milk production starts within a few days. The milk is produced by lobuloalveolar structures and is later modified by the ductal epithelium. The process of lactogenesis and the continuous production of milk (galactopoiesis) require stimulation from pituitary PRL, along with normal levels of other hormones such as insulin, cortisol, and thyroid hormone. While placental estrogen stimulates PRL secretion during pregnancy, the act of suckling by the infant stimulates PRL secretion during breastfeeding. The frequency and duration of suckling directly correlate with PRL levels, and this link is mediated through a neuroendocrine reflex that inhibits dopamine secretion at the median eminence (known as the PRL release inhibitory factor). It is also possible that suckling triggers the secretion of unidentified PRL-releasing hormones.

PRL also inhibits the release of GnRH, leading to lactational amenorrhea (the absence of menstruation) during breastfeeding. This effect of prolactin is often referred to as "nature's contraceptive" and may help in spacing out pregnancies. However, it is important to note that regular nursing over a 24-hour period is required to induce a PRL-induced anovulatory state in the mother. Therefore, lactational amenorrhea is not considered an effective or reliable form of birth control for most women. The inhibition of GnRH by high levels of PRL has significant clinical implications. Prolactinoma, a hormone-secreting pituitary tumor, is the most common form of such tumor and hyperprolactinemia can lead to infertility in both men and women. Additionally, high levels of prolactin can cause galactorrhea, the inappropriate flow of breast milk, in both genders.

Suckling at the nipple also stimulates the release of oxytocin from the pars nervosa. This leads to the contraction of myoepithelial cells, facilitating milk let-down or the expulsion of milk from the breast.

Invasive breast cancer (IBC) is a major cancer affecting women and can be classified into several categories. The most common classification for newly diagnosed IBC is luminal A, which usually originates from the luminal cells of terminal ducts or alveoli. Luminal A IBC shows some epithelial organization, including cell-cell contacts mediated by E-cadherin. This form of cancer is less motile and less aggressive. It also expresses the estrogen receptor α (ERα) and depends on estrogen stimulation for growth. Early diagnosis of luminal A IBC is associated with a good prognosis. The treatment for early, small tumors that have not spread to nearby lymph nodes typically involves surgical removal (lumpectomy), followed by radiation treatment, and then a 5-year daily tamoxifen treatment. Tamoxifen is a selective estrogen receptor modulator (SERM) that opposes the effects of estrogen in the breast.
After parturition, the human breast produces colostrum, which is enriched with antimicrobial and antiinflammatory proteins. In the absence of placental progesterone, normal breast milk production occurs within a few days. The lobuloalveolar structures produce milk, which is subsequently modified by the ductal epithelium. Lactogenesis and maintenance of milk production (galactopoiesis) require stimulation by pituitary PRL in the presence of normal levels of other hormones, including insulin, cortisol, and thyroid hormone. Although placental estrogen stimulates PRL secretion during pregnancy, the stimulus for PRL secretion during the nursing period is suckling by the infant (
Fig. 44.38 ). Levels of PRL are directly correlated with frequency and duration of suckling at the nipple. The link between suckling at the nipple and PRL secretion involves a neuroendocrine reflex in which dopamine secretion at the median eminence is inhibited (the PRL release inhibitory factor; see ). It is also possible suckling increases secretion of unidentified PRL-releasing hormones.
PRL also inhibits release of GnRH, and consequently, nursing can be associated with lactational amenorrhea (see
Fig. 44.38 ). This effect of prolactin has been called “nature’s contraceptive,” and it may play a role in spacing out pregnancies. However, only regular nursing over a 24-hour period is sufficient to induce a PRL-induced anovulatory state in the mother. Thus lactational amenorrhea is not an effective or reliable form of birth control for most women. Inhibition of GnRH by high levels of PRL is important clinically. A prolactinoma is the most common form of hormone-secreting pituitary tumor, and hyperprolactinemia is a significant cause of infertility in both sexes. Hyperprolactinemia can likewise be associated with galactorrhea (inappropriate flow of breast milk) in men and women.
Suckling at the nipple also stimulates release of oxytocin from the pars nervosa (see
Fig. 44.38). Contraction of myoepithelial cells induces milk let-down, or expulsion of milk from the
Invasive breast cancer (IBC) isamajorcancerinwomenandcanbeclassifiedintoseveralcategories.MostnewlydiagnosedIBCisclassifiedasluminal A, whichisusuallyderivedfromluminalcellsofterminalductsoralveoli.LuminalAIBCdisplayssomeepithelialorganization,includingEcadherin–mediatedcell-cellcontacts,andispoorlymotileandpoorlyaggressive.Thisformalsoexpressestheestrogen receptor α (ERα) andisdependentonestrogenicstimulationforgrowth.EarlydiagnosisofluminalAIBCconfersagoodprognosis.Treatmentforearlysmalltumorsthatare“node-negative”(i.e.,havenotspreadtonearbylymphnodes)typicallyinvolvessurgicalremoval(“lumpectomy”),followedbyradiationtreatment,followedby5yearsofdailytamoxifentreatment.TamoxifenisaSERMthatopposesestrogeninthebreast.