Physiology of Lactation
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Direct answer
Lactation is the production and release of milk by the mammary gland, controlled by prolactin, which drives synthesis, and oxytocin, which drives ejection. It proceeds through mammogenesis in pregnancy, lactogenesis I as secretory differentiation during pregnancy, and lactogenesis II, the copious secretion that starts about two to three days after delivery when placental progesterone falls. Sustained supply then depends on galactopoiesis, which is maintained by frequent and effective milk removal.
What you must remember
- Prolactin: an anterior pituitary hormone that drives alveolar milk synthesis; it rises with suckling but does not release milk.
- Oxytocin: a posterior pituitary hormone released in response to suckling, contracting myoepithelial cells around alveoli and ducts to eject milk.
- Let-down reflex: nipple stimulation sends afferent signals to the hypothalamus, which releases oxytocin; stress and pain can inhibit this reflex.
- Progesterone block: high placental progesterone during pregnancy suppresses copious milk secretion until delivery removes its source.
- Colostrum: early milk rich in protein, secretory IgA, lactoferrin and leucocytes, with less lactose and fat than mature milk.
- Mature milk: roughly 7 per cent lactose, 3 to 4 per cent fat and about 1 per cent protein; lactose is the main osmotic determinant of milk volume.
- Feedback inhibitor: a whey-derived protein that accumulates in the breast and slows secretion when milk is not removed, so frequent emptying sustains supply.
- Dopamine: restrains prolactin release, so dopamine agonists such as cabergoline and bromocriptine suppress lactation.
Stages of lactogenesis in order
Lactation is best learnt as a timeline, because each stage depends on the hormone that dominates at that point. Each stage also has a clear hormonal driver, so memorise the driver as well as the name. Exam vignettes usually give the day after delivery and ask which stage is in progress.
- Mammogenesis: ductal branching and alveolar growth during pregnancy, driven by oestrogen, progesterone, prolactin and placental lactogen.
- Lactogenesis I: alveolar cells differentiate into secretory cells during the later part of pregnancy; colostrum is produced, but high progesterone keeps output low.
- Lactogenesis II: after the placenta is delivered, falling progesterone and rising prolactin allow copious milk secretion, usually from day two to three.
- Galactopoiesis: sustained secretion depends on regular milk removal, which clears the feedback inhibitor and maintains output over months.
- Involution: when feeding stops, milk stasis raises the feedback inhibitor and alveolar cells undergo apoptosis.
Where the exam tests the reflexes
Most lactation questions turn on which hormone acts on synthesis and which acts on ejection, and on when progesterone falls.
- Trap: Crediting prolactin with milk ejection Correct: Oxytocin contracts myoepithelial cells, so ejection is the oxytocin reflex, while prolactin drives synthesis.
- Trap: Expecting copious milk during pregnancy because prolactin is high Correct: Prolactin is high throughout pregnancy, yet copious milk waits for delivery because progesterone blocks secretion.
- Trap: Describing colostrum as inferior milk Correct: Colostrum carries secretory IgA and lactoferrin, giving the newborn local immune protection in the first days.
- Trap: Forgetting that dopamine restrains prolactin Correct: Dopamine antagonists raise prolactin and can cause galactorrhoea, while dopamine agonists suppress lactation.
Frequently asked questions
What triggers lactogenesis II after delivery?
Lactogenesis II is triggered by the rapid fall in progesterone after expulsion of the placenta, which removes the block on alveolar secretion. Prolactin then drives copious milk production, and regular milk removal maintains it over the following weeks.
Which hormone causes milk ejection during breastfeeding?
Oxytocin, released from the posterior pituitary in response to suckling, causes milk ejection by contracting myoepithelial cells around the alveoli and ducts. Prolactin does not eject milk; it drives synthesis.
Why is colostrum important for a newborn?
Colostrum is rich in secretory IgA, other immunoglobulins, lactoferrin and leucocytes, which give the newborn local gut protection while its own immune system matures. Its small volume is adequate for the early neonatal stomach.
How do dopamine agonists suppress lactation?
Dopamine normally inhibits prolactin release, so dopamine agonists such as cabergoline and bromocriptine lower prolactin and suppress milk production. They are used when suppression is needed, but their side effects must be weighed, and bromocriptine is no longer advised for routine suppression.