Physiology of lactation for 1st year mbbs 5 marks qn
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Physiology of Lactation
1. Development of the Breast (Mammogenesis)
Breast development for lactation occurs in stages under hormonal control:
- Estrogens (from puberty onward) stimulate growth of the ductal system and fat deposition. During pregnancy, placental estrogens cause the ductal system to grow and branch extensively.
- Progesterone (during pregnancy) drives full development of the lobule-alveolar system - it stimulates budding of alveoli and secretory differentiation of alveolar cells.
- Supporting hormones include: growth hormone, prolactin, adrenal glucocorticoids, and insulin - all involved in protein metabolism needed for breast tissue growth.
2. Initiation of Lactation (Lactogenesis) - Role of Prolactin
Prolactin is the key lactogenic hormone, secreted by the anterior pituitary.
- During pregnancy, prolactin levels rise steadily from the 5th week to 10-20 times the normal level by term.
- Why doesn't lactation occur during pregnancy? Despite high prolactin levels, high estrogen and progesterone from the placenta block prolactin's action on the breast - suppressing actual milk secretion.
- At parturition, the sudden fall in estrogen and progesterone (after placental delivery) removes this inhibition, and prolactin can now act freely - milk secretion begins within 1-7 days.
- The fluid secreted in the first few days before and after birth is colostrum - rich in proteins, immunoglobulins, and lactose, but low in fat.
3. Maintenance of Lactation - Suckling Reflex
Continued lactation depends on the neuroendocrine suckling reflex, involving two key hormones:
a) Prolactin (milk production):
- Suckling sends afferent nerve impulses from nipple mechanoreceptors → hypothalamus → causes a 10-20-fold surge in prolactin lasting ~1 hour.
- This surge keeps alveolar cells secreting milk for the next nursing period.
- Without regular suckling, milk production ceases within ~1 week.
b) Oxytocin (milk ejection/let-down reflex):
- Suckling (and also auditory, visual, olfactory stimuli associated with the baby) → hypothalamic nuclei → posterior pituitary releases oxytocin.
- Oxytocin causes contraction of myoepithelial cells surrounding the alveoli and ducts → milk is ejected through the nipple (let-down reflex).
- Emotional stress can inhibit oxytocin release and thus inhibit milk let-down.
4. Hypothalamic Control of Prolactin
The hypothalamus uniquely inhibits prolactin (unlike its stimulatory control of other pituitary hormones):
- Prolactin Inhibitory Hormone (PIH) = Dopamine, secreted by arcuate nucleus, travels via portal system to anterior pituitary, suppressing prolactin up to 10-fold.
- Suckling suppresses dopamine release → allows prolactin surge.
- Drugs blocking dopamine (e.g., antipsychotics) can cause galactorrhea; dopamine agonists (e.g., bromocriptine) suppress lactation.
- Prolactin Releasing Factor (PRF): TRH also has a minor prolactin-releasing effect.
5. Lactation Amenorrhea
As long as suckling continues:
- Elevated prolactin → inhibits GnRH secretion from hypothalamus → suppresses FSH and LH from anterior pituitary → ovulation is suppressed.
- This is the basis of lactational amenorrhea - breast-feeding acts as a natural (though not 100% effective) contraceptive method.
Fig: Changes in estrogen, progesterone, and prolactin around parturition. Note the sharp fall in estrogen/progesterone at birth, and the intermittent prolactin surges with each nursing episode.
Summary Table
| Phase | Key Hormone | Action |
|---|---|---|
| Breast development | Estrogen + Progesterone | Duct + lobulo-alveolar growth |
| Milk suppression (pregnancy) | Estrogen + Progesterone | Block prolactin action |
| Initiation post-partum | Prolactin (unopposed) | Triggers milk secretion |
| Milk production maintenance | Prolactin surge (suckling) | Keeps alveoli secreting |
| Milk ejection | Oxytocin | Myoepithelial contraction |
| Lactational amenorrhea | Prolactin | Suppresses GnRH → no ovulation |
Sources: Guyton and Hall Textbook of Medical Physiology, pp. 1044-1046 | Ganong's Review of Medical Physiology, 26th ed., pp. 419-420 | Costanzo Physiology, 7th ed., p. 484
Give the answer again
Physiology of Lactation
1. Development of the Breast (Mammogenesis)
Breast development for lactation occurs in stages under hormonal control:
- Estrogens stimulate growth of the ductal system and fat deposition. During pregnancy, placental estrogens cause the ductal system to grow and branch extensively.
- Progesterone drives full development of the lobule-alveolar system - stimulates budding of alveoli and secretory differentiation of alveolar cells.
- Supporting hormones: growth hormone, prolactin, adrenal glucocorticoids, and insulin - all involved in protein metabolism needed for breast tissue growth.
2. Initiation of Lactation (Lactogenesis) - Role of Prolactin
Prolactin is the key lactogenic hormone, secreted by the anterior pituitary.
- During pregnancy, prolactin levels rise steadily from the 5th week to 10-20 times the normal level by term.
- Why doesn't lactation occur during pregnancy? Despite high prolactin, high estrogen and progesterone from the placenta block prolactin's action on the breast - suppressing actual milk secretion.
- At parturition, the sudden fall in estrogen and progesterone (after placental delivery) removes this inhibition - milk secretion begins within 1-7 days.
- The fluid secreted in the first few days is colostrum - rich in proteins and immunoglobulins, low in fat.
3. Maintenance of Lactation - Suckling Reflex
Continued lactation depends on the neuroendocrine suckling reflex, involving two key hormones:
a) Prolactin (milk production):
- Suckling → afferent nerve impulses from nipple mechanoreceptors → hypothalamus → 10-20-fold surge in prolactin lasting ~1 hour.
- This surge keeps alveolar cells secreting milk for the next nursing period.
- Without regular suckling, milk production ceases within ~1 week.
b) Oxytocin (milk ejection / let-down reflex):
- Suckling (and also auditory, visual, olfactory stimuli associated with the baby) → hypothalamic nuclei → posterior pituitary releases oxytocin.
- Oxytocin causes contraction of myoepithelial cells surrounding the alveoli and ducts → milk is ejected through the nipple.
- Emotional stress can inhibit oxytocin release and thus inhibit milk let-down.
4. Hypothalamic Control of Prolactin
The hypothalamus uniquely inhibits prolactin (unlike its stimulatory control of other pituitary hormones):
- Prolactin Inhibitory Hormone (PIH) = Dopamine, secreted by the arcuate nucleus, travels via the hypothalamo-hypophysial portal system to suppress prolactin secretion up to 10-fold.
- Suckling suppresses dopamine release → allows the prolactin surge.
- Drugs blocking dopamine (e.g., antipsychotics) can cause galactorrhea; dopamine agonists (e.g., bromocriptine) suppress lactation.
- TRH has a minor prolactin-releasing effect.
5. Lactation Amenorrhea
As long as suckling continues:
- Elevated prolactin → inhibits GnRH secretion from hypothalamus → suppresses FSH and LH from anterior pituitary → ovulation is suppressed.
- This is the basis of lactational amenorrhea - breast-feeding acts as a natural (though not 100% effective) contraceptive.
Estrogen and progesterone fall sharply at parturition; prolactin shows intermittent surges with each nursing episode (Guyton & Hall, Fig. 83.12)
Summary Table
| Phase | Key Hormone | Action |
|---|---|---|
| Breast development | Estrogen + Progesterone | Duct + lobulo-alveolar growth |
| Milk suppression (pregnancy) | Estrogen + Progesterone | Block prolactin action on breast |
| Initiation post-partum | Prolactin (unopposed) | Triggers copious milk secretion |
| Milk production maintenance | Prolactin surge (suckling) | Keeps alveoli secreting |
| Milk ejection | Oxytocin | Myoepithelial contraction (let-down) |
| Lactational amenorrhea | Prolactin | Suppresses GnRH → no ovulation |
Sources: Guyton and Hall Textbook of Medical Physiology, pp. 1044-1046 | Ganong's Review of Medical Physiology, 26th ed., pp. 419-420 | Costanzo Physiology, 7th ed., p. 484
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