Physiology of Lactation

Question 1

What does milk include?

Answer 1

Energy, lipids, casein, whey, non-protein nitrogen, lactose.

Question 2

What happens to the lobules/alveoli during pregnancy?

Answer 2

They grow to make more milk-producting cells.
Human alveoli are adequately differentiated to produce milk from 16 weeks gestation BUT full lactation doesn’t occur till post partum period.

Question 3

Why doesn’t full lactation occur till post partum?

Answer 3

Prolactin secreted from 16 weeks but our breast tissue is unresponsive.
Withdrawal of oestrogen and progesterone are essential (if no suckling - a small amount of milk is produced for 3 to 4 weeks).
Suckling ensures prolactin secretion - full lactogenesis = copious milk production.

Question 4

How does suckling ensure milk production?

Answer 4

1) Neuroendocrine reflex.
2) Suckling…
…stimulates production of VIP (vasoactive intestinal peptide)
…reduces dopamine release.
3) Strength and duration of suckling determines the amount of prolactin released…which in turn determines the amount of milk available for subsequent feeds

“order food in advance!”

Question 5

Pathway I for milk secretion.

Answer 5

Exocytotic secretion of milk proteins:

Lactose, calcium, aqueous phase components.

Question 6

Pathway II for milk secretion.

Answer 6

Milk fat secretion.

Cytoplasmic lipid droplets move to apical membrane - secreted as a membrane bound milk fat globule.

Question 7

Pathway III for milk secretion.

Answer 7

Vesicular transcytosis.

Proteins - eg immunoglobulins from the interstitial space.

Question 8

Pathway IV for milk secretion.

Answer 8

Transporters.

Direct movement of monovalent ions, water and glucose across apical and basement membranes.

Question 9

Pathway V for milk secretion.

Answer 9

Paracellular Transport.

For plasma components and leukocytes. Open only during pregnancy, involution and inflammation.

Question 10

Explain the milk ejection reflex.

Answer 10

Milk is transported from the alveoli to the nipple.
Neuroendocrine reflex.
Suckling stimualtes paraventricular and supraoptic nuclei to produce oxytocin (PP)
Oxytocin causes contraction of myoeptithelial cells surrounding alveoli full of milk.
Milk is propelled into ducts and intramammary pressure builds up.
Milk spurts through nipple.
Response can be conditioned.