Q5 LONG ANSWER 10M ★ High-yield

Describe the physiology of lactation. Explain why milk secretion begins only after delivery.

Physiology · 1st Professional — Pre-Clinical

Definition

Lactation = the synthesis, secretion and ejection of milk by the mammary gland. It has three distinct phases — mammogenesis, lactogenesis and galactopoiesis.

Mammogenesis — Development of the Breast

Hormone	Action on the breast
Oestrogen	Duct growth and branching
Progesterone	Alveolar (lobular) development
Prolactin	Growth and differentiation of the secretory alveoli
HPL	Assists breast growth in pregnancy
Growth hormone, cortisol, insulin, thyroxine	Permissive

Lactogenesis — WHY Milk Does Not Flow Before Delivery

During pregnancy: prolactin is high → But oestrogen and progesterone are also very high → They block prolactin action at the breast → Delivery → placenta expelled → Oestrogen and progesterone fall abruptly → Prolactin block removed → milk secretion begins (day 2–3)

This is the single most examined point in the topic — the fall in oestrogen and progesterone, not a rise in prolactin, starts lactation
Colostrum is secreted for the first 2–3 days — rich in protein, vitamin A and IgA, low in fat and lactose

Galactopoiesis — Maintenance

Depends entirely on continued suckling
Suckling is the single most important stimulus for both prolactin and oxytocin
Milk left in the breast suppresses further secretion (feedback inhibitor of lactation) — hence demand feeding maintains supply

The Two Reflexes

	Prolactin reflex	Milk ejection (let-down) reflex
Hormone	Prolactin (anterior pituitary)	Oxytocin (posterior pituitary)
Effect	Milk secretion	Milk ejection
Target	Alveolar epithelium	Myoepithelial cells
Conditioned?	No	Yes — the cry or sight of the baby suffices
Inhibited by	Dopamine, bromocriptine	Stress, fear, pain, embarrassment (via adrenaline)

CLINICAL PEARL

Note: prolactin is unique in being under tonic inhibition by hypothalamic dopamine. Section of the pituitary stalk therefore raises prolactin while every other anterior pituitary hormone falls.

Composition of Human Milk

Constituent	Human milk	Cow milk
Protein	1.1 g/dL (mostly whey)	3.3 g/dL (mostly casein)
Lactose	7 g/dL — higher	4.8 g/dL
Fat	4.5 g/dL	3.7 g/dL
Iron	Low but well absorbed (50%)	Low, poorly absorbed
Calcium	Lower	Higher
IgA and lysozyme	Present	Absent
Energy	65–70 kcal/dL	Similar

Human milk is deficient in vitamin K and vitamin D — hence vitamin K at birth and vitamin D supplements

Suckling — the Central Stimulus

Suckling → Afferents from the nipple → spinal cord → hypothalamus → Two simultaneous outputs → ↓ Dopamine → ↑ prolactin (secretion) and ↑ oxytocin (ejection) → Also ↓ GnRH → lactational amenorrhoea

Prolactin rises 10–20 fold within 30 minutes of a feed
The response declines over months, which is why supply falls if feeding is not maintained

Milk Synthesis in the Alveolar Cell

Constituent	Origin
Lactose	Synthesised from glucose by lactose synthase; α-lactalbumin is its regulatory subunit
Casein and whey proteins	Synthesised in the rough endoplasmic reticulum
Fat	From plasma fatty acids and de novo synthesis; secreted as milk fat globules (apocrine)
Water, electrolytes, IgA	From plasma; IgA is actively transported

Milk is isotonic with plasma, with lactose the chief osmotic constituent
Prolactin induces α-lactalbumin, which is why lactose synthesis begins only after delivery

Applied Aspects

Advantages of breast feeding — ideal nutrition, passive immunity through IgA, protection against infection and allergy, bonding, cheap and sterile, aids uterine involution and delays return of fertility
Lactational amenorrhoea — prolactin suppresses GnRH; effective as contraception only if feeding is exclusive, the infant is under 6 months and menses have not returned
Bromocriptine suppresses lactation; metoclopramide and domperidone (dopamine antagonists) increase it
Prolactinoma — galactorrhoea, amenorrhoea and infertility in women; impotence in men
Sheehan syndrome — postpartum pituitary necrosis after severe haemorrhage; failure of lactation is often the first sign
Mastitis and cracked nipples — usually from poor latch; feeding should generally continue
Drugs in breast milk — most pass in small amounts; a few (cytotoxics, radioiodine, lithium) contraindicate feeding
Exclusive breast feeding is recommended for the first 6 months, with complementary feeding thereafter and continued breast feeding to 2 years

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