Lactation
Mammary gland anatomy, mammogenesis, lactogenesis, galactopoiesis, FIL, mammal evolution, and milk physicochemistry (ANSC 224, Lectures 30–31)
Lecture 30A: Mammary Development
Mammary Gland Anatomy
- Parenchyma — secretory tissue: alveoli + duct system (the functional tissue)
- Fat pad (stroma) — adipose + connective tissue that supports parenchyma and guides ductal growth
- Alveolus — spherical cluster of milk-secreting epithelial cells; the basic secretory unit
- Myoepithelial cells — surround alveoli; contract in response to oxytocin to eject milk
- Duct system — collects milk from alveoli → gland cistern → teat cistern → teat orifice
The mammary gland is a modified sweat gland — unique to mammals.
Fetal Mammary Development
Mammary glands develop from ectoderm during fetal life in a defined sequence (before sex hormones act):
| Stage | Description |
|---|---|
| Milk line (mammary ridge) | Bilateral ectodermal thickening along thorax-to-abdomen; specifies nipple positions |
| Placode | Disc-shaped epithelial thickening at each future nipple site |
| Bud | Epithelial cells invaginate into underlying mesenchyme |
| Primary sprout | Elongation of epithelial cord into the fat pad |
| Ductal tree | Branching of the primary sprout; rudimentary duct system present at birth |
At birth, only a rudimentary ductal tree exists — the bulk of mammary development occurs postnatally under hormonal control.
Five Stages of Mammary Development
Rudimentary ductal tree embedded in fat pad. Isometric growth (grows with body, no allometric expansion). Minimal hormonal input.
Estradiol drives duct elongation and branching into fat pad. Then estradiol + progesterone together drive duct maturation (lobuloalveolar development). Allometric growth — mammary grows faster than body.
Simultaneously elevated estrogen + progesterone → greatest mammary mass increase. Exponential growth curve. Full lobuloalveolar development. Additional mammogenic hormones: prolactin, growth hormone (species-dependent).
Initiation of milk secretion. Alveolar cells differentiate and begin secreting milk components. Maintained by ongoing hormonal and mechanical stimuli.
After weaning or dry-off. FIL accumulates → milk secretion inhibited → apoptosis of secretory cells → gland remodels back toward postpubertal state. Can undergo another cycle with subsequent pregnancy.
Lecture 30B: Mammogenesis
Hormonal control of mammary gland growth
Postpubertal Mammary Growth
Phase 1: Estradiol alone
- Duct elongation and branching into fat pad
- Triggered at puberty
- No lobuloalveolar development
Phase 2: Estradiol + Progesterone
- Full duct maturation
- Lobuloalveolar (lobule + alveolus) development
- Occurs during luteal phase and pregnancy
Pregnancy
- Simultaneously elevated estrogen + progesterone → greatest mammary mass increase
- Growth follows an exponential curve during pregnancy
- Growth plateaus just before parturition as progesterone drops
- Alveoli fully develop; parenchyma expands dramatically relative to fat pad
Other Mammogenic Hormones
Beyond estrogen and progesterone, additional hormones contribute to mammogenesis. Importance varies by species.
| Hormone | Role in Mammogenesis | Species Notes |
|---|---|---|
| Estradiol | Duct elongation and branching | Universal |
| Progesterone | Duct maturation; lobuloalveolar growth (synergizes with E2) | Universal |
| Prolactin | Full lobuloalveolar development | Critical for goats, swine; less so for cattle |
| Growth Hormone (somatotropin) | Supports mammary parenchymal growth | Species-dependent; important in bovines for galactopoiesis |
| IGF-1 | Mediates GH effects; stimulates cell proliferation | Produced locally in mammary and by liver |
| Cortisol | Permissive role; required for differentiation | Especially at lactogenesis |
Lecture 30C: Lactogenesis & Galactopoiesis
Initiation and maintenance of milk secretion
Lactogenesis — Initiation of Lactation
Lactogenesis is a two-stage process culminating in full milk secretion at parturition.
Stage 1 — Prelactogenesis
- Differentiation of alveolar secretory cells
- Limited (partial) milk synthesis begins before parturition
- Colostrum components begin accumulating
Stage 2 — True Lactogenesis
- Full secretion of all milk components
- Begins 0–4 days before parturition
- Triggered by drop in progesterone + surge in prolactin
Hormonal Regulation of Lactogenesis
Progesterone
INHIBITS
Blocks prolactin receptor expression on alveolar cells; prevents premature lactation during pregnancy
Prolactin
STIMULATES
Surges at parturition as progesterone drops; drives Stage 2 lactogenesis; secreted from anterior pituitary
Cortisol
STIMULATES
Glucocorticoid; elevated at parturition (stress response); required for differentiation of alveolar cells
Key event at parturition: Progesterone drops sharply (placenta expelled) → prolactin can now act on alveolar cells → Stage 2 lactogenesis begins.
Colostrum
- First milk secreted; produced pre-partum and immediately post-partum
- High in immunoglobulins (IgG) — passive immunity transfer to neonate
- Neonates must absorb colostrum within 24–48 hours (gut closure — enterocytes can absorb intact IgG by pinocytosis only in this window)
- Also high in: growth factors, vitamins, minerals, leukocytes
Galactopoiesis — Maintenance of Lactation
Two most important factors for maintaining milk secretion: hormones and milk removal.
Key Hormones
Prolactin
- Required for non-ruminants (swine, rats, humans)
- Less critical for ruminants (cattle can lactate with low prolactin)
- Secreted in response to suckling/milking stimulus
Growth Hormone / bST
- Critical for ruminants
- bST supplementation increases yield 10–40%
- Mechanism: indirect — stimulates IGF-1 from liver → nutrient partitioning (more glucose/AA to mammary)
- Also reduces extramammary nutrient use
bST Safety Arguments
- Same molecule as the cow's own somatotropin — not foreign
- Biologically inactive in humans (species-specific receptor binding)
- Orally active — degraded by proteases in GI tract
- Not elevated in milk of treated cows
FIL — Feedback Inhibitor of Lactation
- Protein secreted by mammary epithelial cells into the alveolar lumen
- When milk is not removed, FIL accumulates in the alveolus
- High FIL → locally inhibits milk secretion (autocrine/paracrine)
- Increased intramammary pressure (from undrained milk) also suppresses secretion
- Milking/suckling removes FIL → inhibition relieved → secretion resumes
- Milking also decreases intramammary pressure and stimulates a prolactin surge
Practical implication: More frequent milking = less FIL accumulation = greater daily milk yield. 3× milking > 2× milking per day.
Milk Ejection (Let-Down)
- Oxytocin released from posterior pituitary in response to suckling or conditioned stimuli
- Oxytocin binds myoepithelial cells → contraction → milk pushed from alveoli into ducts → cisternal milk
- Stress inhibits let-down: epinephrine (adrenaline) causes vasoconstriction → reduces oxytocin delivery to mammary; also directly inhibits myoepithelial contraction
Lecture 31A: Mammal Evolution & Diversity
Lactation defines mammals — over 5,000 species share this trait
Three Mammal Subclasses
- Egg-laying mammals
- No nipples — milk secreted through pores in skin (modified sweat glands)
- Neonate laps milk from the mother's fur/skin
- Distribution: Australia and New Guinea
Species:
- Duck-billed platypus (Ornithorhynchus anatinus)
- Spiny anteater / Echidna (Tachyglossus)
- Very short gestation → premature birth
- Neonate (joey) crawls to pouch (marsupium) immediately after birth
- Attaches to nipple in pouch; completes development while nursing
- Milk composition changes progressively as joey develops
Species:
- Tammar wallaby (most-studied for lactation)
- Kangaroo
- Koala
- Opossum
- Full gestation with functional placenta
- Neonates born relatively more developed than marsupials
- Singles or multiples (litters)
- Most diverse and numerous mammal group
Examples:
- Cattle, swine, horses, humans
- All domestic livestock
- Rodents, carnivores, primates
Neonate Dependence on Milk
- In utero: fetus completely dependent on maternal blood (via placenta) for all nutrients, oxygen, and waste removal
- At birth: neonate is structurally and functionally immature → dependent on milk
- Milk provides: complete nutrition, passive immunity (colostrum IgG), growth factors, and bioactive compounds
Dairy Mammals
Species from which humans harvest milk for food:
| Species | Products / Notes |
|---|---|
| Cow (dairy breeds) | Fluid milk, cheese, butter, yogurt; high-producing Holsteins ~5,600 gallons/year |
| Goat | Goat milk and cheese; small-scale and artisan production |
| Sheep | Roquefort, pecorino, manchego; high-fat, high-protein milk |
| Water buffalo | Mozzarella di bufala; higher fat than cow milk |
| Camel | Important in arid/semi-arid regions (Middle East, N. Africa); high vitamin C |
| Mare (horse) | Koumiss (fermented); low fat, high lactose; Central Asian tradition |
Lecture 31B: Milk Biology
Composition and physicochemical properties of milk
Milk Composition
Major components of bovine milk (approximate values):
| Component | % in Bovine Milk | Notes |
|---|---|---|
| Water | ~87.5% | Solvent; most abundant component |
| Fat | ~3.5–4.5% | Triglycerides in fat globules; energy-dense; varies with breed/diet |
| Protein | ~3.2–3.5% | Casein (~80%) + Whey (~20%); essential amino acids |
| Lactose | ~4.7–4.9% | Milk sugar; disaccharide (glucose + galactose); energy source |
| Minerals | ~0.7% | Ca, P, K, Na, Mg, Cl — important for neonate bone development |
| Vitamins | trace | Fat-soluble (A, D, E, K) and water-soluble (B vitamins); low in Vit C |
| Immune factors | trace (high in colostrum) | IgG, IgA, IgM; lactoferrin; lysozyme; leukocytes |
Physicochemical Properties
Milk is simultaneously an emulsion, a suspension, and a colloid.
Emulsion — Fat Globules in Aqueous Phase
- Fat globules (core: triglycerides) dispersed in the aqueous plasma phase
- Each globule is surrounded by the Milk Fat Globule Membrane (MFGM): phospholipids + glycoproteins
- Partially stable emulsion: without homogenization, cream (fat) rises to form cream layer (fat less dense than water)
- Homogenization mechanically reduces fat globule size → fully stable emulsion (no cream line)
Suspension — Leukocytes (Somatic Cells)
- White blood cells (leukocytes) are suspended in milk
- Somatic Cell Count (SCC): indicator of udder health and mastitis
- Normal SCC: <200,000 cells/mL; elevated SCC = infection/inflammation
Colloid — Casein Micelles in Serum/Whey Phase
- Casein micelles: large protein aggregates (~200 nm) of α-, β-, κ-casein + calcium phosphate
- κ-casein on surface: hydrophilic "hair" stabilizes micelle via steric and electrostatic repulsion
- Colloid = particles too large for true solution, too small to sediment by gravity
- Disruption of casein micelles → curd formation:
Acid coagulation
Low pH neutralizes κ-casein surface charge → micelles aggregate → soft acid curd (yogurt, cream cheese, cottage cheese)
Rennet (enzymatic) coagulation
Chymosin cleaves κ-casein → removes steric stabilization → micelles aggregate → firm curd (cheddar, mozzarella, Swiss)
Cheese industry: Curd is separated from whey (remaining liquid). Curd is processed into cheese; whey proteins recovered for whey protein products.