LACTATION

Characteristics that define mammals

• possession of hair and mammary glands

• Endothermic (warm-blooded)

• Breathe air with lungs

• Give birth to live young

Protheria

• monotremes

• Egg-laying mammals

• They lack nipples; milk is excreted from ducts onto the skin like sweat

Metatheria

• marsupials

• Give “premature birth”

• The young are born highly altricial and crawl into a pouch to finish developing

Eutheria

• placentals

• Have a complex placenta; the fetus is completely dependent on mother’s blood in utero

• This is the largest group

Characteristics of mammalian newborns that necessitate lactation

• they are altricial (structurally and functionally immature)

• They have immature digestive, metabolic, and immune system

• They cannot find their own food and need a sterile, warm, high-energy source

What does milk provide to neonates?

• water (hydration)

• Energy (fats/sugars)

• Protein (growth)

• Minerals/vitamins

• Protective factors (immunity/growth factors)

Major components of milk

Most variable: fat percentage (varies wildly by species, breed, and stage)

Least variable: lactose (remains steady to maintain osmotic pressure)

Emulsion

Fat globules floating in plasma (oil in water)

Suspension

Large particles like leukocytes/cells in the plasma

Colloid

Casein micelles (protein) in the serum/whey

Solution

Water-soluble items (lactose, vitamins, minerals) dissolved in water

Casein

Structure: casein proteins organized into a “ball” containing Calcium (Ca2+) and Phosphate (PO4)

Disruption: disrupted by enzymes (like rennin) or acid, which causes the micelle to collapse and form a curd (the basis of cheese)

Importance of immunoglobulins in colostrum

• provides passive immunity

• Given so soon after brith because the neonates gut stops absorbing large Ig proteins after 24-36 hours post-birth

Define dairy animal

An animal specifically used to harvest milk for human consumption

Supporting structures of the udder

Median suspensory ligament: the heavy-lifter; attaches the udder to the abdomen

Lateral suspensory ligament: attaches to tendons near the pelvis

Skin: provides very minimal support

Gross anatomy

Teat, streak canal (the only sphincter), teat cistern, gland cistern

Microscopic anatomy

Alveoli are the basic units where milk is secreted and they are surrounded by myoepithelial cells (which squeeze out milk)

Lymph system

Function: drains colorless fluid from tissue spaces, surveys for pathogens, and returns fluid to the heart

Anatomy: lymph capillaries drain into supramammary lymph nodes (which swell during mastitis/infection)

Synthesis and transfer of milk components

• lactose: made in the golgi from glucose/galactose

• Fat: made in the smooth ER (triglycerides)

• Proteins: made in the rough ER (casein and whey)

• Minerals/water: transported from blood into the lumen

Glucose transformation (mother to neonate)

Mothers blood: glucose → mammary cells in the: glucose converted to galactose → glucose + galactose = Lactose → Neonate’s gut: lactase breaks lactose back into glucose + galactose for energy

Two classes of milk protein

Casein: contains bioavailable calcium and phosphate (found in the micelle)

Whey proteins: lactalbumin, lactoglobulin, albumin, immunoglobulins

Protective factors in milk

• cells: leukocytes (fight infection)

• Proteins: lactoferrin, lysozyme (antimicrobial), and Ig (antibodies)

• Growth factors: promote intestinal development in the baby

Why do reproductive hormones control lactation?

Lactation is part of the reproductive strategy. The mammary gland must be ready exactly when the baby is born

• mammogenesis: growth of the gland

• Lactogenesis: initiation of secretion

• Galactopoiesis: maintenance of secretion

Developmental phases and cell interactions

• fetal: formation of the “milk line”

• Prepubertal: minimal growth (mostly fat pad)

• Postpubertal: estrous cycles cause duct elongation (estrogen) and maturation (progesterone)

• Pregnancy: the greatest increase in mass (exponential growth). Driven by simultaneously high estrogen and progesterone

• Crucial interaction: interaction between the fat pad (stroma) and the epithelial cells (parenchyma) is required for growth

Hormones post-puberty

Estrogen (ducts) and progesterone (maturation)

Hormones during pregnancy

Estrogen and progesterone (lobulo-alveolar growth) and prolactin and GH

Stages of lactogenesis

Stage 1: cytological/functional differentiation of cells (before birth)

Stage 2: full secretion of all components (0-4 days before/after birth)

Hormones: progesterone (inhibits); estrogen, glucocorticoids, and prolactin (stimulate)

Galactopoiesis (maintenance)

• hormones: prolactin (critical for non-ruminants) and growth hormone/bST (critical for ruminants)

• BST mechanism: indirectly partitions nutrients to the gland and stimulates IGF-1 from the liver

• Regulatory factor: FIL (feedback inhibitor of lactation). If milk isn’t removed, FIL accumulates and stops production

Inducing lactation in non-pregnant females

Injecting a regime of estrogen and progesterone (to mimic pregnancy/mammogenesis), followed by glucocorticoids (to trigger lactogenesis)

Milk ejection reflex (letdown)

• neuro-endocrine reflex: pressure on teats → nerve impulse to brain → oxytocin releases from posterior pituitary → myoepithelial cells contract → milk ejected

• Time: takes about 30-60 seconds

Mastitis

• inflammation of the gland due to injury or bacteria

• Measured by somatic cell count (an increase in leukocytes (75%) and epithelial cells (25%) in the milk indicates infection)

Mammary involution

The degeneration of alveoli and ducts when milk is no longer removed (becoming “dry”)

• Induced by: cessation of milking/suckling. This allows the gland to rest and repair before the next lactation (cow needs a 45-60 day “dry period”).