Introduction to Lactation

Introduction to Lactation

• Mammary glands are the defining characteristic of Mammalia
• Mammals feed young via lactation, which produces milk
• The mammary gland is part of the Reproductive System

Phases of Lactation

• Mammogenesis is mammary growth and development
• Lactogenesis is the initiation of milk secretion
• Galactopoiesis is the maintenance of milk secretion
• Involution involves anatomical and physiological changes to stop milking

Mammary Gland Development

• Mammary glands are specialized cutaneous glands starting in the embryonic period
• At birth, mammary glands are poorly developed with little difference between sexes
• Mammary glands are arranged as breasts (primates), udders (ruminants), or dugs (other animals)

Mammogenesis Stages

• Fetal life sees isometric growth, growing at the same rate as the rest of the body
• Puberty sees allometric growth with fat development
• Puberty to parturition involves milk-secreting tissue development
  • Fluctuations between estrogen (ductal development) and progesterone (secretory tissue development) occur

Mammary Gland Growth During Pregnancy

• Domestic animal species show increased mammary gland growth during pregnancy
• This growth is reliant on increased estrogen and progesterone plasma concentrations
• Prolactin from the pituitary increases late in pregnancy and is required for full mammary gland development

Mammary Gland Changes During Pregnancy and Lactation

• During puberty, ducts form in the mammary fat pad, followed by lobe formation
• In early pregnancy, ducts branch and elongate, with lobuli and alveoli forming
  • Estrogen primarily drives ductal growth, while progesterone drives secretory tissue development
• Continuous development of lobuli and alveoli continues throughout pregnancy and lactation
  • Alveoli fill with colostrum in late pregnancy
• During lactation, connective tissue is reduced, maximizing space for alveoli

Lactogenesis Definition

• Lactogenesis is a sequence of cellular changes
• It converts mammary epithelial cells from a nonsecretory to a secretory state near the end of pregnancy
• Successful lactation needs prepartum proliferation of alveolar epithelial cells, biochemical and structural differentiation of epithelial cells, and milk component synthesis and secretion

Hormonal Changes at Parturition

• In late pregnancy, high progesterone blocks prolactin receptor synthesis on mammary epithelial cells
• At parturition, progesterone decreases, and prolactin receptors are synthesized
• Prolactin stimulates α-lactalbumin, initiating milk secretion

Galactopoiesis Requirements

• Galactopoiesis needs alveolar cell preservation, high synthetic activity per cell, and an effective milk ejection reflex
• Mammary epithelial cells produce milk as long as suckling or milking continues
• Successful lactation needs regulated metabolism via hormones like glucocorticoids, growth hormone, prolactin, insulin, and glucagon

Hormonal Control of Milk Secretion

• Prolactin, secreted by the anterior pituitary gland, is key for lactogenesis
• Progesterone, secreted by the corpus luteum, is important for udder development during pregnancy
  • It also keeps parturition and lactogenesis in check
• Cortisol is the main glucocorticoid in cattle, which increases during the periparturient period and initiates lactogenesis
• Estrogen induces lactation during pregnancy (important for growth of duct system)
• Oxytocin, secreted by the posterior pituitary gland, enables milk release in response to suckling
• Growth hormones are essential for maintaining lactation, especially in cows

Lactation Inhibition

• In pigs, dogs, and cats, lactation inhibits gonadotropin-releasing hormone (GnRH) secretion
  • Follicle-stimulating hormone (FSH) is regulated by GnRH
  • This keeps FSH low, preventing ovulation
• In humans, horses, and dairy cows, the reproductive cycle often restarts while milk production is high

Involution Process

• Involution follows the cessation of milking
• It leads to structural and physiological changes in the mammary gland
• Secretory activity decreases due to increased pressure and the hormone Feedback Inhibitor of Lactation (FIL)
  • FIL is synthesized by mammary secretory cells and inhibits milk synthesis
• If milk is unremoved, FIL collects and stops further secretion, protecting the gland
• If milk is removed, FIL is also removed, and secretion restarts

Milk Ejection Reflex

• The milk ejection or let-down, is a neuroendocrine reflex
• Oxytocin is released from the posterior pituitary, triggered by tactile stimulation of the teat or conditioned responses
• Tactile stimulation occurs via suckling or mechanical stimulation
• Milk letdown requires continuous teat stimulation for maximum alveolar milk removal
  • Oxytocin has a short half-life so it must be maintained throughout

Milk Ejection Mechanism

• Suckling initiates the milk ejection mechanism
• Sensory neurons in the teat send impulses to the hypothalamus
• Nerves in the paraventricular nuclei stimulate the posterior pituitary to release oxytocin
• Oxytocin travels through the blood to the mammary gland
• Oxytocin targets myoepithelial cells surrounding the alveoli
• Myoepithelial cell contraction squeezes milk out of alveoli into ducts and the gland cistern

Colostrum Contents and Importance

• Colostrum is secreted in the first days post-parturition, synthesized in the mammary gland days before parturition
• It is rich in nutrients for the newborn
• Compared to later milk, colostrum has :
  • higher mineral salts, protein, fat, and vitamins
  • lower ash and lactose
  • higher Ca, Na, Mg, and chloride, but lower K
• Maternal antibodies are transferred exclusively via colostrum in newborn calves

Colostrum Composition

• High protein concentration due to immunoglobulins (Ig's) transferred from the mother's blood
• Ig's provide passive immunity against gut pathogens
• Intestinal absorption of Ig's peaks in the first 6 hours after birth
  • After 24 hours, normal protein digestion occurs, and antibodies can be absorbed without being digested
• Colostrum has higher carotene levels (intense yellow color)

Colostrum in Farm Animals

• Colostrum consumption is crucial for survival in farm animal neonates
• The predominant immunoglobulin is IgG
  • IgM and IgA provide systemic and gut-associated immunity
• Dogs and cats acquire immunoglobulins in utero and via colostrum
• In humans, monkeys, and rabbits, immunoglobulin transfer occurs across the hemochorial placenta

Regular Milk Components

• Milk fat, lactose (milk's sugar), and some proteins are synthesized in mammary epithelial cells (MEC) from precursors absorbed from blood
• These components are released by apocrine, merocrine, or holocrine secretion
• The cells control milk composition by adjusting water, vitamins, and mineral levels, which primarily enter through diffusion
• Milk composition varies between species, based on the neonate's needs
• Breed, environmental factors, stress, and neonatal nursing habits can affect composition

Lactation Curve

• After parturition, milk production rapidly increases to peak yield
• Then, the daily yield gradually decreases until the end of lactation
• The curve is similar across species and differing in: peak height, persistency, and lactation length
  • Milk composition changes throughout:
    • fat and protein % gradually increases
    • fat and protein% are lowest at peak lactation
    • lactose decreases
    • mineral concentrations increases slightly

Udder Structure

• The udder is split into four quarters, each with a teat, which is closed by a sphincter muscle called the streak canal
• A lactating dairy cow's empty udder weighs 10-30 kg (22-66 lbs), depending on breed and lactation stage
• High-yielding dairy breeds can have a total milk storage capacity exceeding 25-30 L (6.6-7.9 gallons)
• Udder size continues to grow until the cow is 6 months old

Blood Supply

• An extensive vascular system is necessary for milk synthesis
• A liter of milk requires 400-500 liters of blood passing through the udder
• In cows, mammary blood flow can be 20% of cardiac output
  • There is a constant ratio between mammary blood flow and milk production
• Blood enters through external pudic and perineal arteries
• Blood exits through: external pudic veins or subcutaneous abdominal veins(milk veins)
• Blood flow increases 2-6 fold in mammary gland 2-3 days prepartum
• Decreased production with lactation advancement is due to loss of secretory cells via apoptosis, not decreased blood flow

Blood Flow in the Udder

• Main arteries divide into cranial and caudal mammary arteries to supply blood to glandular and connective tissues
• Venous blood returns to the heart via subcutaneous veins along the belly
  • These are known as milk veins
• Milk veins empty into the cranial vena cava in the thoracic cavity

Lymphatic System

• The lymphatic system balances fluid and fights infection
• Fluid drains away from the udder, emptying into the blood system
• Fluid enters through open-ended vessels called lacteals
• It passes through superficial inguinal lymph nodes
  • These nodes enlarge during udder inflammation
• During lactation, 1-2 L of lymph returns for each liter of milk produced

Streak Canal

• The streak canal is the main barrier against intramammary infection
• It is kept closed by a sphincter muscle
• When milking, the sphincter muscles relax, which opens the orifice
• The canal stays open an hour afterward, where it is essential to prevent bacteria from contaminating the teat
• Cells lining the canal contain keratin with a sealant effect

Fürstenburg's Rosette

• The rosette is located directly above the canal
• It is made of loose membrane folds that smooth out as milk fills the udder
• These folds aid outflow blocking between milkings
• It is important for detecting bacterial infection and triggering an immune response