Lactation

Mammary gland development Cyclic changes in ovarian steroid hormones –Essential for mammary development Estradiol –Development of mammary alveoli (secretory unit) Progesterone –Development of mammary ducts

Final stage of mammary gland development –Pregnancy Ovarian/placental steroid hormones –Periparturient period GH and prolactin –Placental lactogen? Glucocorticoids

Lactation Parturition –Metabolic shift Fetal growth to synthesis and secretion of milk –Mobilization of nutrients Storage within the body –Fats Triggered by changes in hormones

Lactation and re-initiation of reproductive cycle Lactating women –High prolactin –Low LH and estradiol No ovulation –Lactation-induced anovulation/amenorrhea (infertility) –Women remain anovulatory as long as they breast-feed their babies Increased survivability of the infants

Lactational anestrus in the domestic species –Divert nutrients for lactation Ensure the survival of offspring –Lactation stimuli/physical contact between mother and offspring –Animals remain in anestrus (absence of estrus) until nutrient intake exceeds nutrient demand

Lactation Critical for survival of newborn –Nutrients –Immune system Passive immunity

Milk composition

–Varies greatly among species Frequency of nursing –Increased frequency, lesser the concentrations of fats –Derived via synthesis or filtration Alveolar epithelial cells Components of blood

Changes in milk composition during lactation –Colostrum First week of lactation –High concentrations of proteins –Immunoglobulins –Low in fats and water-soluble vitamins –Concentrations of proteins Decrease as lactation progresses –Increased fat and lactose content

Milk fat –Major source of energy Highly digestible Carrier of fat-soluble vitamins –Synthesized by the smooth ER of alveolar epithelium

Lactose –Promotion of bacterial growth in intestine –Provides component for myelin sheath in neurons –Synthesized by the Golgi apparatus

Milk ejection Process –Transport of milk from alveoli to ducts Nipple or teats –Milk ejection reflex Central nervous system –Regulation of prolactin and oxytocin release

Function of oxytocin Lactation –Critical for milk let-down Oxytocin receptors –Grandular cells in the mammary alveoli –Myoepithelial layers in the mammary ducts Contraction of myoepithelial layer –Secretion stimulated by suckling Tactile response Regulated by the CNS

Suckling stimuli –Increased release of oxytocin Increased synthesis Release of oxytocin –Conditioned Response to factors other than actual suckling stimuli

Inhibition of milk ejection reflex –Stress Physical Psychological –Cause Inhibition of oxytocin release Release of catecholamines –Constriction of mammary gland blood supply

Ejection of milk Combination of factors –Pressure within the nipple/teat from accumulating milk Contraction of alveoli by oxytocin –Expression of milk by the infant Stripping of milk from teat/nipple Aided by suckling

Inhibition of lactation Medical reason –Transmission of infectious agent HIV –Unnecessary Stillbirth Abortion after the first trimester Social issues –Returning to the workforce

Inhibition of lactation –Inhibition of periparturient prolactin surge Dopamine receptor agonists

Termination of lactation Loss of suckling stimuli –Distension of alveoli and ducts Mechanical atrophy Physical destruction of alveolar epithelium Loss of blood supply –Compression of vessels –Hypoxia and ischemia –Loss of alveolar epithelium reduction of alveolar size

Termination of lactation Loss of suckling stimuli –Distension of alveoli and ducts –Loss of alveolar epithelium –Reduction of duct size Reduced lumen diameter Involution of mammary gland –3 months after cessation of lactation