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

Components Human CowWater 88.5 % 87 %Fat 3.3 % 3.5 %Lactose 6.8 % 4.8 %Casein 0.9 % 2.7 %Other proteins 0.4 % 0.7 %Minerals 0.2 % 0.7 %

• 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