Breast and lactation

Question 1

Structure of the breast 1

Answer 1

• Alveoli: fundamental units of lactating breast, synthesize and store milk
• Alveoli drain milk into lactiferous ducts-> lactiferous sinus-> nipple
• Alveoli are arranged in lobules (glands)
• Most of breast growth during puberty is due to fat accumulation, growth during pregnancy is due to glandular growth
• E2 is essential for growth of glands, but inhibits lactation

Question 2

Structure of the breast 2

Answer 2

• Lactiferous ducts are lined by stratified cuboidal epithelium and are very large
• Alveoli are much smaller and are also lined by simple cuboidal/columnar epithelium
• Alveoli are surrounded by myoepithelial cells that contract to eject the milk
• TDLU (terminal ductal lobule unit) is characteristic of most non-lactating mammary glands

Question 3

Breast growth at different ages 1

Answer 3

• Prepubertal: non-functioning gland composed of lactiferous ducts and sinuses
• Puberty: breast development is stimulated primarily by E2, leading to accumulation of fat and CT
• Mammary ducts lengthen, branch, enlarge
• Resting adult: E+P exposure leads to lobule development but few alveoli (blind ended ducts: TLDU)
• Myoepithelial cells become densely packed
• Pregnancy: ducts and alveoli develop, alveoli and ducts expand into the CT space (little CT and lots of alveoli/ducts)

Question 4

Breast growth at different ages 2

Answer 4

• E+P causes expansion of ducts and alveoli, E also stimulates release of prolactin
• However E prevents milk production, thus preventing lactation before parturition
• Prolactin secretion is increased, stimulated by E2, and increases estrogen receptors
• Micro: there is expansion of alveoli, little CT, but no secretory product w/in the alveoli

Question 5

Control of lactation 1

Answer 5

• Lactation begins when E+P levels fall at time of parturition
• When lactation is occurring the mammary glands enlarge w/ milk
• The alveoli become dilated and resemble thyroid gland
• Alveolar epithelial cells synthesize milk proteins (casein) in RER and synthesize fat in SER
• Fat droplet remains surrounded by membrane so it can remain in solution
• In response to suckling myoepithelial cells contract
• Also in response to suckling oxytocin and prolactin are released from pit

Question 6

Control of lactation 2

Answer 6

• Oxytocin stimulates milk-let down (contraction of myoepithelial cells)
• Oxytocin is also stimulated from other stimuli, like baby’s cry or vaginal/cervical stimulation
• Prolactin stimulates further milk production (for next meal)
• Both oxytocin and prolactin release are reflex responses to suckling
• Prolactin is under negative control from DA, thus DA released is inhibited upon suckling for prolactin levels to increase

Question 7

Control of lactation 3

Answer 7

• Prolactin secretion depends on the strength, duration and frequency of suckling
• Lactation: regulated by supply/demand, more suckling increases production and infrequent suckling decreases production
• E inhibits lactation (women who are breastfeeding should not be taking OCPs), but women will resume menstrual cycle while breastfeeding (too little E to inhibit lactation)
• Inhibition of milk let down: psychological and physiological stress
• Breast feeding provides a limited, localized immune protection to the infant via IgA in milk
• Colostrum (higher protein, lower sugar) is produced for the first 2-3 days, after which mature milk is produced

Question 8

Lactational amenorrhea

Answer 8

• High levels of prolactin inhibit FSH and LH, thus many women remain amenorrheic during first few mos postpartum
• Since lactation amenorrhea is dependent on prolactin, it is influenced by suckling frequency
• Most women resume menses w/in 12 mo, non-breastfeeding women resume menses earlier