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Flashcards in lecture 18: lactation Deck (30):
1

What is lactation?

  • primary source of nutrition and energy for newborn mammal 
  • immune protection 
  • mammary glands change - hormones and development 
  • variation between species: e.g. 
    • number of glands: human 2; wallaby 4; pig 18; cow 4 (apposed in single udder) 
  • in a way lactation is the thing that characterises mammals 

2

What is an example of variation in milk composition between species?

  • human:
    • water = 90 g/100mL 
    • protein = 1.1 g/100mL 
    • carbohydrate = 7.5 g/100mL
    • lipid = 4.2 g/100mL
    • energy (MJ/L) - 3.3
  • elephant seal:
    • water = 35
    • protein = 10 
    • carbohydrate = 2 
    • lipid = 55 
    • energy = 23
  • elephant seals require more energy from the mother in order to survive the cold 
  • the pups have to develop a thick layer of insulating blubber very quickly 
  • dramatic differences between species that relate to environmental constraints and all sorts of other things

3

What is the structure of a mammary lobule? 

  • e.g. cluster of alveoli in the goat 
    • capillaries 
    • venule 
    • arteriole 
    • myoepithelial cells 
    • alveolar epithelial cells
    • alveolus 
    • milk
    • milk duct 
  • want to maximise surface area so there are a number of epithelial cells that can produce secretions → package into alveoli that are interconnected by milk ducts 
  • these cells are very metabolically active 
    • some high yielding dairy cows might produce two udders worth of milk a day → a lot of milk 
  • fantastic vascular system 
  • capillary beds surrounding each of the alveoli 
  • huge amount of opportunity for exchange of nutrients between blood and epithelial cells 
  • milk ducts coalesce in different lobules → down towards teat 

4

What is the structure of mammary lobules and alveoli?

  • note vascular supply around alveolus 
  • myoepithelial cells surrounding alveolus contract, increasing intra-mammary pressure at milk let-down 
  • not just alveoli and blood vessels
  • connective tissue 
  • ducts 
  • adipose tissue → fat → most of human mammary tissue 
  • pregnant women → mammary glands get bigger but the amount of fat probably stays the same 
  • alveolus
    • secretory epithelial cells 
    • surrounded by a cluster of cells called myoepithelial cells → flat, squamous sort of cells, muscle-like characteristics, respond to oxytocin
    • when you fill your alveolus with milk oxytocin causes contraction of myoepithelial cells → increases the intraalveolar pressure which squishes the milk out 
    • highly specialised epithelium
    • basement membrane → gives structural support 
    • specialised junctions between epithelial cells 
    • milk is not just things from the blood oozing through leaky membrane   

5

What are duct systems in different species?

  • rat 
    • alveoli 
    • lactiferous ducts 
    • galactophore 
    • nipple
  • rabbit 
    • galactophore → multiple milk ducts ending on the teat 
    • nipple
  • woman 
    • lactiferious sinus
    • areola 
    • nippe
    • 15 - 20 mammary lobes dilating as lactiferous sinus emerging at nipple 
  • ruminant 
    • galactophores
    • gland cistern → can get a lot of milk out very quickly
    • teat cistern 
    • teat canal

6

How do mammary glands develop in the foetal mouse? 

  • normal female → development of gland, nipple etc 
  • ovariectomised female → mammary glands still differentiate therefore don't need ovarian hormones to develop mammary gland 
  • normal male → remnant of mammary gland, epidermis shows no sign of nipple formation 
  • castrated male → development of mammary glands 
  • androgens cause regression of the mammry primordia 

7

What is postnatal mammary growth in rat?

  • pre-pubertal
    • atrophic ducts and very little mammary development 
    • oestrogen + growth hormone → adrenal steroids → 
  • pubertal
    • duct growth 
    • oestrogen + progesterone + prolactin + growth hormone + adrenal steroids 
  • post-pubertal
    • lobulo-alveolar growth
    • cyclic changes through oestrous/menstrual cycles 
    • prolactin + adrenal steroids →
  • late-pregnancy → lactation 
    • milk secretion 
  • oestrogen, progesterone, cortisol, growth hormone, placental lactogen and prolactin needed for mammary growth during first pregnancy 

8

What is the mammary gland cycle?

  • virgin → pregnancy → lactation → involution → pregnancy etc
  • duct structures in virgin female have little development
  • pregnancy → development
  • lactation → secretory etc
  • involution → milk production shuts down after relatively shuts down 

9

Summary of structure and development of the mammary gland?

  • structure of mammary gland – variations on a theme
    • ducts and cisterns (cisterns can offload a lot of milk in a short time, helpful when you feed infrequently)
    • alveoli open into milk ducts
    • ducts join and empty via nipple or teats
    • galactophores are common ducts leading to teat: rabbits 6–8, man 15-20; agile wallaby approx. 20 
    • supernumerary teats
  • foetal development 
    • present in both sexes but poorly developed in men
    • regress in male mice and rats as a result of androgens 
  • development after birth 
    • full growth not achieved until end of puberty or in early lactation 

10

How does hormonal regulation of milk secretion differ between species?

  • to maintain high milk production after removal of the pituitary gland: 
    • rats need: prolactin, Adrenocorticotrophin (ACTH)
    • sheep and goats need: growth hormone, prolactin, ACTH, thyroptrophin 
    • rabbits need: prolactin 
  • ACTH maintains cortisol secretion from the adrenal gland 
  • TSH maintains thyroid hormone secretion from the thyroid gland 

11

What is the hormonal control of milk secretion?

  • mammary gland transplanted to the neck of a goat (Jim Linzell's experiment) 
    • separated from nerve supply 
    • continued milk secretion 
  • control is hormonal, not neural
  • however milk removal necessary for continued lactation (Malcolm Peaker)  

12

What is the composition of breast milk at diffrent time points?

  • colostrum (onset of lactation)
    • high protein 
    • rich in immunoglobulins
  • mature milk 
    • 7% lactose 
    • 4% fat
    • 1% protein 
    • + minerals, vitamins etc 
  • two 25 mL samples of human breast milk
    • foremilk: the watery milk coming from a full breast 
    • hindmilk: the creamy milk coming from a nearly empty breast
  • late pregnancy
    • high levels of progesterone, oestrogens
    • prolactin, hPL (hPL drops off dramatically at birth) 
  • milk secretion turns on after birth, maybe a small amount before 
  • prolactin stays high during lactation
  • sensory stimuli through teats 

13

What are the hormones involved in pregnancy and peripartum in regards to lactation?

  • pregnancy → mammary gland development
    • high progesterone and oestrogens 
    • hPL (hCS) 
    • prolactin 
  • peripartum → transition to lactogenesis 
    • fall in progesterone and oestrogens 
    • no hPL 
    • slow fall in PRL → needs nipple stimulation (suckling) to maintain PRL and milk production 

14

What is local regulation of lactation?

  • FIL - feedback inhibitor of lactation 
  • small protein secreted in alveolus 
  • if not removed by emptying alveolus, it acts to suppress further milk secretion 

15

What are mammary secretory processes?

  • exocytosis: proteins made via RER and Golgi; lactose 
  • apocrine secretion of lipids: vesicle membranes → phospholipids 
  • trans-membrane: water; small molecules; drugs 
  • trans-cytosis: immunoglobulins; some hormones and growth factors 
  • paracellular: immune cells; leakiness may be high in disease states increasing transfer of interstitial fluids 

16

What are the phases of lactation?

  • milk secretion 
    • initiation of milk secretion begins in pregnancy 
    • prolactin and other hormones 
    • synthesis of milk constituents within alveolar cells
    • intracellular transport of milk componenets 
    • discharge of constituents into alveolar lumen 
    • local control – FIL 
  • milk removal 
    • passive removal of milk from cisterns and large ducts 
    • stimuli 
      • sucking 
      • sights, smells, sounds, tactile stimuli associated with suckling 
    • reflex ejection of milk from alveoli ('let down' or 'draught') 
    • oxytocin 

17

What is the importance of sucking?

  • in milk secretion
  • sucking induced release of prolactin 
  • prolactin
    • VIP = vasoactive intenstinal peptide
    • potent Prl releasing factor 
  • dopamine from TIDA nerves
    • DA is a Prl release inhibitory factor 

18

What is the milk ejection reflex?

  • sucking-induced release of oxytocin 
  • hypothalamus-hypophyseal tract: increased firing rate after nipple or reproductive tract stimulation causes increased oxytocin secretion 

19

What is the effect of anticipation of milking on oxytocin concentrations in cows?

  • PM: preparation for milking 
  • MA: application of teat cups
  • S: stripping 
  • C: control level 

20

What regulates the response to milk ejection?

  • oxytocin receptors 

21

What is the Hottentot method of inducing milk let down?

  • Ferguson Reflex → oxytocin positive feedback in uterine contractions 

22

What is the hormonal control of lactation?

  • consists of milk secretion and milk removal stages 
  • maintenance of lactation usually requires several anterior pituitary hormones 
  • prolactin is critical in non-ruminants, and growth hormone in ruminants 
  • oxytocin essential for milk ejection 
  • milk removal essential for continued lactation 
  • responses mediated by receptors 

23

How does milk composition changes?

  • changes in peripartum period 
  • milk composition in women pre- and post-partum 
  • 0 indicates time of birth 

24

What are variances in milk composition?

  • colostrum: post-partum secretion high in protein, sodium and chloride
    • also antibodies (IgG and IgA)
  • milk: large species differences in concentrations of milk fat, lactose, protein and water 
  • milk fat mixture of lipids: triglycerides, diglycerides, monoglycerides, free fatty acids, phospholipids and sterols
  • arctic, aquatic, desert mammals produce milk with 75% energy in lipid fraction
  • frequent nursing species produce milk with lower nutrient density 

25

What is lactation in marsupials?

  • milk composition changes in marsupials
  • high in carbohydrates early 
  • middle levels of proteins and lipids 
  • ultimately high in lipids, v low in carbohydrates 
  • protein levels remain similar 

26

What is milk ejection in the agile wallaby?

  • concurrent asynchronous lactation 
  • difference in response to OT allows milk ejection in gland with PY continuously attached 
  • sucking by young at foot causes ME in both glands 

27

What is prolactin related suppression of reproductive cycle?

  • dopamine and agonists (bromocriptine) inhibit prolactin synthesis 
  • high levels of prolactin in lactation inhibits GnRH 
  • pituitary LH response to GnRH reduced, failure of positive feedback 
  • ovarian response to LH same 
  • naloxone (opiate inhibitor) also inhibits prolactin release (alternate pathway) 

28

What are the contraceptive effects of breastfeeding?

  • cumulative % probability of pregnacy 
    • non-lactating, no contraception → relatively high even immediately post partum 
    • lactating, no contraception 
      • low initially, increases
    • post partum, lactating + post menstrual contraception 
      • low with v moderate increase 
  • monthly % risk of pregnancy vs months of lactational amenorrhoea 
    • increases with time 
  • contraceptive effects of extended periods of lactational amenorrhoea 

29

What is the lactational control of reproduction?

  • prolactin release important in suppressing cyclic release of gonadotrophins 
  • endogenous opiates (beta endorphins) are also involved
  • frequency of suckling very important in lactational amenorrhoea 

30

summary 

  • nutrition, energy, water, immune protection for newborn 
  • mammary gland has multiple lobules 
    • alveoli → secretory epithelium 
    • myoepithelial cells → OT and milk ejection 
    • well vascularized 
  • development induced by hormones - prolactin, Prog, E2 etc 
  • placental lactogens in pregnancy 
  • initially colostrum (immunoglobulins) → mature milk
  • oxytocin and Fergusson reflex + CNS 
  • lactation control of reproductive cycles - role of endorphins 
    • anoestrus 
    • diapause