lactation and its control Flashcards
what pH is milk
6.6-6.9
which vitamins and minerals are contained in milk
- Na
- K
- Cl
- HCO3
- Ca
- Mg
- HPO4
- SO4
- citrate
outline how milk composition changes throughout lactation
explain what milk cell count means
- milk containes leukocytes and dislodged mammary epithelial cells
- in healthy cows, 30000-300000 cells/ml is normal
- too low = increased risk of mastitis
- during inflammation the number of cells will increase 10-100 fold due to neutrofil invasion
- cell density is a parameter of milk qulity
explain milk production regulation
- regulated by number of offspring
- or amount emptied from udder in production
- in production animals, udder is emptied 2-3 times per day to prevent milk inhibition (body will down regulate amount of milk produced so isnt wasting)
how is milk made
- epithelial cells synthesise the majority of milk components
- high number of mitochondria
- mitochondria synthesise fatty acids and non-essential amino acids
- rough ER synthesises secretory proteins
- smooth ER syntesises phospholipids and triglycerides
- protein is secreted in the form of granules by exocytosis (merocrine or eccrine secretion)
- lactose is secreted with protein
- lipid is secreted in the form of membrane limited lipid droplets by incorporation into the cell membrane. may contain elements from the cytoplasm
- water follows the lactose by osmotic pressure
- triglycerides is made from fatty acids in chylomicrons and LDL in the blood
- and from glucose or acetat/beta hydroxybuterate in ruminants
- pathway of fatty acid synthesis in mammary gland are the same as in other tissues
discuss how metabolism changes during lactation
- during pregnancy and lactation, changes in metabolism occur in many tissues
- coordinate re-direction of metabolic resources
- Rheostasis =moving set point of homeostasis
- reduced fat synthesis and increased lipolysis
- decreased uptake of glucose in muscle and adipose tissue - increased mammary uptake
- increased uptake of amino acids in the mammary glands (breakdown of muscle protein is not sufficient)
- these changes are regulated by prolactin, growth hormone and insulin
what hormonal switches occur at parturition that induce lactation
progesterone domination
- laying down of fat
- increases arachidonic acid stores
- inhibits prolactin
oestrogen domination
- conversion of arachidonic acid to pgf2a
- myometrium expresses oxytocin receptors
- does not block prolactin = lactation starts
how is milk secretion initiated
- increase in udder volume during pregnancy follows increase in oestrogen and placental lactogen (mimic prolactin)
- onset of milk secretion (lactogenesis) conicindes with drop in circulatin progesterone and rise of cortisol at arturition
what hormones affect galactopoesis
- prolactin
- oestrogen
- progesterone
- cortisol
density of prolactin receptors is regulated by thyroid, adrenal and ovarian hormones
- prolactin regulates the production of a-lactabumin
- progesterone competes with prolactin
- cortisol is necessary for growth and differentiation in the golgi which is where lactose is synthesised
glactopoesis = maintenance of milk secretion
how does a-lactalbumin control lactogenesis
- the enzyme galactosyltransferase binds a-lactalbumin to become lactose synthase
- progesterone inhibits synthesis of a-lactalbumin therefore inhibits production of lactose
- prolactin induces synthesis of a-lactalbumin and therefore stimulates production of lactose
milk secretion is maintained by
- regular suckling
- secretion depends on prolactin (except in cattle and goats where GH is more important)
- release depends on oxytocin
- insulin, growth hormones and thyroid hormones regulate metabolism and ensure adequate energy supply to the udder
prolactin:
sucking stimulus –> paraventricular neclues –> reduction in dopamine releases inhibition on prolactin release from adenohypophysis
increased vasoactive intestinal polypeptide stimulates prolactin release
tactile stimulation of mammary causes prolactin release
only requires 2-3x suckling for maintenance of prolactin
milk secretion is reduced by:
- lack of suckling (reduces prolactin release)
- accumulation of milk in alveoli increases concentrations of feedback inhibitor of lactation
how is oxytocin involved in lactation
stimulates milk let down
- neuroendocrine reflex to induce emptying of the mammary gland (milk forced from alveoli into the teat)
- requires sensory activation, neural activation, oxytocin release, contraction of alveoli and ducts, mechanical transfer of milk
Nerves in the paraventricular nuclei are stimulated by afferent neurons and terminals of the neurohypophysis which releases oxytocin (3)
Oxytocin:
Increase pressure in the alveoli.
Reduce resistance in the small excretory ducts (myoepithelial cells are oriented along the long axis-ducts become shorter and wider).
Reduce resistance in the teat canal.
Resulting in milk flow to teat cistern.
what stimualtes myoepithelial cells and what is their function
Myoepithelial cells function to squeeze milk out of alveoli into the ducts. They contract in response to oxytocin.
Express oxytocin receptors which have been induced by E2.
how does lactation effect reproduction
lactation reduces gonadotrophin secretion, inducing lactational anoestrus
- suckling induces opioid production which blocks GnRH secretion, preventing LH surge (ovulation)
- weaning removes suckling stimulus, prolactin is reduces and LH increases
