(6&1/3) Maintenance of Lactation (Galactopoiesis)

Question 1

(1)

(Non-Ruminants)

1. In lab animals the minimum requirements for continued lactation are what?
2. Glucocorticoids regulate the activity of several enzymes in the milk synthesis pathway by doing what?

Adrenalectomy causes what change in milk production?

3. Prolactin acts in a general fashion to maintain enzyme levels and protein synthesis.

It increases gene transcription rates and the half-lives of the resulting mRNAs

The effect of prolactin on some enzymes is enhanced by what?

While in other their effects appear to be what?

4. prolactin also functions to prevent what?

and maintain what?

Answer 1

1. prolactin, corticosteroids and oxytocin
2. controlling their transcription rates

40-50% decrease (not simply due to reduction in the enzyme levels as none become rate limiting)

(There are complex interactions between the glucocorticoids and prolactin.)

3. glucocorticoids (here the two hormones are synergystic)

only additive

4. cell death (apoptosis) of mammary epithelium

maintains tight junctions between cells

Question 2

(2)

(Ruminants)

Ruminants are the most widely studied species outside of lab rodents and there are several differences in their physiology.

1. There is no (or very minimal) effect of adrenalectomy on maintenance of lactation. It does not reduce milk production or enzyme levels, thus there is no requirement for what?
2. There IS a requirement for what in the maintenance of ruminant lactation?

has been shown to maintain what?

It also causes greater rates of what?

Note that growth hormone does not just act directly on the mammary gland (that has relatively few receptors for it), but also by increasing what?

3. The importance of prolactin for maintenance of lactation is more controversial. Up till recently evidence suggested it was not important, but studies with more specific and potent inhibitors have shown roles in what?

Answer 2

1. glucocorticoids
2. somatotropin (growth hormone)

mammary secretory cell numbers (major reason that bST treatment of dairy cows gives increased persistency of lactation (it doesn’t decline as fast))

milk synthesis

IFG production primarily in the liver (which then circulates to the mammary gland to give the effects.)

3. epi cell proliferation, function, and prevention of apoptosis

Question 3

(3)

(Ruminants and Non-ruminants)

1. what is required for removal of milk from the alveolus?
2. In its absence the mammary glands do what?
3. so what is necessary?

Answer 3

1 Oxytocin

2. degenerate/involute due to milk accumulation
3. repeated suckling/milking

Question 4

(Local control of milk synthesis)

Increasing the frequency of milk removal increases milk production while decreasing milking frequency reduces production. This is seen in several species, including women.
In dairy cows, increasing milking frequency from 2x to 3x daily increases production ~ 10%. Increasing to 6x daily gave a 21% increase. Conversely, reducing frequency from 2x to 1x daily reduced yield by up to 20%.
In cows there is both this acute effect but also a sustained effect: you see increased production throughout lactation even if you only milk 4x daily in the first 3 weeks then go back to 2x daily. There appears to be a 3-week window at the beginning of lactation where there is plasticity for setting potential milk production for the lactation, and you “use it or lose it” based on how much you demand from the gland.

1. The controls for these local effects are being worked out, the sustained one appears to involve changes in what?
2. For the more acute changes there appear to be several factors that act in concert to exert local control of milk production, including what?
3. FIL is an autocrine regulator: it acts to inhibit the same cell that secretes it. FIL appears to bind to a receptor on the apical plasma membrane and has immediate effects to inhibit what?

Answer 4

1. genetic imprintin (ie it is an epigenetic effect)
2. “Feedback inhibitor of lactation” (FIL), casein fragments, and serotonin among others.
3. protein and lactose secretion

Question 5

(5)

(Milk Ejection (Let-Down))

1. This is the process whereby milk that has been synthesized and stored in the mammary alveoli is released by suckling young or the milker.
2. Milk is secreted into the alveoli relatively continuously from the epithelium. It is stored within the lumen of the alveolus and in expansions of the duct system in species where these exist. The duct storage system is large in ruminants (gland cistern) but still only stores a minority of milk in the udder. Species such as the rat and rabbit have no such storage structures, whereas it exists but is much smaller in women (sinuses). While milk within the storage ducts may be available passively to the suckling offspring, or in the case of cows, to the milking machine; the majority of milk is stored within the alveoli and smaller ducts. Here the effect of surface tension means the milk cannot be removed solely by suckling, access to this larger store requires that it is actively ejected. In a cow immediately after milking there is essentially no milk stored in the cistern. The fraction stored here gradually increases and when the udder is full (~12 hours from last milking), about 20% of milk is passively available (stored in the cisterns and larger ducts) but the remaining 80% is in the alveoli and requires active ejection. The cisternal fraction is also larger during peak lactation than it is during late lactation. The size of the cistern gradually increases with lactation number so there is a greater fraction of milk passively available in older cows, but still well under 30%.
3. millk ejection is controlled by what?
4. nipple is densely innervated with what?
5. These synapse with nerves in the spinal cord which pass up the cord and after going through several synaptic relays the stimulus arrives at the what?

Answer 5

3. neuroendocrine reflex
4. intradermal sensory afferent nerves
5. hypothalamus

Question 6

(6)

(Milk Ejection (Let-Down)

These synapse with nerves in the spinal cord which pass up the cord and after going through several synaptic relays the stimulus arrives at the hypothalamus.

1. Here the information passes to the paraventricular and supraoptic nuclei, which contain what?

2 Oxytocin circulates in the blood and causes contraction of what?

which does what?

Answer 6

1. oxytocin producing neurones

(These are synchronously activated and oxytocin pulses are released from nerve endings located in the posterior pituitary gland.)

2. myoepithelial cells

which ejects milk from the alveoli into the duct system from where it is available for removal.

Question 7

(7)

Milk-Ejection (Let Down)

The milk ejection reflex may be conditioned to other stimuli, thus many dairy cows will let-down in response to the sight and sounds of the milking parlor. A similar phenomenon occurs in other species, including women, when they are preparing to nurse.
Likewise milk ejection can be inhibited by stressful situations (first-time heifer, unfamiliar dairy, rough handling) and this is due to central abolition of oxytocin release in response to tactile stimulation of the teat.

Under natural conditions the frequency of nursing (and thus letdown) varies from continuous attachment in the marsupials to once/week in the seals. The pattern of letdown during nursing also varies. In pigs and rabbits there is a single episode of let-down in response to suckling. In rats the young may stay attached for hours and during this time there are regular episodes of oxytocin secretion and milk
letdown.

Question 8

(8)

(Colostrum)

1. After the alveoli appear during mammary development in pregnancy, the lumen begins to accumulate fluid with progressively larger quantities of what?
2. Colostrum is vital for providing passive immunity for neonates that cannot get it via the placenta during pregnancy. It is generally considered that animals with more tissue layers between maternal and fetal blood - a histologically more complex placenta - are more or less likely to get significant immunoglobulin transfer from the mother prior to birth?

Answer 8

1. serum-derived proteins

(This will form the basis of the immunoglobulin-rich “first milk” - the colostrum.)

2. less likely

(Thus species like primates with hemochorial placentation have little requirement for colostral immunoglobulin, but species like ruminants with synepitheliochorial and horses with epitheliochorial placentation, have an absolute requirement for colostrum in order to achieve a level of passive immune protection. Species with intermediate placental thicknesses such as carnivores (endotheliochorial), receive IgG from both sources (5-10% of neonatal IgG may be obtained transplacentally in dogs and cats).)

Question 9

(9)

(Colostrum cont)

1. In species requiring colostrum for passive immunity the major immunoglobulin present in colostrum is what?
2. When the colostrum gives way to milk within days after parturition this changes so that the major immunoglobulin in milk is what in most species?

exceptions are what?

In those species requiring no colostrum the major immunoglobulin throughout is what?

Answer 9

1. IgG
2. IgA

ruminants and cat (where IgG remains the predominant Ig type.)

IgA

Question 10

(10)

(Colostrum cont)

1. Colostrum quality generally refers to its what content?

since this is all important in its primary function of supplying passive immunity. Several things can affect this quality:

what are they (look at pic)

Answer 10

1. IgG

Question 11

(11)

(Colostrum cont)

1. The factors controlling the onset of colostrum formation (colostrogenesis) are poorly understood.
2. Colostral leukocytes may also be absorbed intact by the neonate and appear to play a role in neonatal immune function, especially what?
3. Colostrum also has an important nutritive function for the neonate. The energy content of colostrum is important, especially for animals born into cold environments. This is largely due to what?
4. Colostral proteins are used for what?
5. In ruminants which vitamins do not cross the placenta and are absorbed from colostrum?
6. Colostrum, and later milk are involved in establishing the mother-offspring bond - “reward” to the offspring.

Answer 11

1. macrophages and T-lymphocytes.
2. colostral fat content
3. protein synthesis and gluconeogenesis.
4. A, D, and E

Question 12

(12)

(Functions of Milk)

In addition to its nutritional role, milk also continues to provide local passive immunity, influences development of active immunity by the neonate, provides factors that influence growth and development of the GI tract and provides anti-inflammatory agents.

(In general terms)

1. % protein is proportional to what?
2. more fat = ?
3. more lactose = ?
4. faster growers need more what?

5 Most what are adequate?

6. Digestive enzymes: Milk also comes with certain means of digestion pre-packaged to help the neonate.

what is one of these?

Answer 12

1. growth rate (or inversely related to time taken to double weight)

(Source of peptides, amino acids and nitrogen for growth of the infant.)

(Protein degradation products may influence intestinal motility (opioid-like peptides). Depending on species, may enhance mineral absorption.)

2. faster gain
3. slower gain (since attracts more water ⇨ milk will be less nutrient dense)
4. ash (Calcium, phosphate and magnesium are required for skeletal growth.)
5. fat-soluble vitamins
6. bile-salt stimulated lipase

Question 13

(13)

(functions of milk)

(passive immunity)

1. Milk is rich in factors that help protect the infant from pathogens.

most act where to prevent what?

2. Lactoferrin (an iron binding protein) is found in high levels in human and sow milk - but low in what?

it does what?

also does what to g-

Answer 13

1. at mucoseal surfaces to prevent first stage of infection - pathogen adhesion

(Breast fed human babies have lower incidences of intestinal and respiratory infections than formula fed babies.)

2. cows milk

competes with some bacteria for iron and thus stops their proliferation

causes release of lipopolysaccharide from the cell wall of gram negative bacteria, increasing their susceptibility to lysozymes.

(Since lysozyme are also present in milk they may function together.)

Question 14

(14)

(functions of milk)

(passive immunity cont)

1. Milk mucins from fat globules act as receptor analogs for pathogenic bacteria, hence preventing what?
2. Digestion of milk lipids forms fatty acids and monoglycerides, which have protective effects against what?
3. Oligosaccharides (mostly modified lactoses) act as soluble factors to bind what and prevent what?

(immunoglobulins)

IgA and IgG (depending on species) function to prevent adhesion of bacteria to mucosal surfaces and thus prevent upper respiratory (by protecting the pharynx) and gastrointestinal infections. They can also neutralize some bacterial toxins.

(Leukocytes)

Leukocytes are present in milk, mostly macrophages and neutrophils plus some lymphocytes (mostly T-cells). These cells are competent but their exact function is largely unknown. They appear to help with immunocompetence and also secrete cytokines, IgA and growth factors which are involved in maturation of the gastrointestinal tract.

Answer 14

1. binding of bacteria to mucosal surfaces (e.g. E. coli)
2. viruses and some parasites.
3. bind pathogenic bacteria and prevent binding to mucosa

(In humans they are active against organisms such as Hemophilus influenzae (the most common cause of otitis media) in the pharynx and Vibrio cholerae in the proximal intestine.)

Question 15

(15)

(Fucntions of Milk)

(active Immunity)

1. Breast feeding in humans is also associated with the development of the infants own (active) immune system. Components in milk promote what production by the infant?

thus enhancgin what?

2. Long term effects are also suggested by data that show breast fed infants have lower incidences in later life of insulin-dependent diabetes mellitus (IDDM), Crohn’s disease, malignant lymphoma and atopic conditions than those on formula or cows milk.

Answer 15

1. IgA

mucosal defenses

( In some studies, breast fed infants have better responses to vaccinations.)

Question 16

(16)

(Fucntions of Milk)

(Hormones and Growth Factors)

1. Many hormones, enzymes and growth factors are present in milk (>30), and they appear to have roles in promoting and supporting infant growth and development.
2. They function, among other ways, to accelerate maturation of what?

Answer 16

2. GI tract and internal organs

(For example: epithelial growth factors function to mature the intestinal epithelial barrier of the neonate, which limits penetration of foreign antigens.)

Question 17

(17)

(Fucntions of Milk)

(Anti-inflammatory Agents)

1. Lactoferrin inhibits complement and prevents inflammatory cytokine release from what?
2. There are also many antioxidants, some of which may be absorbed for systemic effects.

Answer 17

1. activated macrophages

Question 18

(18)

(It’s not all Good - Disease Transmission in Milk and Colostrum)

1. Several diseases can be transmitted from dam to offspring via the milk. This may be especially true for those involving what?

(Some examples include…)

(Johne’s Disease (paratuberculosis))

2. especially cows in the later stages of infection may shed the organism in milk and colostrum.
3. which is worse - milk or colostrum?

(Caprine Arthritis-Encephalitis Virus (CAEV))

4. what is the main means of transmission for this disease?

(Bovine Leukiemia Virus (BLV))

5. colstrum and milk main means of spread?

((HIV))

Considered the one absolute contraindication for breastfeeding in the developed world. Not all infants born to HIV positive mothers are infected in utero, and of the negative ones not all will be infected by breastfeeding, however it is not worth the risk given the available alternatives and high survival of bottle fed infants.
In the developing world the benefits are currently debated since 50% of bottle fed babies die of more common infections in the first year.

Answer 18

1. cell-associated organisms (maternal leukocytes are secreted in milk)
2. Colostrum is worst with 36% of asymptomatic shedders secreting it in colostrum, 3 times as many as secreted it in milk.
3. ingestion of virus laden colostrum or milk

(Control measures center around preventing kids sucking - remove from dam immediately (prior to suckling) and feed heated colostrum and pasteurized milk.)

5. no (but can be important on individual farms.)

Question 19

(19)

(Involution of the Mammary Gland)

1. Once the offspring are weaned there is no further need for lactation so the mammary gland regresses
2. The process is one of organized what?

initiated when?

3. what form in the alveolar epi cells?

a result of fusion of what?

4. Normal secretory mechanisms appear to become inactive in what time after milk removal ceases?
5. In animals that are not again in late pregnancy at the time weaning occurs the process of involution is rapid and alveolar epithelial cell sloughing commences by day 3. Extensive cell loss and gland remodeling results in loss of lobuloalveolar structure and duct regression so the resultant gland is similar to that of a virgin animal.

Answer 19

2. apoptosis

12-24 hours after milk removal ceases

3. large vacuoles

secretory vesicles and fat droplets

4. ~ 48 hours (RER and Golgi decline at this time)

Question 20

(20)

Commercial lactating dairy cows are already 7 months pregnant at the time they are dried-off (“weaned” off the milking machine) and this means they don’t undergo the full involution described above. Alveolar structure is generally maintained and while there is apoptosis and turnover of alveolar epithelial cells, extensive sloughing is not a feature. Similarly, being pregnant with the next litter/offspring retards full involution in other species (the mammary gland is being given involution signals as a result of weaning but these are opposed by development signals from the pregnancy - if it is advanced enough).