Slide Set 6: Involution

Question 1

Define involution

Answer 1

• The regression of the mammary gland following the end of a lactation cycle
• The mammary gland returns to a non-lactating state

Question 2

What are the characteristics of involution?

Answer 2

• Decrease in mammary cell number and activity
• Reduction in galctopoietic hormones (systemic factors)
• Accumulation of local factors (inhibition of lactation)

Question 3

When does involution begin?

Answer 3

Around 4-5 months after parturition

Question 4

What are the main features of involution?

Answer 4

Ø A decrease in cell number due to cell death
(apoptosis)
Ø A decrease in cell activities duo to tissue remodelling

Question 5

What are the types of involution?

Answer 5

1-Gradual involution
2-Initiated involution
3-Senile involution

Question 6

What is gradual involution and how does it occur?

Answer 6

Ø Occurs during normal lactation
Ø Results in gradual reduction in milk production
throughout lactation (i.e. less suckling)
Ø Decline in hormonal stimulus (galactopeiotic hormones)
Ø Accumulation of milk and local inhibitory factors in the mammary gland

Question 7

What is initiated involution and how does it occur?

Answer 7

Ø Induced by humans on domestic animals
Ø sudden cessation of milk removal
Ø Changes in the mammary gland tissues more abrupt than changes observed during gradual involution
Ø Example: dry period in dairy cows
Ø Both gradual and initiated involution are reversible if
pregnancy occurs

Question 8

What is senile involution and how does it occur?

Answer 8

• irreversible involution

Ø Occurs at the end of the reproductive cycle of the female (aging)

Question 9

What is complete involution?

Answer 9

Ø Complete involution: rapid loss of secretory functions; complete loss of the epithelial secretory cells (virgin-like mammary gland)
Ø Epithelial cell components are degraded by lysosomal enzymes released into the cytoplasm
Ø Complete involution is common in some mammalian species such as rats and mice

Question 10

What is incomplete involution?

Answer 10

Ø Incomplete involution: collapsing of the alveolar structure but the secretory cells are still intact (e.g. dairy cows)

Question 11

What are the changes that occur during involution?

Answer 11

• Changes in mammary tissues
• Changes in mammary cell secretions
• Changes in mammary cell organelles

Question 12

What are the changes in mammary gland during involution?

Answer 12

Ø Progressive regression from the peripheries towards the gland cisterns
Ø Entire lobes & lobules degenerate instead of single epithelial cells

Question 13

What are the changes in mammary gland during involution (cow)?

Answer 13

Ø Decrease in alveolar size
Ø Decrease in # of alveoli per lobule
Ø Decrease in # of secretory cells per alveolus
Ø Reduction in luminal volume
Ø Connective tissue become more prominent
Ø Limited disengagement of epithelial cells from basement membrane (unlike rodents)

Despite the fact that the lobular structure is reduced, it is still recognizable (unlike virgin mammary gland)

Question 14

What are the changes in milk volume during involution?

Answer 14

• Accumulation of milk in mammary cell
• Increased intra-mammary pressure
• Rupture & damage of the basement membrane
• Milk moves into interstitial space
• Milk components broken-down by phagocytosis

Question 15

What are the local factors that are responsible for milk inhibition?

Answer 15

• milk accumulation
• FIL
• Serotonin
• Lactoferrin

Question 16

What is the role role of plasminogen / plasmin in involution

Answer 16

Ø ensures normal functioning of the circulatory system
Ø maintains lumen fluidity in vascular system by cleaving excessive / unwanted clots (fibrin)
Ø Widely distributed in the body
Ø In mammary gland the concentration of plasominogen / plasmin increases as lactation progresses

Question 17

What is the role of role of serotonin in involution?

Answer 17

• Potent inhibitor of lactation (rats, women, bovine)
• Immune response
• Junctional disruption (tight junctions)
• Cells death
• Inhibits milk protein synthesis

Question 18

Explain the changes in milk volume during involution (cows)

Answer 18

Ø Milk volume increases during the first days of involution
Ø Milk volume declines substantially between day 3-7 following cessation of milk removal
Ø Further reduction occurs by day 30

Question 19

What happens to the cell organelles during involution?

Answer 19

Ø Milk synthesis in the mammary secretory cell stops within 48 h after cessation of milk removal
Ø Appearance of large vacuoles after 48 h of milk cessation
Ø Vacuoles are formed by the fusion of secretory vesicles & fat droplets
Ø Organelles involved in milk biosynthesis (Golgi apparatus & RER): Almost disappear 48 hours following cassation of milk removal
Ø Mitochondria, ribosomes, nucleus: remain intact throughout involution

Question 20

Which organelles remain intact during involution?

Answer 20

Mitochondria, ribosomes, nucleus: remain intact throughout involution

Question 21

Explain the changes in milk constituents during involution

Answer 21

Ø Milk specific components decline
Ø Blood serum components increase
Ø Milk fat and lactose decrease
Ø concentration of total protein increases in early days of involution (high lactoferrin and high serum albumin)
Ø Milk export proteins (casein, α-lactalbumin, β- lactoglobulins) decreases
Ø Concentrations of other serum proteins (lactoferrin, immunoglobulins) increases

Question 22

What happens to the lactoferrin during involution?

Answer 22

Ø iron-binding protein secreted by the mammary gland
Ø Synthesis of lactoferrin increases during milk stasis
and involution (~100 fold)
Ø Synthesis declines after ~14 d of involution
Ø Suppresses casein expression (inhibits secretion)
Ø Effect of lactoferrin on involution is reversible (based on duration of milk stasis)
Ø antibacterial properties
Ø Lactoferrin is higher in human milk than in cow milk

Question 23

What happens to the leukocytes during involution?

Answer 23

Ø Leukocytes # in secretion increase during early involution and remain elevated (non-inflammatory reaction
Ø Involve in phagocytosis of residual milk constituent
Ø Involve in protecting the mammary gland from bacterial invasion

Question 24

What happens to the leukocytes (phagocytes) during involution?

Answer 24

Ø Play a major rule in removing fat droplets, cellular debris
ØTwo types
1-Neutrophils (polymorphonuclear)
ØDominant during the initial few days of involution
2-Macrophages & lymphocytes (mononuclear)
ØDominate after neutrophils

Question 25

What are the two types of phagocytes that are found in milk contituents? What are their functions?

Answer 25

1-Neutrophils (polymorphonuclear) ØDominant during the initial few days of involution 2-Macrophages & lymphocytes (mononuclear) ØDominate after neutrophils

Question 26

What happens to the lysosomal enzymes during involution? Give examples

Answer 26

Ø Activities of several lysosomal enzymes increases during involution Example: Ø Lactoperoxidase Ø Proteases Ø Plasminogen activators

Question 27

Involution in rodents (complete involution) is characterized by:

Answer 27

Ø Rapid loss of tissue function Ø Degeneration of alveolar structure Ø Massive loss of epithelial cells (>50%) Involution remains reversible for 30-36 h Ø The cell loss is due to programmed cell death (apoptosis)

Question 28

What is apoptosis?

Answer 28

* Programmed (active) cell death * Cell shrinks, losses contact with neighboring cells and displays intracellular protein on its surface * Chromatin condenses and DNA cleaved into small fragments * Small membrane bodies break off containing intracellular materials (apoptotic bodies) * Secondary necrosis (breakdown) will follow

Question 29

What is necrosis?

Answer 29

* Uncontrolled (passive) cell death * Cell swells and mitochondria is damaged (rapid energy depletion) * Breakdown of homoestatic control * Cell membrane lysis and release of cellular contents * Inflammatory response and damages to surrounding cells

Question 30

Explain the involution in rodents

Answer 30

- A rapid process (8-10 days) where the epithelial cells of the lactating gland is returned to a rudimentary state (virgin-state) - Can be divided into 2 stages (based on reversibility): 1- Reversible stage (2 days): Gland can return to lactating state if suckling is re-introduced 2- Irreversible stage(3-10days): Gland is unable to return to lactation without being re-stimulated by pregnancy

Question 31

What are the 2 stages of involution in rodents? What is it based on?

Answer 31

- Can be divided into 2 stages (based on reversibility): 1- Reversible stage (2 days): Gland can return to lactating state if suckling is re-introduced 2- Irreversible stage(3-10days): Gland is unable to return to lactation without being re-stimulated by pregnancy

Question 32

What are the detailed categories of involution in rodents?

Answer 32

1- Milk stasis (inhibition) 2- Epithelial regression (via cell death) 3- Immune response and clearance of debris 4- tight junction and barrier disruption 5- Extra cellular matrix (ECM) remodeling 6- Adipose tissue remodeling 7- Vascular remodeling

Question 33

Define dry period

Answer 33

Period between the end of one lactation and the start of the next (2 month prior to parturition or calving)

Question 34

What does life of a cow look like

Answer 34

Calf --> Bred (14-18 months) --> 1st calving (23-25 months) --> dried off (305-320 days in milk) --> calf for 2nd time --> calf or culled re-bred

Question 35

Explain the lifecycle of cottontail rabbit

Answer 35

* Becomes pregnant immediately after parturition * Pregnant and lactate for 18 days * Dry period of 8-10 days after weaning * Cycle is repeated 4-6 times a year * Rarely repeated for more than 2 seasons

Question 36

What is the importance of dry period?

Answer 36

Ø Allow mammary gland to go through a period of Involution Ø Ensures maximum number of secretory cells in Early lactation Ø Minimize metabolic disorder

Question 37

What are the characteristics of unique involution?

Answer 37

- Involution during the dry period in markedly less than in other species - Few epithelial cells are lost, no detachment of cells from basement membrane - Area occupied by epithelium does not change and alveolar structures remain intact

Question 38

What are the justifications for the dry period?

Answer 38

1- Old cells produce less milk than new cells 2- Life span of old cells is shorter than new cells 3- # of proliferative cells will increase during the dry period

Question 39

What are the stage of drying-off?

Answer 39

Ø Build-up of intra-mammary pressure Ø Rapid decline of lactose biosynthesis (within few hours) Ø Water transportation to the epithelial cells stops Ø Aveolar and lumenal areas decline (within 7 days) Ø Aveolar system considerably collapses by day 28 but does not disappear

Question 40

Physiology of the mammary gland during the dry period can be divided into three phases:

Answer 40

1- Active involution 2- Steady state involution 3- Redevelopment and colostrogenesis

Question 41

Explain active involution

Answer 41

Ø Transition phase from lactating to non-lactating stage Ø Duration: 21-30 days Ø Changes in milk constituents, cell organelles and cell Secretions (discussed earlier)

Question 42

Explain steady state involution

Answer 42

``` Ø Transition between active involution &re- development Ø Little fluid in the gland Ø High concentrations of leukocytes Ø Little milk fat or casein debris left Ø Immunoglobulin concentrations are high ```

Question 43

Explain redevelopment and colostrogenesis

Answer 43

Ø Transition between non-lactating to lactating phase Ø Begins 3-4 weeks prepartum Ø Selective increase in uptake of immunoglobulins Ø Increased concentrations of milk components (~2 weeks pre-partum, increased markedly 3-5 pre- calving)