View
6
Download
0
Category
Preview:
Citation preview
MAMMARY GROWTH AND DEVELOPMENT
Involution
Dry Period
■ Period between milk removal cessation and
subsequent calving and lactation
■ Recommended duration: 45 to 60 days
■ Milk yield in next lactation decreases if:
– < 40 days
– > 80 to 90 days
Bovine Dry Period
Rat Dry Period
Why Do Cows Go Dry?
■ Suckling or milking required to maintain lactation
– Mammary gland stimulation results in oxytocin
secretion from the posterior pituitary
– Oxytocin travels via the blood to the mammary gland
and causes contraction of the myoepithelial cells
surrounding the alveolus
– Results in expulsion of luminal milk from alveoli into
ducts and out of the gland, resulting in the physical
removal of milk from the alveoli
■ Without frequent emptying of the mammary gland, milk
synthesis will not persist despite adequate hormones
■ Maintenance of consistent milking will not maintain
lactation indefinitely
Suckling
■ Rat mammary gland still in secretory state if teats
are ligated but suckling continues
■ Involution inhibited from one side (with ligated
teats) if suckling occurred on other side
■ Involution inhibiting effects of prolactin
■ What happens to a cow if not milked for short
periods of time?
FL Power Outage (5-6 milkings)
Involution Characteristics
■ Decreased metabolic activity of mammary gland
■ Decreased size and number of alveoli
■ Decreased total alveoli and lobule volume
■ Alveolar collapse
■ Regression of glandular tissue
■ Decreased number of organelles
– Mitochondria swelling (=degradation)
■ Increased numbers of:
– Cytosomes (vacuoles containing cellular debris)
– Cytosegrosomes (autophagic vacuoles containing
organelles and debris)
– Lysosomes
:Lactating
Dry:
Lactating
Involuting
Involution
■ Milk composition changes
– Rapid decline in lactose synthesis
– Total protein concentrations increase in early involution,
partially because of water resorption from the secretion
and partly due to increased concentrations of lactoferrin,
serum albumin, and immunoglobulins
– Lactoferrin synthesis increases in contrast to milk-
specific proteins (e.g. casein) whose synthesis decreases
■ Lactoferrin has a number of potential functions in the
mammary gland, particularly as a nonspecific
disease resistance factor
– Iron-binding protein, thought to compete with
bacteria for iron (bacteriostatic)
Secretory Changes in Bovine Dry Period
Milk Components Active Involution Steady State
Involution
Redevelopment
/Colostrogenesis
Lactose Decreasing Low Increasing
Protein Decreasing Low Increasing
Fat Decreasing Low Increasing
Total volume Decreasing Low Increasing
Leukocytes Increasing High Low
Lactoferrin Increasing High Low
Immunoglobulins Increasing High Increasing
Structural Changes In Cows■ Changes in bovine mammary cells begin within 48 hours
after cessation of milk removal
– Formation of large stasis vacuoles in the epithelial
cells, a result of intracellular accumulation of milk fat
droplets and secretory vesicles
■ Alveolar lumenal area declines, while interalveolar stromal
area increases
■ Fluid volume in the gland decreases between day 3 and 7
of involution
■ Collapsed alveolar structures remaining are considerably
smaller than during lactation by day 28, with a very small
lumen
■ General alveolar structure is maintained throughout
involution
Structural Changes In Cows
■ No extensive tissue degeneration like other species
(e.g. rodents)
■ Change in secretory state rather than a change in
tissue regression
■ Possible cause: Cows are typically pregnant during
involution and pregnancy might play a role in the
inhibition of epithelial cell apoptosis
– But we don’t completely know if inhibition of
apoptosis is the role pregnancy plays on involution
– Apoptosis = programmed cell death
■ Necrosis = cell death from pathological cause
Changes In Cows: Summary
■ In summary, histological changes occurring in the involuting
bovine mammary gland involve:
– Little loss of alveolar epithelial cells
– Invasion of leukocytes
– Little histological evidence of apoptosis
– Increase in interalveolar connective tissue and
fibroblasts
– Increase in remodeling of the extracellular matrix
– Decrease in intra-alveolar area
– Decrease of cytoplasmic organelles involved in milk
protein synthesis (e.g., Golgi apparatus) and secretion
function (e.g., vesicles)
– Limited change in organelles with metabolic function
(e.g., ribosomes)
The Dry Period is a Working Vacation – Tissue Remodeling
Lactating cells
Involution
Lactating cell
Milk stasis
Autophagic process
Involuted cell
Cellular Migration
■ Very few cells (less than 2%) in mammary secretions
during involution are epithelial cells
■ Most cells are leukocytes
■ Total leukocyte concentrations in mammary secretions
increase rapidly in early involution
■ Types of leukocytes found in mammary secretions
during the dry period:
– PMN (polymorphonuclear neutrophils)
– Macrophages
– Lymphocytes
Cellular Migration
Cellular Migration
Cellular Migration
PMN
■ Phagocytic leukocytes
■ Predominant for the first 3-7 days
■ If quarter is also infected, they will predominate
throughout the early stages of infection
https://www.youtube.com
/watch?v=JnlULOjUhSQ
PMN migration
PMN migration
Macrophage
■ Predominant cell type after 7 days
■ Phagocytes
■ Ingest fat droplets and other debris
■ Play a major role in removing large quantities of fat
and cellular debris, including dead PMN
■ Also predominant cell type in the colostrum
Lymphocytes
■ Always present
■ B-cells become plasma cells (produce antibody)
■ T-cells aid in antibody response and cell-mediated
functions
■ Specific function in the involuting mammary gland
is not well known
Lymphocyte Riding a Macrophage!
New IntramammaryInfections During Early Dry Period■ Milk is no longer periodically removed from the gland
■ Milk is an excellent growth medium for bacteria
■ Accumulation of a large volume of milk in the gland (for the
first few days)
■ Milk leakage
■ Teat-end disinfection is stopped
■ Phagocytic Leukocytes (Somatic Cells) and Other Defenses:
■ Leukocytes begin entering the gland by day 1 after dry-off
– Busy ingesting milk fat, casein and debris
Recommended