Download Section 1 Lactation Physiology

‫فیزیولوژی تولید و ترشح شیر‬
Lactation Physiology
(part 1)
By: A. Riasi
(PhD in Animal Nutrition & Physiology)
 At
the end of this section student will be able to reply:
 What is the supportive system in udder?
 What is the forstenberg’s rosette in papilla mammae?
 What are the important parts of a secretory cell?
 Which barriers are between the circulatory primary substance and milk?
 What are the transporter systems in mammary epithelial cells?
 Where is the lactose production site in the alveolar epithelial cells?
 What are the pathways for milk fat globule transit and secretion from
mammary epithelial cells?
 How is the de novo fatty acid synthesis in mammary cells?
 What is the milk fat depression syndrome?
 What
is a mammary gland?
 Is modified sweat gland.
 Serves a reproductive function; nourishment of the neonate.
 Can repeatedly undergo growth, functional differentiation, and
regression.
 Relies on same endocrine support for development and function.
 Example: gonadal steroids, prolactin, etc.
 What
is the difference between the animal udder?
Anterior
(thoracic)
Intermediate
(abdominal)
Posterior
(inguinal)
Total
Goat, sheep, horse
guinea pig
0
0
2
2
Cattle
0
0
4
4
Cat
2
2
4
8
Dog
4
2
2 or 4
8 or 10
Mouse
6
0
4
10
Rat
6
2
4
12
Pig
6
6
6
18
proboscideans,
primates
2
0
0
2
Species
www.wikipedia.org
 What
is the difference between the animal udder?
 Cow: Four glands and four teats
 Sheep and goats: Two glands and two teats
 Sow: 12-14 teats and two glands per teat.
 Mare: Four glands and only two teats.
 The
udder is a complex system
 A supportive system
 A secretory system composed of epithelial cells
 A duct system for storage and conveyance of milk
 Blood, lymph, and nerve systems
 The
udder of cows
 The
weight of empty cows udder is about 12-30 kg.
 The
udder weight is affected by:
 Age
 Stage of lactation
 Amount of milk in the udder
 Inherited differences among cows
 The
supportive system of udder
 There
are seven tissues that provide support for the udder:
 Skin (covering the gland is only of very minor support)
 Superficial fascia or Areolar subcutaneous tissue
 Coarse areolar or cordlike tissue
 Subpelvic tendon
 Superficial layers of lateral suspensory ligament
 Deep lateral suspensory ligament
 Median Suspensory Ligament
An illustrated view of the ligaments that permit udder suspension
(Courtesy of Iowa State University)
 Teat
structure
Annular (cricoid) rings
Furstenburg’s rosette

Interior anatomy of the Mammary Gland
 The interior structure of mammary gland:
 Connective tissue (Stroma)
 Ductular system
 Secretory tissue

Mammary duct system

Secretory tissue
(Adapted from Akers & Denbow, 2013)
Mammary alveolus. This diagram illustrates the three dimensional structure of the mammary
alveolus. The hollow center of the alveolus provide a space for the accumalation of milk
components that have been synthesized and secreted by the secretory cells that compose the
internal wall of the structure. The outside of the alveolus has a network of myoepithelial cells
that contract in response of release of oxytocin at the time of milking. This forces stored milk
into the terminal duct, which exits the lumen the alveolus. The milk progresses through larger
ducts to be emptied at the nipple or teat end. (Adapted from Akers & Denbow)
A photomicrograph of a developing mammary duct. Taken from a Holstein calf, this tissue
stained with specific cytokeratin 18 (red, a marker specific for epithelial cells), CD10 (green , a
marker of myoepithelial cells), and Ki67 (yellow, a protein produced in nuclei of cells that are
about to divide). The tissue section is from a study to evaluate the effects of the ovary on
ontogeny of myoepithelial cells in the bovine mammary gland. (Adapted from Akers & Denbow,
2103)

Secretory tissue
 A lactating secretory cell is the basic unit of milk synthesis
 Milk precursors are taken from the blood into the cell
 The secretory cell have two kind of junctions with neighbor cells:
 Tight junction around the apical portion
 Gap junction in lateral portion

Major component of a secretory epithelial cell
Apical membrane
Secretory vesicles
Golgi apparatus
Tight junction
Rough Endoplasmic Reticulum
Nucleus
Lysosomes
Cytoplasm
Basement membrane
Gap junction
Smooth Endoplasmic Reticulum
Basal and lateral membranes

Milk synthesis and secretion
 The product of mammary gland depends on two mode of
secretion:
Apocrine
Merocrine
 Other components are derived by passage of soluble molecules
across (transcellular) and sometimes between (paracellular) the
cells.

Milk synthesis and secretion
 Physically, milk is a complex solution of:
 Salts
 Carbohydrates
 Miscellaneous compounds with dispersed proteins and protein
aggregates
 Casein micelles
 Fat globules
 Milk osmolarity generally equals blood and the pH of 6.2-7.0.

Milk synthesis and secretion
 During the established lactation, function of the mammary gland
is closely linked with:
 Some hormones
 Growth factors
 Local tissue regulators
 Along with mammary cell-specific constituents, milk contains a
myriad of minor components.
 Many of these molecules are important nutrients or regulators of
the neonate.

Milk synthesis and secretion
 Molecules are transported into the milk by several possible
routes.
 Mammary epithelial cells are able to maintain substantial
gradients for Na+, K+, and Cl− ions across the cell membrane.
Concentrations of Na+ inside (~ 43 mM) the cells are typically
lower than outside (150 mM).
The gradient for K+ is the opposite (143 mM inside compared with
4.5 mM outside).
Concentration of Cl− is higher inside the cells.

Milk synthesis and secretion
 Milk is a rich source of calcium.
 The calcium in the milk exists as:
 Casein-bound calcium
 Calcium associated with various inorganic anions
 For example, citrate and phosphate
 Free calcium

Milk synthesis and secretion
 The rate of calcium influx into the cell is matched by a
corresponding uptake of calcium by cellular organelles.
 An ATP-dependent calcium pump on Golgi membranes
 The uptake of Ca by the epithelial cells probably dependent to:
 Parathyroid hormone-related protein
 1,25-(OH)2 vitamin D3

Precursors of Milk
 Precursors of milk come from the blood stream and the primary
substrates extracted from blood are:
 Glucose
 Amino acids
 Fatty acids
 Minerals
 Acetate *
 βHB *

Precursors of Milk
 Several materials in milk come unchanged from the blood:
 Minerals
 Hormones
 Immunoglobulins

Synthesis of milk proteins
 There are several specific systems for amino acids absorption
through the basal membrane.
 Inside the cell, amino acids are covalently bound together to
form proteins at the polysomes (Poly-ribosomes).
 Proteins sythesized at RER include:
 Casein
 β-lactoglobulin
 α-lactalbumin
 Membrane bound proteins
 Membrane boding enzymes

Synthesis of milk proteins
 Synthesized proteins are transferred the golgi apparatues (GA).
 Casein is secreted as micelle, which is formed in the GA from:
 Casein molecules
 Calcium
 Phosphorus
A: a submicelle; B: protruding chain; C: Calcium phosphate; D: κ-casein; E: phosphate groups

Synthesis of milk lactose
 Glucose enters the cells via the basolateral membrane by specific
transport system.
 Some glucose is converted to galactose in the cell.
 Both glucose and galactose enter the GA and react resulting in the
formation of lactose.

Synthesis of milk fat
 The sources of milk FA:
 Blood FA
 De novo FA
 Glycerol
 Monoacylglyceride (MAG)
 Acetate *
 β-hydroxybutyrate *
 Milk
fat
triglycerides
are
synthesized
endoplasmic reticulum and form small droplet.
on
the
smooth

Synthesis of milk fat
 The protein coat on the milk fat globule membrane comprises:
 Mainly butyrophilin (BTN) *
 Xanthine oxidoreductase (XDH) *
 Adipophilin (ADPH)**
 Mucin 1
 CD36
 Periodic acid/Schiff
 PAS III
 FABP
Pathways for milk fat globule transit and secretion from mammary epithelial cells

Synthesis of milk fat
 The properties of milk fat:
 Milk fat composed of different fatty acids:
 Short chains (4-8 C)
 Medium chains (10-14 C)
 Long chains (≥16 C)

Synthesis of milk fat
 The properties of milk fat:
 TAG (more than 95% of milk fat)
 DAG (2%)
 Phospholipids (1%)
 Cholesterol (0.5%)
 FFA (0.1%)
 Ether lipid, Fat soluble vitamins., etc.

Synthesis of milk fat
 The properties of milk fat:
 Saturated FAs (~70%)
 Palmitic acid
 Myristic acid
 Stearic acid
 Monounsaturated FA (~25%)
 Oleic acid
 Vaccenic acis
 Polyunsaturated FA (~5%)

Synthesis of milk fat
 There are two sources of FA for milk fat synthesis:
 The de novo FA synthesis in mammary epithelial cells
 Short chain (4-8 C)
 Medium chain (10-14 C)
 ~ 50% of 16 C
 Preformed FA uptake from blood circulation
 ~ 50% of 16 C
 > 16 C

De novo fatty acid synthesis
 In ruminants, the substrates for de novo FA synthesis in
mammary epithelial cells are:
 Acetate produced by rumen fermentation
 β- hydroxybutyrate produced by the rumen epithelium

Preformed fatty acid uptake
 Long-chain FA taken up by the mammary gland are imported
from plasma:
 Released from circulating lipoproteins by lipoprotein lipase
 NEFA bound to albumin
There is evidence showing that the membrane transport of long-
chain FA is a facilitated process.
 Some factors might play a role in FA uptake and transport:
 Cluster of differentiation 36 (CD36)
 Fatty acid binding protein 3 (FABP3)

Properties of milk TAG
 Fatty acids are not esterified randomly to the sn-1, -2, and -3
positions of glycerol backbone.
 The distribution of FA is dependent on the distinct binding
affinities of the acyltransferase enzymes for substrate FA.
 Transport
of milk components
Adapted from McManaman and Neville, 2003
Abbreviations: SV, secretory vesicle; RER, rough endoplasmic reticulum; BM, basement membrane;
N, nucleus; PC, plasma cell; FDA, fat depleted adipocyte; JC, junctional complex containing the tight
and adherens junctions; GJ, gap junction; ME, myoepithelial cell.
 Transport
of milk components
 Pathway I depicts exocytotic for:
 Protein secretion by alveolar cells
 Water
 Lactose
 Oligosaccharides
 Phosphate
 Calcium
 Citrate
 Transport
of milk components
 Pathway II depicts milk fat secretion.
 Milk lipids, primarily triacylgycerides and phospholipids, are
synthesized in the smooth endoplasmic reticulum in the basal region
of the cell.
 Newly synthesized lipid molecules form cytoplasmic lipid
droplets and are secreted by a unique budding process (MFGs).
 Transport
of milk components
Milk fat globule membrane is known to contain numerous
enzymes, including oxidases, reductases and hydrolases with
relatively high specific activities.
 In particular milk fat globule membranes are highly enriched in
the purine oxidizing enzyme xanthine oxidoreductase (XOR).
 Transport
of milk components
 Pathway III depicts transcytotic pathways for transport of
proteins and other macromolecules.
 Transcytotic secretion of immunoglobulin A in rabbit mammary
glands has been shown to occur.
 Prolactin and transferrin transcytosis have been detected
 Transfer of labeled low-density lipoprotein (LDL) from blood to
milk has been reported.
 Considering that xenobiotic agents, including carcinogens and some
drugs, can bind to and be transported by lipoproteins.
 Transport
of milk components
 Pathway IV depicts transport of:
 monovalent and polyvalent ions
 glucose
 amino acids
 Transport
of milk components
 Ion transport: Transporters or channels for sodium, potassium and
chloride have been identified on the basal and apical plasma
membranes of alveolar cells.
 Phosphate and iodide transporters appear to be limited to the basal
membrane.
 Transport
of milk components
 Glucose transport: Glucose transport systems have been detected
in the mammary gland at both the apical and basal plasma
membrane, and on Golgi and secretory vesicle membranes.
 Two distinct glucose transport mechanisms have been identified in
the mammary gland:
 GLUT1 transporter mechanism
 A sodium dependent glucose transporter
 Transport
of milk components
 Amino acid transport: Both sodium-dependent and sodium
independent amino acid transport mechanisms analogous to those
found in other organs have been demonstrated at the basolateral
component of the mammary epithelium.
 Other
agents:
The
presence
of
higher
than
expected
concentrations of certain drugs in milk have raised the possibility
that alveolar cells may have active transport mechanisms for such
compounds.
 Transport
of milk components
 Pathway V depicts transport the paracellular pathway for direct,
bi-directional, extracellular movement of both low-molecular-weight
substances and macromolecular solutes.
 This pathway is closed during lactation in humans and most other
species by the presence of very tight-junction.

Milk fat depression (MFD)
 Several theories have been proposed to explain the physiology
behind this reduction in fat synthesis.
 Lower production of acetic and butyric acids in the rumen caused
less fat production in mammary gland.
 The greater proportionate production in rumen increases the blood
insulin, which partitions nutrients away from the mammary gland.
 A more current theory is that the combination of high grain and
high unsaturated fatty acids in the diet causes the microorganisms in
the rumen to produce more trans fatty acids.

Milk fat depression (MFD)
 Avoiding milk fat depression
Proper cooling of cows
Control the amount of polyunsaturated fatty acids in the diet
 Balance dietary carbohydrates
 Buffer and alkalinizing agents
 Ionophores
 Feeding Management