Other Systems in the Mammary
Gland: Support, Nervous,
Circulatory, and Lymphatic
What is this mammal?
Support Systems
• Variation across species
Litter-bearing Species
• No large suspensory ligaments
• Fibrous connective tissue and skin are the
primary support
• Examples: pigs, dogs, cats, rodents
Humans and Litter-Bearing Species
• Gland is on top of muscle tissue
• Strong, fibrous connective tissue
separates it from the muscle
• Suspensory ligaments are either
attached to the muscle or the
connective tissue
• In litter bearing species, each
half is also separated from the
other
• Midline
• Skin also provides protection
and a little support
The Cow
• Many different tissues…
* Median suspensory ligament
* Lateral suspensory ligament
- Fine connective tissue
- Coarse connective tissue
© The Babcock Institute
* Skin
- Superficial fascia
- Subpelvic tendon
Connective Tissue & Skin
• Fine connective tissue
– Attaches skin to underlying tissue
• Coarse connective tissue
– Attaches front quarters to abdominal wall
• Skin
– Little support
– Protection from pathogens
• Front and rear are also separated by connective
tissue
– No internal crossover between any of the quarters
Median Suspensory Ligament
• Primary support of the
udder
• Two adjacent heavy
sheets of tissue
• Mostly elastic, some
fibrous tissue
• Attaches to the
abdominal wall
• Divides the udder into
halves (left and right)
– Glands on each half
are divided by sheets
of tissue
© Biology of Lactation, Schmidt
Lateral Suspensory Ligaments
• Like a “hammock” around the udder
– From the pelvis to the median suspensory
ligament
• Mostly fibrous tissue
– Collagen
• Attaches to the alveolar tissue
– Provides internal framework
• Does not connect at the base of the udder
How much support is enough?
•
•
•
•
High producing Holstein cow
Empty Udder = 50 lb.
Milk = 60 lb.
50 + 60 = 110 lb. !!!
Importance
• Udder support can significantly affect
milking ability and useful life
– 10 – 25% heritable
• Pendulous udders are more likely to
suffer injury
– Especially teats
– Increased mastitis
– Affect on offspring
Nervous System
• Few nerves go into
the gland
• Like other skin
glands…no
parasympathetic
innervation
Nervous
System
Autonomic
(Involuntary
control)
Sympathetic
Somatic
(Voluntary
control)
Parasympathetic
Sensory Nerves
• Skin and teats
• Positive stimulation of
teats and surrounding
area initiates milk letdown reflex via
oxytocin
– Critical!
© Biology of Lactation, Schmidt
Sympathetic Nerves
• Associated with arteries in the gland
– Control blood flow to the gland
– Carry oxytocin to the gland
– Innervation of sphincters muscles
in teats
– Stress causes vasoconstriction decreasing
milk secretion and let-down
• There are no nerves to myoepithelial cells
or alveolar cells
– Contraction is regulated by oxytocin
Nervous System
• In species such as pigs, nerve supply to
the abdominal mammary glands is
different than inguinal mammary glands.
Circulatory System
• All milk precursors
come from the blood
• No crossover
between each side of
the mammary gland
• Venous circle
– Prevents pinching off
of areas of venous
outflow when animal is
lying down
Circulation
• Elongation and proliferation of
growing vessels
– Angiogenesis
– Vasculogenesis
• Complex interactions between
• Endothelial cells
• Extracellular matrix
• Specific stromal cells
• Requires dramatic reorganization of
surrounding tissue
• Growth factors affect rates of endothelial cell
proliferation and degree of vessel formation
– VEGF-vascular endothelial growth factor
•
•
•
•
Major regulator
Maintains viability
Stimulates mitogenesis and chemotaxis
Changes permeability
• Many growth factors in different isoforms
• Enzymes alter surrounding tissue
Alveolus
arteriole
Lumen
venuole
Myoepithelial Cell
Duct
Sphincters
• Capillary beds
– alveolus ‘hairnet’
• Sphincters constricted
– no blood flow
• Sphincters open
– blood flow to alveolus
Local control of blood flow
High pO2 = high blood flow
Need ATP
ATP needs oxygen
High pO2 = high ATP
= sphincters constricted
Terminal arteriole
Metarteriole
Large vein
sphincters
Post capillary venule
Other local controllers of blood
flow
* Oxygen
* Carbon Dioxide
• Adenosine
• Lactic Acid
• pH
• Hormones
• Nitric Oxide
• Alveolar cells each control their own
blood and nutrient supply
So how much blood is
enough?
500 Liters blood = 1 Liter milk
1 truck of blood = 100 lbs. milk
Hot area of research:
mammary tumors
Depends on angiogenesis for survival
Killer of females and males
Estrogen
• Mediates synthesis/secretion of
local tissue growth factors
– Direct effect?
• Changes in angiogenesis of ovary,
placenta, endometrium
• Mammary gland is a reproductive
organ
• Other hormones?
Tumors
•What’s u-PA?
–Activator of the zymogen plasminogen
–Plasmin degrades extracellular proteins
–Increases angiogenesis and mammary
development
• High concentrations of u-PA in
breast cancer patients
Thermogenesis
Hot spot
Infra-red thermography
Lymphatic System
• Molecules go out of the capillaries but not
all can get back in
– Disrupt the normal osmotic pressure
• Functions
– Pick up extracellular fluids and bring back to
the circulatory system
– Transport of leukocytes
– Transport of immune cells
Lymphatic System
• 2 Supramammary Lymph Nodes
– Each above ½ the udder
– Filter tissue fluids
– Remove foreign material
– Vaccination?
1.6 L LYMPH = 1 Lb MILK
Udder Edema
• Parturition
• Accumulation of secretions
– Pressure
– Passive flow is hindered
• Tight junctions do not function well
– Fluid enters interstitial space
– Alters osmolarity
– Alters hydrostatic pressure
– Less fluid is removed
• Connective tissue spaces swell
• Causes???