LACTATION Structure of Mammary Glands by MikeJenny


									Department of Animal & Poultry Science
University of Saskatchewan
ANSC 56.6 Animal Breeding & Genetics

Lactation is the production of milk by the mammary gland, and it is a characteristic of all
mammals. The mammary gland has two functions:
       i) To provide milk to nourish offspring.
       ii) To provide passive immunity to the offspring via colostrum.

Structure of Mammary Glands
The mammary glands are composed of collections of spherical sacs called alveoli. These sacs
are made up of a single layer of glandular cells that actually synthesize milk and secrete it into
the lumen of the alveoli.
Each alveolus has its own blood supply from which milk constituents (e.g. fat, protein and water)
are obtained. Milk production is directly related to the amount of blood flowing through the
mammary gland. A cow producing 30 kg milk per day has a blood flow of about 9 L per min,
while a cow producing only 10 kg milk per day has a blood flow of about 3 L per min. One L of
milk is obtained from about 600 L of blood.

Each alveolus is also surrounded by myoepithelial (smooth muscle) cells. During milk let-down,
these contract and force the milk out of the lumen of the alveoli into a duct system. The
hormone that causes the myoepithelial cells to contract is oxytocin.

The alveoli are arranged in groups called lobules and the lobules are in turn grouped into lobes.
Milk is forced into the duct system and collects in a large reservoir called the udder cistern. From
the udder cistern, milk goes to the teat cistern.

11Lactation                                                                                   Page 1
                                       There are species differences in the detailed structure of the
                                       duct system carrying the milk from the alveoli to the teat.
                                       Instead of a large udder cistern with a single canal through
                                       the teat, in some species large ducts from a number of
                                       lobes lead directly to the nipple (e.g. rat, pig and human).
                                       The only provision for milk storage prior to removal is
                                       enlargements of the ducts called ampullae.

                                       Development of the Mammary Gland
                                       The bovine udder is composed of four (4) separate
                                       mammary glands. An important component of the udder is
the supporting suspensory ligaments. The udder of a high producing dairy cow may weigh 20 kg
and may contain another 20 kg of milk. The weight of the milking cluster and the vacuum effect
may be 15 kg. Therefore, the total udder weight can be 55 kg and has to be supported by the
suspensory ligaments.

The Suspensory System of the Udder
At birth, the bovine udder has both teat and gland cisterns but
the major duct system is little developed. From birth, mammary
development can be divided into four (4) phases:
i) From birth to the immediate prepubertal period, mammary
gland growth is isometric (i.e. grows at the same rate as the rest
of the body). The increase in size during this period is due largely
to increases in connective tissue and fat with a little duct
development. Over-feeding during this period will greatly
increase the amount of fat laid down and so restrict the space
available for subsequent gland development. In dairy cows, this
can result in a reduction in subsequent milk yield of up to 30%, and the effects may persist into
the third lactation.
ii) At the time of puberty, the developing ovarian follicles are producing estrogen, which
stimulates further growth of the udder cistern and the duct system. Following ovulation, the
corpus luteum produces progesterone, which stimulates growth of the alveoli. Estrogen and
progesterone act indirectly by regulating other hormones. If the adrenal or pituitary glands are
removed, estrogen and progesterone have no effect. Other hormones important for mammary
development include growth hormone, thyroid hormones (T3 and T4) and adrenal steroids.
iii) During pregnancy, mammary development continues under the influence of estrogen and
progesterone. The duct system develops during the major part of pregnancy. The development
of the alveoli occurs mostly towards the end of pregnancy.
iv) Around the time of calving, the secretory cells of the alveoli start to produce milk. During
parturition, there is a large surge of prolactin and this is the signal to start lactation. The
important hormones for milk production and the maintenance of lactation are prolactin, insulin-
like growth factor (IGF)-1 and growth hormone.

11Lactation                                                                                       Page 2
                                                                 Milk Let-Down
                                                                 Milk let-down is under the control
                                                                 of a neuro-hormonal reflex. The
                                                                 suckling stimulus or massaging of
                                                                 the udder stimulates somatic
                                                                 nerves in the teat, which send a
                                                                 signal to the posterior pituitary
                                                                 gland and causes the release of
                                                                 the hormone oxytocin.
                                                                 Oxytocin travels through the
                                                                 blood stream to the udder and
causes the myoepithelial (muscle) cells around the alveoli to contract. This contraction
squeezes the alveoli and forces the milk out into the duct system. For efficient milking, there are
several important factors to remember.

•   After a good massage, it takes about one (1)
    minute before milk let-down occurs. The degree
    of milk-let down depends on the amount of
    oxytocin released, which again depends on the
    extent of the suckling stimulus or udder massage
    (afferent neural input).
•   The maximal effect of oxytocin occurs during the
    first 2 to 3 minutes of milk let-down. After this
    time, the effect of oxytocin gradually wears off.

After 6 to 8 minutes, all of the oxytocin has been broken down so that milk let-down ceases.
                                                                  Stress during cow preparation or
                                                                  during milking will inhibit
                                                                  oxytocin release. Stress causes
                                                                  the release of epinephrine from
                                                                  the adrenal gland, which
                                                                  constricts the blood vessels of
                                                                  the alveoli and so prevents
                                                                  oxytocin from reaching the
                                                                  myoepithelial cells. There is also
                                                                  inhibition of oxytocin release
                                                                  from the pituitary. These effects
                                                                  can reduce or even prevent
                                                                  milk let-down.

11Lactation                                                                                     Page 3
If the milking cups are put on late, the cow may not be milked out. The udder must then be
stripped manually. The force required for stripping may damage the teat. Such teats are
predisposed to infection and mastitis may result. (Mastitis will be discussed later)

In the adult cow, there are four (4) phases of the mammary gland during the lactation cycle:
i) Dry period. Development of the small ducts and alveoli, especially near calving.
ii) Around calving (-4 to +4 days). Differentiation of alveoli cells into actively secreting cells.
iii) Lactation. All cell activity directed towards milk synthesis and no further mammary growth.
iv) Involution of the mammary gland. This is the gradual but irreversible regression of the gland
(i.e. a reduction in the numbers of active alveoli). This starts after the peak of lactation, but is
more pronounced during late lactation.
                                                              Milk Yield and Composition
                                                              Daily milk production typically
                                                              increases during the first few weeks of
                                                              lactation. Peak milk yield is usually
                                                              achieved at about 4 to 8 weeks, and
                                                              then decreases slowly during the rest
                                                              of lactation (due to mammary gland
                                                              involution). This pattern of milk yield is
                                                              also observed in sows and mares.

                         Average Milk Composition of Different Species (%)
    Species            Solids           Fat         Protein        Lactose         Minerals
    Cow                 12.7            3.9           3.3             4.8              0.7
    Ewe                 18.4            6.5           6.3             4.8              0.9
    Sow                 21.8            9.6           6.3             5.0              0.9
    Goat                12.4            3.7           3.3             4.7              0.8
    Mare                10.5            1.2           2.3             5.9              0.4
    Human               13.3            4.5           1.6             7.0              0.2
    Reindeer            33.7           18.7          11.1             2.7              1.2
    Whale               50.0           36.1          10.6             2.1              1.2

Milk composition varies greatly with species, as shown. The table shows values for mature milk.
In the first 24 hours of lactation, different milk is produced called colostrum.

11Lactation                                                                                       Page 4
Colostrum, the first milk produced upon delivery of the newborn, is important to the offspring.
Colostrum from cows may have 1.5 times as much fat and up to 40 times as much protein. Most
of this protein is globulins, which are antibodies necessary for passive immunity against disease
until the young develops its own immunity. It contains lymphocytes and monocytes that protect
against exposure to infection. It also contains a greatly increased amount of protein and high
levels of vitamins A, E, carotene, and riboflavin. However, lactose, vitamin D, and iron are all low
compared to normal milk.

Colostrum contains antibodies against diseases to which the dam has been exposed, so it is
extremely important that the newborn animals drink the colostrum so they obtain passive
immunity until they can develop their own antibodies.

The small intestine of some neonatal animals has the ability to absorb macromolecules including
intact protein molecules from the colostrums. This receptive period lasts approximately 1 day in
the horse and pig and up to 3 days in the ruminant.

During the first 24 hours after birth, immunoglobulins (antibodies) are absorbed intact from the
gut rather than being digested. After this time, "gut closure" occurs and then the
immunoglobulins are considered just an ordinary protein and are digested to amino acids,
which are then absorbed.

If bacteria or other foreign material gains access to the body, an immune response is initiated by
the lymphocytes. The foreign body is called an antigen and must be sufficiently large and
complex in order to be able to cause a response. Small molecules may evade detection (e.g.
snake venom). To immunize against small molecules, the small molecule must first be joined to a
larger molecule, the combination then being large enough to cause an immune response.
On first exposure to the antigen, there is a lag time while the body selects the appropriate
lymphocytes for antibody production. These cells then multiply and produce the required
antibody (this is called the primary response). Once the foreign body has been dealt with, most
of the selected lymphocytes are removed. However, some remain in the body for many years
to provide a memory of the foreign body. If the same organism attacks again, the memory cells
are immediately available to initiate a response (the secondary response). Compared to the
primary response, the secondary response occurs faster, has a greater magnitude and lasts

11Lactation                                                                                 Page 5
Factors Affecting Milk Yield and Composition
A cow producing 30 kg of milk per day is giving almost 4 kg of dry matter daily. A beef steer
gaining weight at 1.4 kg per day is producing only about 0.4 kg of dry matter per day.
Therefore, the nutrient requirements of a lactating cow are much greater than those for a dry
cow. The primary determinant of milk yield is genetics. Management is very important to allow
the expression of genetic potential.

In monogastric (non-ruminant) species, energy from the diet is obtained mostly as glucose. In
ruminants, energy is in the form of volatile fatty acids (VFA) including acetate and propionate.
Acetate is the precursor for fat and propionate the precursor for glucose. The amounts of the
different VFA produced depend on type of diet fed and acidity (pH) in the rumen. Grain
feeding results in less acetate and more propionate, which is converted into glucose, which
again stimulates the release of insulin. Elevated insulin in dairy cattle promotes fat deposition in
the carcass, rather than butterfat yield in milk. High forage feeding results in more acetate
production, and thus lower glucose and insulin levels. Dairy cattle fed primarily forage produce
milk with higher butterfat content.

The factors affecting milk yield include:
i) Feed intake: The more feed that is consumed, the more nutrients that are available for milk
production. Total nutrient intake can be increased by:
        a) concentrate feeding
        b) increased feeding frequency

ii) Milking frequency: There may be a 5 to 10% increase by milking three (3) times daily.

iii) Litter size: If the female is nursing its young, the level of demand by the young will affect milk
yield. The greater the demand, the greater the milk yield. The greater the litter size, the greater
the demand. If particular teats are not suckled the associated mammary gland will begin to
involute. These mammary glands will not be able to "come back" even if the demand is again

iv) Hormones: An important controller of lactation is growth hormone. Milk yields can be
increased by 10 to 15% by injections of growth hormone (also known as bovine somatotropin or
bST). An improved milk yield will only occur under conditions of good nutrition and low disease.
Growth hormone can not be used to correct poor management. Other hormones that are
effective in sows are the thyroid hormones (T3 and T4). They do not appear to be effective in

v) Disease: Good health is important for good performance. For example, mastitis in dairy cows
can reduce milk yield by 30% or more.

11Lactation                                                                                     Page 6
Mastitis is the costliest disease of the dairy industry today.
Epithelial cells synthesize milk constituents (protein, fat, lactose). Mastitis is a bacterial infection
that destroys these milk-secreting cells. Scar or connective tissue replaces the milk secreting
tissue, which results in a permanent loss of productive ability.
Mastitis organisms enter the udder through the teat end and streak canal. The streak canal is
held closed by a circular muscle that holds milk in and foreign matter out. Also, the streak canal
is lined with keratin, which traps and kills organisms that attempt to invade through the teat end.
Losses can be reduced greatly by following an effective control program, which should include
the following items:
1. Handle cows gently to achieve highest production.
2. Follow proper milking procedures, milk clean, dry udders, apply milking units properly, and
   make adjustments to prevent the admittance of air into the teat cup liners and prevent liner
3. Dip cow's teats after milking to prevent new infections.
4. Treat all cows going dry with an approved dry cow drug to eliminate existing infections and
   prevent new infections during the dry period.
5. Cull chronically infected cows.

The California Mastitis Test (CMT) is a simple, inexpensive and rapid screening test, which
estimates the number of somatic cells present in milk. Somatic cells are a normal constituent of
milk and only when they become excessive do they indicate a problem. Somatic cells are
composed of approximately 75 percent leucocytes (white blood cells) and 25 percent epithelial
cells (secretary and lining cells). Leucocytes increase in milk in response to infection or injury.
They are the body's primary defense against microorganisms and disease. Epithelial cells, on the
other hand, increase as a result of infection or injury. They indicate that damage to body tissue,
particularly udder tissue, has occurred. They are in fact dead cells, which have been sloughed
from the alveoli and canals within the udder.

Fat Levels in Milk
The amount of butterfat in the milk is influenced by the relative amounts of the VFA (acetate
and propionate) leaving the rumen. A higher acetate production favors fat production. To a
large extent, the ratio of VFA is controlled by the pH (acidity/alkalinity) of the rumen. If pH falls,
this favors propionate over acetate and so a lower milk fat results. Higher acetate (and so
higher milk fat) is favored by:

High fiber diets: it is easier to maintain a neutral pH with high forage diets.

Greater feeding frequency: feeding "little and often" again allows the rumen to more easily
maintain pH and so favors acetate production.

11Lactation                                                                                        Page 7
Use of buffers: include buffers (NaHCO3) to maintain rumen pH when feeding high grain diets.

Reduced processing of the feed, in particular the forage component.

Weaning or Drying-Off
i) Dairy cows. It is important to dry-off a cow before the next calving to avoid a reduction in
milk yield in the next lactation. After peak milk yield, milk production gradually declines during
lactation. If the cow is not dried-off, milk yield will increase during the next lactation, but
production will be lower. A drying-off period is necessary to allow normal renewal of alveoli.
Drying-off is initiated about 60 days before the next calving and is achieved by reducing diet
quality (stop feeding concentrates), stop milking and then milk one last time 3 to 4 days later
with high producing cows (i.e. >16 kg/day), use poor forage and limit water to reduce
                                                              production and then dry-off.

                                                              ii) Beef cows. Bring in from fall pasture
                                                              and separate cows and calves (house
                                                              in adjacent pens to avoid fence
                                                              damage). Give the cows poor quality

                                                              iii) Sheep. Stop grain feeding 7 days
                                                              before the lamb is weaned. If feeding
                                                              alfalfa, reduce from 2.0 to 0.7 kg per
                                                              day. From the day of weaning, cut-off
                                                              water for 48 hours and feed poor
                                                              quality forage.

iv. Sows. Abrupt removal of piglets. There should be no feed or water restriction.

Following weaning or drying-off, maximum alveoli break-down occurs after 24 hours. The alveoli
are non-responsive to oxytocin after 36 to 48 hours.

11Lactation                                                                                       Page 8

To top