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					      Babcock Institute for International
      Dairy Research and Development                                                             Dairy
      University of Wisconsin-Madison

                                               Michel A. Wattiaux
                                                   Babcock Institute

    MAMMARY GLAND STRUCTURE                                       Secretory and duct system. The udder is
  The udder of a cow is an organ designed                       known as an exocrine gland because milk is
to produce and offer a newborn calf easy                        synthesized in specialized cells grouped in
access to its mother’s milk. It is suspended                    alveoli, and then is excreted outside the
outside the wall of the rear abdomen and                        body through a duct system that functions
thus it is not restrained, supported, or                        like the tributaries of a river.
protected by any skeletal structures.                             Blood supply and capillary structures.
  The udder of a cow is made up of four                         Milk production demands a lot of nutrients
mammary glands or “quarters.” Each                              that are brought to the udder by the blood.
quarter is a functioning entity of its own                      To produce 1 kg of milk, 400 to 500 kg of
which operates independently and delivers                       blood must pass through the udder. In
the milk through its own teat. Generally,                       addition, the blood carries hormones that
the rear quarters are slightly more                             control udder development, milk synthesis,
developed and produce more milk (60%)                           and the regeneration of the secretory cells
than the front quarters (40%). The major                        between lactations (during the dry period).
components of the udder are listed here                           Lymph system. Lymph is a clear fluid
with a short explanation of their                               that comes from tissues highly irrigated by
importance and function.                                        blood. The lymph helps to balance the fluid
  Support system. A set of ligaments and                        flowing in and out of the udder and helps
connective tissue maintain the udder close                      to combat infections. Sometimes the
to the body wall. Strong ligaments are                          increased blood flow at the onset of
desirable because they help to prevent the                      lactation leads to an accumulation of fluid
occurrence of pendulous udder, minimize                         in the udder until the lymph system is able
the risk of injuries, and avoid difficulties                    to remove the extra fluid. This condition,
when using milking equipment.                                   referred to as udder edema, is more

                                                                                   Fine connective membranes
                               Pelvis                                              (separate front and rear quarters)
                               Connective tissue                                                Left              Front
                               (attaches udder to the
                                abdominal wall)
                               Lateral suspensory
                                                                                   Rear                       Right

                                                                    Median suspensory
                                                                    ligament (separates left from right quarters)
Figure 1: The support system of a cow’s udder
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Dairy Essentials - Lactation and Milking


                      0.1 to 0.3 mm                                             Lobe
          Arterial blood                                                        Connective
                                                                                tissue capsule
                                                                                Lobules containing
     Secretory cell
                                                                                Secondary duct
                                                                                Support lamella

        Lumen                                                                   Primary duct
                                      cells                                     Gland cistern (400 ml)

                                      Duct                                      Teat cistern (30-40 ml)
 Venous blood                                                                   Sphincter muscle
                                                                                Streak canal
Figure 2: Alveoli and ducts form the milk secretory system
prevalent in first-calf heifers and older cows        also plays a minor role in the support and
with pendulous udders.                                stabilization of the udder.
  Innervation of the udder. Nerve                       The median suspensory ligament is an
receptors on the surface of the udder are             elastic tissue that attaches the udder to the
sensitive to touch and temperature. During            abdominal wall. Viewed from behind, the
the preparation of the udder for milking,             inter-mammary groove, a distinct mid-line
these nerves are triggered and initiate the           groove in the udder, marks the position of
“milk let down” reflex that allows the                the median suspensory ligament. Because
release of milk. Hormones and the nervous             this tissue is elastic, it acts as a shock
system are also involved in the regulation            absorber and accommodates changes in the
of blood flow to the udder. For example,              size and weight of the udder that occur
when a cow is startled or feels physical              with milk production and age. Damage or
pain, the concerted action of adrenaline and          weakness of the this ligament cause the
the nervous system decreases blood flow to            udder to stretch downward, making it more
the udder, inhibits the “milk let down”               difficult to milk and increasing the
reflex and lowers milk production.                    likelihood of injuries, especially to the teats.
                                                      Genetic selection for a strong suspensory
  Support system                                      ligament is effective in minimizing these
  In modern dairy cows, the udder may                 problems in the herd.
weigh more than 50 kg because of the large              In contrast to the suspensory ligament,
amount of secretory tissue and milk that              the lateral suspensory ligament is a rather
accumulates between milkings. The main                inflexible fibrous tissue. It reaches down
structures that support the udder are the             the sides of the udder from the tendons
median suspensory ligament and the lateral            around the pubic bones to form a sling.
suspensory ligament (Figure 1). The skin

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                                                           20 - Milk Secretion in the Udder of a Dairy Cow
  Duct and milk secretory systems                           defense mechanism against mastitis-
  The alveolus is a functional unit of                      causing bacteria. Differences in teat
production in which a single layer of milk                  structures, particularly the diameter and
secretory cells are grouped in a sphere with                the length, are related to susceptibility to
a hollow center (Figure 2). Capillary blood                 infection.
vessels and myoepithelial cells (muscle-like
                                                                     MILK SECRETION IN THE
cells) surround the alveolus, and the
                                                                       SECRETORY CELLS
secreted milk is found in the internal cavity
(lumen). The functions of the alveolus are:                   Milk secretion by the secretory cells is a
  • To remove nutrients from the blood;                     continuous process that involves many
  • To transform these nutrients into milk;                 intricate biochemical reactions. During
  • To discharge the milk into the lumen.                   milking, the rate of milk secretion is
                                                            somewhat depressed, but it never stops
  The milk leaves the lumen through a                       completely.       Between milkings, the
collecting duct. A lobule is a group of 10 to               accumulation of milk increases the pressure
100 alveoli drained by a common duct.                       in the alveoli and slows down the rate of
Lobules are themselves organized into a                     milk synthesis.       As a result, it is
larger units called lobes. The lobes                        recommended that high-producing cows be
discharge the milk into larger collection                   milked as close as possible to 12 hour
ducts that lead to the gland cistern, which                 intervals (the highest milkers should be
lies directly above the teat of the gland                   milked first in the morning and last in the
(Figure 2).                                                 evening). More frequent ejection of milk
  The udder is thus composed of billions of                 reduces the pressure build-up in the udder,
alveoli where milk is secreted. The ducts                   and for this reason milking three times a
form channels of drainage in which milk                     day can increase milk yield by 10 to 15%.
accumulates between milking. However, it                      The secretory cell is a complex factory.
is only when the myoepithelial cells that                   Figure 3 presents a summary of the
line the alveoli and the smaller ducts
                                                 BLOOD             SECRETORY                    ALVEOLUS
contract in response to the hormone CONSTITUENT                        CELL                       CAVITY
oxytocin (milk let-down reflex) that milk                                                        (LUMEN)

flows into the galactophores and the gland Water
                                              Minerals &                                       Minerals &
cistern.                                      vitamins                                         vitamins
  The teat forms a passageway through Immuno-                                                  Immuno-
                                              globulins                                        globulins
which the milk can be withdraw from the Amino              Amino                      Caseins (α,β,κ)
gland. It has a smooth skin covering and a Acids           Acids
rich blood and nerve supply. The teat tip                           β-Lactoglobulin
                                                                    α-Lactalbumin               proteins
closes with a smooth muscle ring or
sphincter called the “streak canal.” At its                       Energy
upper end, the teat is separated from the Glucose          Glucose                  Lactose
gland cistern by only a series of delicate                        Galactose
folds of sensitive cells particularly                             Glycerol
vulnerable to damage. These folds of tissue Acetate &      Acetate &                Triglycerides
                                                                                    Short chain FA
are also found at the other extremity of the Butyrate      Butyrate
                                                                                    Long chain FA
                                              Fatty acids  Fatty acids
teat directly above the streak canal (rosette
of Furstenburg). The teat is thus designed                                                                of milk
as a effective barrier to invading bacteria. Figure 3: Overview of milk secretion in the
Preservation of the normal teat structure is secretory cells (crossed circles are key regulatory
essential to the maintenance of the natural steps).

University of Wisconsin-Madison                                                                               79
Dairy Essentials - Lactation and Milking
mechanisms and the origin of nutrients             3) The glucose available influences the
needed for milk synthesis.                            amount of lactose synthesized in the
  The use of glucose by a secretory cell.             mammary gland;
Although glucose in the diet is entirely           4) The lactose available influences the
fermented in the rumen in volatile fatty              amount of milk produced per day.
acids (acetic, propionic and butyric acids), it
is needed in large amounts by the lactating         Synthesis of protein. The caseins found
udder. The liver transforms propionic acid        in the milk are synthesized from the amino
back into glucose which is transported by         acids taken up from the blood under the
the blood to the udder where it is taken up       control of the genetic material (DNA).
by the secretory cells. Glucose can be used       These proteins are packed in micelles
as a source of energy to the cells, as the        before they are released in the lumen of the
building block of galactose and                   alveolus. Genetic control of milk synthesis
subsequently lactose, or as the source of the     in the alveoli comes from the amount of
glycerol needed for the synthesis of fat.         α-Lactalbumin synthesized by the secretory
  Synthesis of lactose. The synthesis of          cells. As described above, this enzyme is an
lactose is controlled by a two-unit enzyme        important regulator of the amount of
called lactose synthetase. The sub-unit α-        lactose and milk produced per day.
Lactalbumin is found in the milk as a whey          The immunoglobulins are synthesized by
protein.                                          the immune system, and these usually large
  Regulation of milk volume. The amount           proteins are drawn from the blood into the
of milk produced is controlled primarily by       milk. The permeability of the secretory
the amount of lactose synthesized by the          cells to immunoglobulins is high during the
udder. Lactose secretion into the cavity of       synthesis of colostrum, but decreases
an alveolus increases the concentration of        sharply with the onset of lactation.
dissolved substances (osmotic pressure)             Synthesis of fat. Acetate and butyrate
relative to the other side of the secretory       produced in the rumen are used, in part, as
cells where the blood flows. As a result, the     the building blocks of the short-chain fatty
concentration of dissolved substances on          acids found in milk. The glycerol needed to
each side of the secretory cells is balanced      “unite” three fatty acids into a triglyceride
by drawing water from the blood and               comes from glucose. About 17-45% of the
mixing with the other milk components             fat in the milk is built from acetate and 8-
found in the cavity of the alveolus. For          25% from butyrate. Diet composition has a
normal milk, a balance is reached when            strong influence on milk fat concentration.
there is 4.5 to 5% lactose in the milk. Thus      Lack of fiber depresses the formation of
lactose production acts as a “valve” that         acetate in the rumen, which in turn results
regulates the amount of water drawn into          in the production of milk with a depressed
the alveoli and therefore the volume of milk      concentration of fat (2-2.5%).
produced (crossed circles in Figure 3).             Lipids mobilized from body reserves in
  The effect of the diet on milk production       early lactation are another building block
may be easily seen:                               for milk fat synthesis. However, in general,
  1) The amount of energy (i.e.,                  only half the amount of fatty acids in milk
      concentrates) in the diet influences        fat is synthesized in the udder, the other
      propionate production in the rumen;         half comes from the predominantly long-
  2) The propionate available influences          chain fatty acids found in the diet. Thus
      the amount of glucose synthesized by        milk fat composition may be altered by
      the liver;                                  manipulating the type of fat in the cow’s

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