Introduction to Mammary Gland
Biology and Lactation
Definition of Mammals
• Mammals/Mammalia are a class of
vertebrate, air-breathing animals who are
characterized by the possession of mammary
glands and by hair and/or fur. Some mammals
have sweat glands.
Definition of Mammals Cont.
• Mammals are divided into three main infraclass taxa depending
how they are born. These taxa are: monotremes, marsupials and
placentals. Except for the five species of monotremes (which lay
eggs), all mammal species give birth to live young. Most mammals
also possess specialized teeth, and the largest group of mammals,
the placentals, use a placenta during gestation. The mammalian
brain regulates endothermic and circulatory systems, including a
four-chambered heart.
• There are approximately 5,400 species of mammals, distributed in
about 1,200 genera, 153 families, and 29 orders[1] (though this
varies by classification scheme). Mammals range in size from the
30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter
(108-foot) Blue Whale.
Definition of Mammals Cont.
• Mammals are divided into two subclasses: the Prototheria,
which includes the oviparous monotremes, and the Theria,
which includes the placentals and live-bearing marsupials.
Most mammals, including the six largest orders, belong to
the placental group. The three largest orders, in descending
order, are Rodentia (mice, rats, porcupines, beavers,
capybaras, and other gnawing mammals), Chiroptera
(bats), and Soricomorpha (shrews and moles). The next
three largest orders include the Carnivora (dogs, cats,
weasels, bears, seals, and their relatives), the
Cetartiodactyla (including the even-toed hoofed mammals
and the whales) and the Primates to which the human
species belongs.
Platypus (Monotremes) Development
and Lactation
Wallaby (Marsupials) Development
and Lactation
Mammary Development and Lactation
in the Pig (Placentals)
Skin Adaptations and the Mammary
Gland
• All mammals have mammary glands
• Only mammals have mammary glands
• Mammals can have numerous skin glands
adaptations such as hair follicles, sweat glands,
sebaceous glands, and mammary glands.
• The mammary gland is a highly evolved skin
gland
• Like skin it develops from the ectodermal layer of
the mammalian embryo
Germ Layers of the Developing Embryo
Ectoderm: Skin, Nervous System, Mammary Gland
Mesoderm: Skeleton, Muscle, Kidney, Heart, Blood
Endoderm: Gut, Liver, Lungs
Glands and Secretion
• A gland is a secreting organ
• The secretion may be poured out onto the
surface of the animal, poured into a cavity, or
taken into the blood stream without appearing
externally
• Glands can have a relatively simple structure
(sweat glands) or a more detailed structure
(mammary gland)
• The mammary gland has a compound, branched
tubulo-alveolar structure
Types of Secretion
• Apocrine: Components are synthesized by the
cell in the gland and secreted without
disintegration of the cell itself
• Eccrine: A salty solution is secreted, but cells are
not lost as part of the process
• Holocrine: The secretion consists of the
disintegration of cells of the gland itself
• Merocrine: The gland is repeatedly functional
and the cells are not destroy during secretion
Mammary Secretion
The Mammary Gland
• Has a compound, branched tubulo-alveolar structure
and the merocrine/apocrine mode of secretion means
the secretion (milk) is synthesized and secreted in an
organized manner from epithelial cells in a tissue
consisting of alveoli. The alveoli are connected to a
duct system through which milk flows to the exterior of
the animal. There is considerable external innervation
of the skin covering the gland, but not much internal
gland innervation other than nerves that control blood
vessels. Because it is a skin gland, its blood supply
must leave and return to the body cavity through
points where blood vessels exit the body cavity to
reach the ventral portions of the skin.
Innervation is Not Needed for Mammary
Gland Function and Milk Secretion
Location of Mammary Glands
Number and Location of Complex
Mammary Glands in Various Species
Thoracic
Region
Abdominal
Region
Inguinal
Region
Cattle
-
-
4
Goat, Sheep
-
-
2
Horse
-
-
2
Pig
6
6
4
Cat
4
2
2
Dog
4
4
2
Rat
6
2
4
Mouse
6
-
4
Guinea pig
-
-
2
Elephant
2
-
-
Whale
-
-
2
Human
2
-
-
Species
Variation in Gland Structure Between
Species
Species
Number of Complex Glands
Openings per Teat
Total Simple Gland
Cattle
4
1
4
Goat, Sheep
2
1
2
Horse
2
2
4
Pig
12-14
2
24-28
Cat
8
4-8
32-64
Dog
10
8-22
80-220
Rat
12
1
12
Mouse
10
1
10
Guinea pig
2
1
2
Elephant
2
10-11
20-22
Whale
2
1
2
Human
2
10-20
20-40
Uterine to Birth Transformation
• The fetus is maintained in a sterile, protected,
moist and warm environment and is provided all
necessary nutrients, oxygen and developmental
factors. All of its metabolic, digestive, sensory
functions and waste removal are set up for a
uterine environment.
• At birth, the neonate is exposed to a cold/hot,
dry, non-sterile environment. The neonate
undergoes a dramatic metabolism and growth.
Importance of Milk to the Offspring
• Mammalian species have relatively large and
complex brains and complex body systems
• Development of the offspring is much longer
than most other types of animals
• Part of development of occurs intrauterine,
however much of the development is
extrauterine and is supported by milk
Milk and the Neonate
• Milk supplies everything to the neonate except air.
• The composition of milk is generally balanced for the
nutrient needs for the rapidly developing young of that
species.
• Milk of most species contains a number of protective
factors
• These factors include anti-microbial proteins and lipids
as well as immunoglobulins (antibodies)
• Some of these factors are produced by the mammary
gland and others are taken up directly from the blood
and transported to the milk.
Colostrum and the Survivability of the
Neonate
• Colostrum is the first milk taken from the mammary gland
and typically contains very high levels of immunoglobulins
(antibodies).
• The importance of colostrum to the neonate is determined
by the type of placental structure for most mammals.
• Colostrum is vital for survival of placental mammals that
lack the ability to transfer antibodies via the placenta, but is
also important for other placenta mammals to protect the
gut and add additional immunity.
• Mammals that lack that ability must obtain the initial
“immune” system from colostrum (passive immunity)
Antibody Concentrations in Colostrum
What is Milk?
•
•
•
•
•
•
•
Water
Lactose (Sugar/CHO)
Fat
Protein
Minerals
Vitamins
Other
Nursing Habits and Milk Composition
• A possible relationship exists between nursing habits and
composition of milk. Mammals nursing on demand tend to produce
milk with lower nutrient content than those nurse infrequently.
Three groups of mammals can be distinguished:
1) Mammals that nurse on demand. These include marsupials,
primates, perisodactyls, and some artiodactyls.
2) Mammals that nurse on schedule. These include most rodents,
many carnivores and many artiodactyls.
3) Arctic, aquatic and desert animals. These include whales, beavers,
and seals.
• In general, total solids are highest in group 3, intermediate for
group 2 and lowest for group 1. In group 1, carbohydrate and ash
make up more than half of the solids not fat. In group 3, fat is the
major constituent
Milk Fat Percentage in Various Species
Milk Composition of Various Species
SPECIES
Antelope
Ass (donkey)
Bear, polar
Bison
Buffalo, Philippine
Camel
Cat
Cow:
Ayrshire
Brown Swiss
Guernsey
Holstein
Jersey
Zebu
Deer
Dog
Dolphin
Elephant
Goat
Guinea Pig
Horse
Human
Kangaroo
Mink
Monkey
Opossum
Pig
Rabbit
Rat
Reindeer
Seal, gray
Sheep
Whale
1.3
1.2
31
1.7
10.4
4.9
10.9
6.9
1.7
10.2
4.8
5.9
3.7
11.1
4
6.9
0.5
5.7
4.3
5.1
3.4
1.3
0.45
1.2
0.96
0.8
0.7
---
TOTAL
SOLIDS
%
25.2
10.2
42.9
13.2
21.5
14.4
25.4
4.1
4.0
5.0
3.5
5.5
4.9
19.7
8.3
14.1
15.1
3.5
3.9
1.6
4.5
2.1
8
3.9
6.1
8.2
12.2
14.8
22.5
53.2
5.3
34.8
3.6
3.6
3.8
3.1
3.9
3.9
10.4
9.5
10.4
4.9
3.1
8.1
2.7
1.1
6.2
7
2.1
9.2
5.8
10.4
11.3
10.3
11.2
5.5
13.6
4.7
5.0
4.9
4.9
4.9
5.1
2.6
3.7
5.9
3.4
4.6
3
6.1
6.8
Trace
6.9
5.9
3.2
4.8
1.8
2.9
2.5
2.6
4.6
1.8
0.7
0.7
0.7
0.7
0.7
0.8
1.4
1.2
--0.76
0.79
0.82
0.51
0.2
1.2
0.7
2.6
1.6
0.63
2
1.5
1.4
0.7
0.9
1.6
13.1
13.3
14.4
12.2
15.0
14.7
34.1
20.7
30.4
26.9
12
15.8
11
12.6
9.5
22.6
14.5
24.5
19.9
26.4
31.7
36.7
67.7
16.3
51.2
FAT PROTEIN LACTOSE ASH
%
%
%
%
Relationship Between Milk Yield and
Body Weight in Mammals
Reproduction/Lactation
• Lactation is considered part of the reproductive
process of mammals.
• Mammals reproduce multiple times during there
life and need to provide nutrients to offspring
intermittently
• Because lactation requires so much energy,
continuous lactation is not a favorable concept.
• A mammary gland/lactation that is closely
integrated with the reproductive cycle of the
species is much more efficient.
Lactation Biology Definitions
• Lactation biology-involves the production of milk,
development and function of the mammary gland, the
metabolic and reproductive status of the female, and the
developmental status of the nursing young.
• Mammary glands-are compound, tubulo-alveolar skin
glands that are located on the ventral surface of the animal.
• Milk secretion is the synthesis of milk by the mammary
epithelial cells into the alveolar lumen
• Milk removal is the passive removal of the cisterns and the
ejection of milk from the alveolar lumen.
• Alveoli are the basic milk secreting structures in the
lactating mammary gland.
The Lactation Cycle
• Mammogenesis is the structural development of the
mammary gland.
• Lactogenesis is the start of milk synthesis and secretion and
coincides with the formation of colostrum and parturition.
• Galactopoiesis is the maintenance and enhancement of
lactation.
• Mammary gland involution is the process of the gland
ending lactation and ceasing to produce milk.
• Lactational anestrous a period of inhibition to the normal
reproductive cycle of the female caused by lactation.
Presence and length of a lactational anestrous varies
between species
Linkage between Reproduction and
Lactation in Mammals
A major characteristic of mammalian species is that they employ
lactation as a critical part of their reproductive strategy