ANSCI 308

Lactation Biology

Comparative Mammary Gland Anatomy

W L Hurley
Department of Animal Sciences
University of Illinois
Urbana-Champaign


This lesson covers:

Comparative Mammary Gland Anatomy

Monotremes

Marsupials

Eutherian Mammals

General Observations
Goats and Sheep
Horses
Pigs
Human Breast Anatomy

Mammary Gland Location

 

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Monotremes

- Monotremes are egg laying mammals. They are considered to have the most primitive mammary glands.


Duck-billed Platypus (Ornithorhynchus):

Lays eggs and hatches them in an underground tunnel much like a bird. The eggs hatch in 10-14 days. Young live entirely on milk for 3 4 mos. The mother has no nipples; milk extrudes from 100-150 separate gland tubes that open at the base of a stiff hair. The glands are paired (left and right sides). Secretory tubes have 2 cell layers, an inner secretory layer and an outer contractile layer. There is no internal storage of milk. Milk is secreted onto the hairs and is lapped off by the young.

Porcupine (Australian) anteater (Tachyglossus aculeatus):

Sometimes called the echidna or spiny anteater. Lays eggs which are transferred to a pouch in the ventral abdominal area. Mammary glands are located in the pouch, under the abdominal surface of the skin. She has no nipples. Milk oozes from several ducts into indentations in the skin where it is lapped up by the young. There is no storage reservoir. There is a milk ejection response.


Marsupials


- In marsupials, the young are born after a short gestation: the mother has a placenta but it is not considered a true placenta.

Tammar Wallaby - a kangaroo (Macropus eugenii)


Eutherian Mammals

General Observations

Below is a brief description of the mammary anatomy of goats, sheep, horses and pigs. These should be compared with the more detailed description of mammary anatomy of cattle, as described above.

First, some generalizations and observations:


Goats and Sheep

The lactating goat (nanny) and sheep (ewe) each have two glands, each drained by a single teat with a single streak canal. They are located in the inguinal region. The teats vary considerably in size. Goat teats and udder are generally larger than the sheep's. The nanny’s teats are wide at the base and protrude like a funnel from the udder (no specific point of connection). There are fine hairs on the teats. Ewes have short cylindrical teats more similar to those of a cow. The mammary blood and lymph systems for goats and sheep are similar to the cow's.


Horses

Glands are located in the inguinal region. Blood and lymph systems are like a cow's. There are two teats that are flat, broad and blunt, each with two streak canals and two teat cisterns. Each cistern leads to a separate gland. Therefore, the mare has 4 mammary "quarters" like a cow, but only 2 teats, each with 2 streak canals (like the pig).


Pigs

Sows average 12-14 complex glands (6-7 on each side), but the range may be 6-32. Glands are in two parallel rows, one on each side of the ventral midline. They may or may not be spatially paired (left to right). The number on each side may be even or odd. Each visible gland is composed of 2 simple glands, each with a streak canal in the teat; therefore the pig’s teat has 2 orifices which are aligned anterior-posterior in the teat end. Each simple gland is separate, that is the secretory tissue is independent from the adjacent gland. Heritability of teat number is low (.1-.2). Also, there is little relationship between teat number and maternal performance. Nevertheless, gilts are still chosen for breeding based partially on the number of teats. Most breed associations require 12 functional teats for pure bred registry.

Sows can have supernumerary teats. Teats usually have 2 streak canals, but may have 3-4. "Inverted nipples" is an inherited condition where the nipples fail to project from the surface of the gland. This makes it impossible to suckle that gland, even though the sow begins to lactate (the secretory tissue is normal); and the gland soon involutes because there is no milk removal. This condition can be on one or any number of nipples. Blood is supplied to the glands by two major arterial supplies. The external pudic (similar to cow) which passes forward and the external thoracic which passes backward. Venous drainage is parallel to the arteries.

Milk production of a sow is about 6-10 lbs/day, reaching a maximum at 2-4 weeks of lactation. She will produce ~500 lbs/lactation. Piglets suckle about every 45 min to one hour. The anterior glands generally produce the more milk than the posterior glands.


Human Breast Anatomy

he secretory apparatus of the human breast consists of approximately 10 to 15 ducts extending from the nipple, and coursing through the mammary fat pad to terminate in clusters of alveoli. Each duct serves a specific lobule. Lobules are separated and supported by thick connective tissue septae and, in the breast of the non-pregnant, non-lactating woman, by large amounts of adipose tissue. Blood vessels, nerves and lymphatics run in the septae which merge imperceptibly with the fascia at the anterior thoracic wall.

The nipple, which serves as the termination point for the lactiferous ducts, is surrounded by an area of pigmented skin, called the areola, which contains sebaceous glands (Montgomery's follicles) and sweat glands. The areola serves as the termination point for the fourth intercostal nerve which carries the sensory information about suckling to the spinal cord and brain. This is extremely important in the regulation of secretion of oxytocin from the posterior pituitary and prolactin from the anterior pituitary. The mammary ducts expand slightly to form sinuses beneath the areola. The entire areola is positioned in the infant's mouth forming a teat that extends nearly to the soft palate. During sucking, milk removal is accomplished not so much by suction as by the stripping motion of the infant's tongue against the hard palate and "milking" the contents of the sinuses out of the openings in the nipple.


Location of Mammary Glands

Mammary glands are invariably located ventrally and lateral to the midline on all species. However, there is considerable variability in the number of glands and location along the midline among mammalian species.

Generally species can be grouped by mammary gland location according to:

Anterior glands: include primates, elephants, seacows, bats

Posterior glands: include ungulates, whales

Glands extending anterior to posterior: include litter bearing species (pigs, rats, mice, etc.).

 

Comparison of Mammary Glands from Various Species

A simple gland is defined as the mammary tissue from which the milk empties through a single opening or orifice at the surface.

A complex gland is defined as the gland(s) which empty from a single teat or nipple.

Teats or nipples may have multiple openings, each draining a functionally separate simple gland.

 


Species

Complex Glands

Thoracic
Region

Abdominal
Region

Inguinal
Region

Openings per Teat

Total Simple Gland

Cattle

4

-

-

4

1

4

Goat, Sheep

2

-

-

2

1

2

Horse

2

-

-

2

2

4

Pig

12-14

6

6

4

2

24-28

Cat

8

4

2

2

4-8

32-64

Dog

10

4

4

2

8-22

80-220

Rat

12

6

2

4

1

12

Mouse

10

6

-

4

1

10

Guinea pig

2

-

-

2

1

2

Human

2

2

-

-

10-20

20-40


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