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MAMMARYGLAND|Anatomy

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Nickerson SC and Akers RM (2011) Mammary Gland | Anatomy. In: Fuquay JW,Fox PF and McSweeney PLH (eds.), Encyclopedia of Dairy Sciences, Second

Edition, vol. 3, pp. 328–337. San Diego: Academic Press.

ª 2011 Elsevier Ltd. All rights reserved.

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MAMMARY GLAND

Contents

Anatomy

Growth, Development and Involution

Gene Networks Controlling Development and Involution

32

AnatomyS C Nickerson, University of Georgia, Athens, GA, USA

R M Akers, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

ª 2011 Elsevier Ltd. All rights reserved.

Introduction

Dramatic development of the mammary gland duringgestation and subsequent differentiation of alveolar cellsto allow onset of milk synthesis and secretion in precisecorrespondence with parturition is indeed a biologicalmarvel. The initial mammary secretion produced afterparturition is called colostrum. Colostrum and the maturemilk subsequently produced provide the neonate with aspectrum of nutrients and antibodies necessary for goodhealth and early development. Nutritionally, milk of allmammals contains variable amounts of proteins, carbo-hydrates, and fats suspended in an aqueous medium.Thus, milk provides each of the major classes of nutrientsto the neonate. Although there are species differences inmilk composition, having the birth of the offspring andfunctionality of the mammary gland coincide is clearlycritical.

The mammary gland evolved in all mammalian spe-cies to nourish the newborn young. However, in dairyanimals such as the cow, through genetic selection andadvances in milking technology, the mammary gland orudder now yields far more milk than a calf can consumeand far greater quantities than the original organ wasdesigned to accommodate. The selection for greatermilk production and the removal of the product bymachine milking impose unnatural stresses on the bovineudder. Thus, a basic understanding of mammary glandanatomy, supporting structures, milk storage, and theprocesses involved in milk secretion, letdown, andremoval from the udder should aid in the developmentof procedures to efficiently harvest large volumes of milkfrom the mammary gland. For the placental mammals, thenumber of mammary glands varies markedly between

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classes and species. However, among those studied todate, each mammary gland has a teat or nipple. It isnonetheless worth remembering that only a few of theknown mammals have been studied. Because the dairycow is the most important milk-producing animal froman economic standpoint, the following information isprimarily based on the bovine mammary gland.However, some consideration is given to differencesin udder development among dairy ruminants.

Gross Anatomy

Regardless of the specific arrangement or number ofmammary glands for a given mammal, milk synthesisand secretion require development of a functionallymature mammary gland. In reproductively competentanimals, the mature mammary gland consists of a teat ornipple, associated ducts, which provide for passage ofmilk to the outside, and alveoli composed of epithelialsecretory cells and supporting tissues. The epithelial cellsare arranged to form the internal lining of the sphericalalveoli, and these cells synthesize and secrete milk.Secretions are stored within the internal space of thehollow alveoli and larger ducts between sucklingepisodes.

Given the variety of mammals and the environmentalniches occupied, it is no surprise that there is muchvariation in the number of mammary glands, location,and composition of secretions. Unlike common dairy spe-cies (cows, goats, or sheep), aquatic mammals, especiallythose in cold environments, produce milk very high inlipid content with relatively less lactose. High lipid con-tent is essential for the suckling young to rapidly produce

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Table 1 Variation in location, number, and nipple openings of mammary glands of some common species

OrderCommonname

Position of glandsTotalglands

Openingper teatThoracic Abdominal Inguinal

Artiodactyla Cattle 4 4 1

Artiodactyla Goat 2 2 1

Artiodactyla Pig 4 6 2 12 2

Artiodactyla Sheep 2 2 1Carnivora Domestic dog 2 6 2 10 8–14

Carnivora House cat 2 6 8 3–7

Cetacea Whale 2 2 1Lagomorpha Rabbit 4 4 2 10 8–10

Marsupialia Opossum 13 13 8

Marsupialia Red kangaroo 4 4 15

Perissodactyla Horse 2 2 2Primate Man 2 2 15–25

Proboscidea Elephant 2 2 10–11

Rodentia House mouse 4 2 4 10 1

Rodentia Norway rat 4 4 4 12 1

Mammary Gland | Anatomy 329

a layer of insulating fat to protect them from the cold andto provide a source of metabolically derived water. Thisillustrates the relevance of lactation to provide a strategyfor survival of offspring and for securing reproductivesuccess. Table 1 illustrates some of the variations foundin the number and location of mammary glands in somecommon species.

Although the basics of mammary development aregenerally similar among species, the unique anatomy ofthe udder deserves special attention. In the cow and otherruminants, the mammary glands are clustered togetherinto groups of two (goats or sheep) or four (cattle) mam-mary glands to create the udder. This arrangementprovides a practical advantage. Because the mammaryglands and teats are close together, the portion of themilking machine attached to the animal (teat cups andteat cluster) can be relatively compact. For those notfamiliar with milking and management of modern dairycows, the udder of a lactating Holstein cow for examplecan be rather massive. It is not unusual for a single cow toyield 25 kg or more of milk at a single milking. Combinedwith the mass of the udder tissues, this means that theconnective tissue elements and supporting structures ofthe mammary glands have to support as much as 70 kg oftissue and stored milk just before milking. Given the ventralinguinal orientation of the udder, this is no trivial matter.Support is provided by strong, flat suspensory ligaments,which are attached to the pelvic bone and to the strongtendons of the abdominal muscles in the pelvic area.

In the cow and other ruminants, the udder is dividedinto two distinct halves, separated by the medial or med-ian suspensory ligaments, which provide most of thestrength to hold the udder attached to the ventral bodywall. Fibers of the lateral suspensory ligaments are con-tinuous with the median ligaments but spread over either

Encyclopedia of Dairy Sciences, Sec

side of the udder so that the gland appears to be held in asling of connective tissue. The median ligaments aresomewhat elastic but the lateral ligaments are not. Asthe milk accumulates in the udder, the normally verticalorientation of the teats is lost as teats progressively pro-trude laterally. As animals age, excessive degradation ofthe fibers of the median suspensory ligament can reduceits support capacity so that the udder becomes pendulousirrespective of time relative to milking. This can lead toproblems with milking, that is, difficulty maintainingattachment of teat cups as well as problems with teatinjury and increased mastitis risk. The mammary glandsof the udder are directly connected to the abdominalcavity only via passage through the inguinal canals.These are paired narrow oblique passages through theabdominal wall on either side of the midline, just abovethe udder. These canals allow passage of blood and lymphvessels and nerves into the udder.

Interestingly, the two halves of the udder can easily bedissected by cutting along the median suspensory liga-ment, but there are no evident gross anatomical barriersbetween the front and rear glands (quarters) on either sideof the udder; only a thin connective tissue septum ispresent. Additionally, there are no direct connectionsbetween front and rear quarters. This is easily demon-strated following the injection of dye into the teat openingof one of the mammary glands. The dye stains only thetissue of the gland that is injected. This demonstrates thatthe mammary glands of the udder are independent. Thisis sometimes an advantage in some experimental situa-tions since one mammary gland, or more often, one udderhalf, can be given an experimental treatment with theopposite side serving as a control. This of course is rele-vant only if treatments can be shown to have only localeffects.

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Figure 1 Diagrammatic cross section of the four quarters ofthe udder illustrating the gross anatomy.

330 Mammary Gland | Anatomy

The surface epidermis of the udder is composed of astratified squamous epithelium and is covered with finehair; however, the teats are hairless. Although the foreteats are usually longer than the rear teats, the capacity ofthe rear quarters is greater than that of the fore quarters;

the ratio is approximately 60:40 (Figure 1).The dairy goat and dairy sheep industries in the United

States are not extensive, but in many parts of the world,these ruminants provide a much greater portion of the milk

and dairy products to the local economy than dairy cows. Inthe case of dairy sheep, international protocols for evalua-tion of the udder were developed in the early 1980s. Usingstandardized protocols, the udder structure and develop-ment in many dairy breeds have been systemically studied,especially relative to machine milking. Milk productionand milk composition are of course critical elements, butudder shape, teat length and size, and ease of machine

milking in sheep are also important. Comparisons of exter-nal udder morphology and typology are used to standardizegroups of ewes for machine milking, choice of animals tocreate a milking flock, or for culling of breeding animals.A number of researchers have suggested that an idealudder of a lactating dairy sheep should have the followingcharacteristics: (1) large volume with a globular shape andclearly defined teats; (2) soft and elastic tissues, with an

evident, palpable gland cistern; (3) moderate udder height,no lower than the hock; (4) an apparent demarcation orgroove between udder halves; and (5) teats of medium size(length and width), oriented in a nearly vertical position.When morphological traits are related to milk production,udder width and height are usually positively correlatedwith milk yield, confirming the importance of uddervolume to milk yield. Interestingly, mammary cistern cav-

ity size in some dairy sheep is nearly as large as that in cows,but of course, total gland size is much smaller than that incows. This does, however, suggest that a proportionallygreater amount of the milk obtained at milking for dairy

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ewes comes from cisternal storage rather than the alveolarstorage. Differences in udder anatomy reflect greater dailymilk yields in Lacaune (1.9 l day�1) sheep compared withManchega sheep (0.9 l day�1). For example, cisternal milkvolume and tissue area are more than doubled in Lacauneewes but alveolar milk yields are essentially identical.This suggests that differences in udder anatomy areimportant determinants of lactation performance, and inthis case, the cistern capacity is especially important.Moreover, it may be that the relative need for oxytocinrelease at milking might differ among dairy sheep breedsor perhaps among various dairy ruminants depending onthe proportion of milk obtained from alveolar comparedwith cisternal storage.

Patterns of milk flow during an individual milking canbe characterized as occurring in 1, 2, or 3, or more peaks.However, there seems to be little if any relationshipbetween the number of peaks and total milk yield.There are also apparent differences in the patterns ofoxytocin release among animals within breed as well asaverage differences in oxytocin release among breeds inresponse to machine milking. Regardless, it is difficult todefine an optimal pattern of oxytocin release since someanimals in both high-yielding and low-yielding breedsshow minimal secretion of oxytocin but apparently nor-mal milk yields. On the other hand, it is generallyaccepted that the volume of milk obtained during theprimary phase of machine milking (prior to stripping) isgreater in animals with more oxytocin release and if thereis a bimodal release of oxytocin. These observations sim-ply indicate that relationships among udder anatomy(alveolar vs. cisternal space), effectiveness of udder sti-mulation or milking to cause oxytocin release, andlactation performance are complex. Considering differ-ences in the degree of selection for milk yield amongbreeds and differences among dairy ruminants (cows,goats, sheep, and camels), this finding is hardly surprising.

In the dairy cow, each udder half is nearly independentand has its own vascular system, nerve supply, and suspen-sory apparatus. Teats vary in shape from cylindrical toconical, and teat length is extremely variable. Both char-acteristics are independent of the shape or size of the udder.The teat skin is thin and devoid of sebaceous glands;however, supernumerary teats are commonly found,mainly on rear quarters. It has been estimated that 40%of cows have one or more supernumerary teats, which tendto be nonfunctional and should be removed because theycan become infected with mastitis-causing bacteria.

Supporting Structures

The mammary gland is attached to the cow’s body wallunder the pelvis by several strong, flat ligaments.Median and lateral suspensory ligaments provide the

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main support by forming a sling for the udder. The

median suspensory ligaments are attached to the pelvic

bone and to the tendons of the external oblique abdo-

minal muscles in the region of the pelvis. These

ligaments run parallel to each other and pass ventrally

between the two udder halves, forming the intermam-

mary groove at the ventral surface, which separates the

right and left quarters. These two layers of ligaments are

joined by loose areolar connective tissue. The ligaments

then separate to cover the anterior, posterior, and ventral

areas of the glandular tissue on each udder half but

terminate at the base of the teats. The two median

suspensory ligaments fuse with the two lateral suspen-

sory ligaments at the anterior, posterior, and ventral

borders of each udder half (Figure 2).The lateral suspensory ligaments of each udder half

originate at the subpubic and prepubic tendons of the

body wall and travel vertically, covering the outer sides

of the mammary gland. Both the median and lateral

suspensory ligaments have lateral branches (lamellar

plates) that are inserted into the glandular tissue and

become continuous with the connective tissue stroma

supporting the lobules and lobes of parenchyma. The

median ligaments are composed of both yellow elastic

and fibrous connective tissues. Because of the elastic

fibers, this ligament will stretch to absorb the shock as

the cow moves about. In addition, the elasticity of these

ligaments allows for the increase in udder size between

milkings. As a cow matures and the udder increases in

weight, the median suspensory ligaments often stretch,

weaken, and lose tone, allowing teats to point outward.

The lateral ligaments are mainly composed of white

fibrous connective tissue (collagen) and do not stretch as

much; hence, they provide support in the absence of

much elasticity. If both the median and lateral suspensory

ligaments weaken, the udder becomes pendulous and is

vulnerable to injury and mastitis.

Figure 2 Diagram of longitudinal section of the cow’s udder

illustrating gross structure and suspensory apparatus.

Encyclopedia of Dairy Sciences, Sec

Microscopic Anatomy

Synthetic and Secretory Tissues

Each of the four quarters functions as a separate gland

within the udder and has its own milk secretory (parench-

ymal) tissues. The parenchyma is composed of alveoli,

ducts, and connective tissue; the connective tissue sup-

ports and protects the delicate synthetic tissues. The

millions of alveoli are the milk-producing units of the

udder (see Mammary Gland: Growth, Development and

Involution). These are microscopic globe-like structures

that are 50–250 mm in diameter, depending upon the

volume of accumulated milk. A single layer of cuboidal

to columnar epithelial cells lines the peripheral borders.

Milk component precursors are absorbed from blood

capillaries adjacent to the alveoli by mammary epithelial

cells and are converted into milk protein, lactose, and

butterfat. These components are released with other

milk components into the lumen or interior of the alveo-

lus for storage between milkings (Figure 3).As milk accumulates in the alveolar lumenal spaces

between milkings, the pressure on the epithelial lining

causes the secretory cells to become flattened. This sig-

nals the cells to stop synthesizing milk and releasing it

into the lumen. In addition, capillaries surrounding the

alveoli collapse because of the expanding lumenal space,

and the supply of milk precursors is reduced. Just prior to

milking, approximately 60% of the milk synthesized by

the udder is held in the alveoli and small ducts, and 40%

is stored in the cisterns and large ducts. After milking, the

alveolar lumena are no longer filled with milk, and secre-

tory cells assume a columnar shape as the alveolar lining

collapses; capillaries also assume their normal shape. A

network of smooth muscle cells called myoepithelial cells

immediately surrounds each alveolus. Myoepithelial cells

also surround the small ducts, running in a lengthwise

direction, and upon contraction they shorten the ducts,

thereby increasing the diameter of the ductal lumena,

which permits maximum milk flow.The alveolar epithelial cells, limited by a cell mem-

brane, contain the organelles necessary to convert

precursors absorbed from the blood into milk constitu-

ents. The interior of the cell is composed of cytoplasm in

which organelles such as the nucleus, rough endoplasmic

reticulum, mitochondria, and Golgi apparatus are dis-

persed. The portions of the cytoplasm adjacent to the

basement membrane and near the nucleus are occupied

by parallel cisternae of rough endoplasmic reticulum

(ergastoplasm). The Golgi apparatus is located in a supra-

nuclear position, between the nucleus and apical cell

membrane, and is composed of parallel cisternae of

smooth-surfaced endoplasmic reticulum with terminal

swellings that pinch off as casein-containing secretory

vesicles. Butterfat droplets and secretory vesicles

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Figure 3 (a) Diagram of a mammary quarter illustrating the glandular tissue (alveoli are drawn out of scale), ducts, gland and teatcisterns, and teat canal. (b) Diagrammatic cross section of an alveolus illustrating mammary epithelial cells, myoepithelial cells, and

capillary network.

Figure 4 Diagram of an alveolar epithelial cell typical of the

lactating bovine mammary gland illustrating an extensive rough

endoplasmic reticulum (R), secretory vesicles (S) andnumerous casein-containing Golgi secretory vesicles (G)

typical of the active milk-producing cell. Other structures

include mitochondria (M), microtubules (Mt), nucleus (N),

microvilli (Mv), and myoepithelial cells (My). The casein micelles(Cm) and lipid droplets (L) are synthesized within the cell

cytoplasm and released into the alveolar lumen for storage

between milkings.

332 Mammary Gland | Anatomy

populate the apical cytoplasm, and microtubules are

oriented perpendicular to the plasma membrane to

guide the flow of secretory products toward the alveolar

lumen. Mitochondria and free ribosomes are found

throughout the cytoplasm (Figure 4).Milk protein, most of which is casein, is composed of

amino acids that are taken up by cells from the blood.

Casein is synthesized in the rough endoplasmic reticulum

and transported to the Golgi apparatus, where it is con-

centrated and packaged in secretory vesicles for export

from the cell to the alveolar lumen. Lactose is synthesized

in the Golgi apparatus and is secreted from the cells in the

same vesicles that transport casein. Calcium, magnesium,

and other ions are also secreted via secretory vesicles

originating from the Golgi apparatus. Butterfat is synthe-

sized in areas of the cytoplasm occupied by rough

endoplasmic reticulum. The size of fat droplets increases

from the basal to apical cytoplasm, and many small dro-

plets probably coalesce to form larger droplets. During

secretion, the droplets push through the apical cell mem-

brane and are pinched off and released into the lumen,

with each droplet limited by a unit membrane that origi-

nated from the apical cell membrane. For a more

complete discussion of component synthesis and secretion

as well as an electron micrograph of a lactating cell, see

Mammary Gland, Milk Biosynthesis and Secretion:

Secretion of Milk Constituents.

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Figure 5 Diagrams of longitudinal sections of the teathighlighting the teat canal keratin (a) and sphincter muscle (b).

Mammary Gland | Anatomy 333

The alveoli are drained by small ducts that possesssome synthetic activity. Some alveoli have a commonopening into a duct, or they may open directly into otheralveoli. A cluster of alveoli separated from other clustersof alveoli by fibrous connective tissue is referred to as alobule, and the ducts draining alveoli converge into acommon larger intralobular duct. A cluster of lobulesforms a lobe that is drained by a common interlobularduct, and the lobes make up the glandular tissue of aquarter. Each lobe is surrounded by fibrous connectivetissue to separate it from other lobes. Within each lobe,the intralobular ducts merge to form a single intralobarduct, which becomes the interlobar duct as it emergesfrom the lobe. This combination of alveoli and the tub-ular ducts supported in a connective tissue framework(stroma) classifies each quarter as a tubulo-alveolargland.

The ducts draining lobes of milk-producing tissues arecomposed of a double-layered epithelium and are sur-rounded by myoepithelial cells. These ducts convergeinto larger ducts that eventually drain into the collectingspaces (cisterns) near the ventral surface of the quarter.From 5 to 20 large ducts empty into the gland cistern ofthe udder. Gland cisterns are extremely variable in sizeand shape within an udder, and hold from 100 to 2000 mlof milk. The shape of the gland cistern ranges from aspherical hollow cavity to one composed of folds or divi-sions, exhibiting a honeycomb appearance. A double-layered epithelium forms the lining of the gland cistern,and lobes of secretory tissue are found immediately adja-cent to the lining.

Teat

The gland cistern empties ventrally into the teat cistern,and, at their union, there may be a slight constrictionknown as the annular fold. The teat cistern is also linedby a double-layered epithelium; however, the superficial(lumenal) epithelial cells are more columnar than cuboi-dal, and the basal cells are smaller and cuboidal. Thiscistern holds 10–50 ml of milk, and the surface structurevaries greatly. It may be smooth or it may exhibit long-itudinal and horizontal folds, giving a pocketed orhoneycombed appearance. Lobules of secretory tissueare sometimes present adjacent to the teat cistern lining,which drain directly into the teat cistern (Figure 5).

The teat cistern terminates distally at the teat canal,the opening through which milk is removed. The teatcanal terminates distally at the teat meatus or orifice.Just above the union of the teat cistern and teat duct,the 6–10 longitudinal folds of the cistern lining convergeto form Furstenberg’s rosette. The tissue folds appear toprovide no mechanical function in preventing milk leak-age as previously theorized. The increased epithelial

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surface area and connective tissue stroma provided bythe folds, however, appear to recruit protective leukocytepopulations, especially lymphocytes and plasma cells,which may function in the local defense against mastitis-causing organisms.

Teat Canal

The teat canal is 5–13 mm in length and averages about8.5 mm. The diameter ranges from 0.4 mm at the distalend to 1.63 mm at the proximal end and averages 0.46 mmat its midportion. With advanced lactation age, the teatcanal lengthens and increases in diameter. At the union ofthe teat cistern and teat canal at Furstenberg’s rosette, thedouble-layered epithelium abruptly changes ventrally toa stratified squamous epithelium, which is continuouswith that of the outer teat skin. Continued desquamationof the cells surrounding the teat canal lumen results in theformation of keratin, which occludes the canal lumenbetween milkings, serving as a barrier to bacterialpenetration.

If keratin is lost or removed, the effective barrier iscompromised, and the teat canal may be unable to resistbacterial invasion. For example, if the milk flow-inducedshear stress is excessive because of prolonged machinemilking time, excessive vacuum, or improper pulsation,some of the keratin may be lost. In addition, the keratinbarrier may be compromised by the method of infusingantibiotics into a mammary quarter to treat mastitis. Fullinsertion of the antibiotic treatment syringe cannula maypush portions of keratin colonized by bacteria into theteat cistern and induce an intramammary infection inaddition to the one for which therapy was directed. Inaddition, keratin could be forced against the interior teat

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duct wall by the syringe cannula, creating a larger thannormal opening, thereby enhancing bacterial penetration.The conventional syringe cannula averages 3.1 mm indiameter, and teat duct diameters range from 0.46 to1.63 mm for distal through proximal portions of the duct.Full insertion of a commercial cannula can result intemporary dilation of the duct lumen beyond the normaldiameter (up to 8 times). Likewise, tissue trauma causedby full insertion of the cannula may cause gaps or spacesin keratin, providing areas in which bacteria can adhereand colonize. A comparison of histological cross sectionsof teat ducts that were inserted with a syringe cannula bypartial or full insertion revealed that teat ducts insertedpartially had a thicker keratin layer compared with teatducts infused by full insertion. The latter exhibited partialloss of keratin, which decreased resistance to intramam-mary infection. Commercial syringes are now available toaccommodate partial insertion by providing a twist-offtip, which when removed allows the protrusion ofapproximately 3.0 mm of the syringe cannula, and at thesame time, forms a seal with the teat orifice to providesupport during infusion and to ensure upward movementof the antibiotic.

The teat canal is surrounded by bundles of smoothmuscle fibers. Fibers are arranged longitudinally imme-diately adjacent to the epithelial lining and in a circularfashion around the canal deeper in the connective tissue.The circular smooth muscles in their contracted statefunction to maintain tight closure of the canal betweenmilkings to prevent leakage and to keep keratin occludingthe canal lumen compressed as an aid in preventingbacteria from progressing upward into the teat cistern.Teats with weak, relaxed, or incompetent circular smoothmuscle bundles (sphincters) are termed ‘patent’ or ‘leaky’.Cows having such teats milk out in 2–3 min, but theincidence of mastitis is higher in quarters with patentteat canals. Cows having teats with tight sphincters arecalled ‘hard milkers’ because milk is expressed as a finespray and milk flow is very slow, thereby extendingmilking time.

Figure 6 Diagram of the arteries supplying the udder with

blood.

Vascular System

Arterial Supply

The vascular system reaches the udder via the right andleft inguinal canals in the abdominal wall. Arterial bloodfrom the heart is supplied initially through the posteriordorsal aorta, which becomes the abdominal posteriordorsal aorta after entering the abdominal cavity. Thisvessel runs parallel to the vertebral column until itreaches the sixth lumbar vertebrae, and then it divergesinto the right and left iliac arteries, which in turndiverge into the internal and external iliacs. The exter-nal pudendal or mammary artery arises from the

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external iliac and passes through the inguinal canal tothe dorsal surface of the udder. Upon emerging from theinguinal canal, the mammary artery and the associatedmammary vein follow a tortuous route forming anS-shaped curve. This allows for the lengthening of theblood vessels as the median suspensory ligaments stretchto accommodate the full and distended udder that gravit-ates downward.

The mammary arteries enter the right and left halvesof the udder just anterior to the rear teats and divideinto the anterior and posterior mammary arteries,branching into arterioles that supply the fore and rearquarters, respectively. The subcutaneous abdominalartery usually arises from the mammary artery beforeit divides into the anterior and posterior branches. Thisartery supplies blood to the anterior dorsal portion ofeach side of the udder. The anterior and posteriormammary arteries spread vertically through the paren-chyma of the fore and rear quarters of each side,respectively, and divide, ultimately terminating in capil-laries that form a network surrounding the alveoli(Figure 6).

The mammary arteries also give rise to the papillaryarteries of the teats. The vascular tissues of the teatcomposed of the papillary arteries and venous plexis arecollectively termed the corpus cavernosum. The right andleft udder halves generally have their own arterial supply;however, some small arterial connections pass from onehalf to the other. Blood also reaches the udder, to a lesserdegree, via the cranial epigastric and perineal arteries thatsupply, in part, the anterior and posterior portions of theudder, respectively. The arterial blood flow pathwayleads from the heart to the udder.

The primary purpose of the arterial system is to pro-vide a continuous supply of nutrients to the milk-synthesizing cells so as to produce milk. The arterialvessels have heavy muscular walls, which aid in drivingblood away from the heart to peripheral tissues. In a500 kg cow, about 71 000 l of blood flow through the

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udder each day. Approximately 400 volumes of blood pass

through the mammary gland to produce 1 volume of milk.

Venous Drainage

After passing through the capillaries surrounding the

alveoli, and the interchange between blood and tissue

fluids takes place, blood reaches the small veins or

venules. These venules run in a dorsal direction and

unite to form the larger mammary veins at the base of

the udder, forming the venous circle. Veins have thin

connective tissue walls and exhibit little change in dia-

meter because venous pressure does not vary greatly.

Papillary veins of the teat also course upward to meet

the mammary veins and converge upon the venous circle

at the base of the udder. The external pudendal vein

follows the course of the external pudendal artery, passes

through the inguinal canal, and becomes the external iliac

vein, which then drains into the posterior vena cava

(Figure 7).Anterior extensions of the mammary veins on both

sides of the udder are the very prominent and turgid

subcutaneous abdominal veins, also known as milk veins

in the mature lactating cow. These travel along the

ventral surface in a rather tortuous route under the

skin but exterior to the abdominal wall. The two veins

from each side form an anastomosis in front of the udder

and enter through the rectus abdominis muscle near the

breast bone to become the internal abdominal veins.

They penetrate the diaphragm to become the internal

thoracics, which drain into the anterior vena cava. The

two main routes by which blood exits the mammary

gland are the external pudendal and the subcutaneous

abdominal veins. Approximately two-thirds of the blood

exits the udder via the external pudendal veins and one-

third exits via the subcutaneous abdominal veins. Some

blood may leave the rear quarters via the perineal veins.

The pathway of venous blood flow is from the udder to

the heart.

Figure 7 Diagram of the veins draining blood from the udder.

Encyclopedia of Dairy Sciences, Sec

Lymphatic System

Interstitial fluids originating from capillaries that nourishmammary parenchymal cells recirculate via the lympha-tic system, which carries waste products away from theudder. The composition of lymph is similar to bloodplasma but has half the protein and no red blood cells.Lymph vessels are very thin walled and begin as smallcapillaries dispersed among the connective tissues of theteat and milk secretory parenchyma. These small vesselsconverge upon larger lymphatics toward the dorsal por-tions of the udder, terminating at the supramammarylymph nodes on the right and left halves of the mammarygland. These nodes are located dorsal to the rear quarters,and each side of the udder may have from one to threenodes. The nodes serve as filters that remove or destroyforeign substances and also provide a source of lympho-cytes to fight infection. Lymph is filtered through thenodes by entering at the peripheral border, passingthrough a network of sinuses, and exiting at the hilus vialarge vessels that pass through the inguinal canal. Vesselsmay then branch, and the fluid is passed through theinguinal, iliac, and prefemoral lymph nodes before joiningthe lumbar lymph trunk. The fluid continues to the thor-acic duct and empties into the anterior vena cava(Figure 8).

Movement of lymph in vessels of the udder is alwaysin a dorsal direction, toward the supramammary lymphnodes. When the udder becomes edematous during theperiparturient period, the udder surface can be mas-saged in the direction of the supramammary lymphnodes as an aid in alleviating the subcutaneous buildupof lymphatic fluids. Lymphatic vessels are equippedwith one-way valves to maintain the direction of flow;however, movement is slow because there is no pump tocirculate the fluid. The forces behind lymph flowinclude muscle movement, swaying of the udder as thecow moves about, and breathing; for example, with eachinspiration, lymph is drawn forward and emptied intovena cava.

Figure 8 Diagram of the mammary lymphatic system. 1, vena

cava; 2, thoracic duct; 3, cisterna chyli; 4, lacteals; 5, lumbartrunk; 6, internal iliac node; 7, inguinal node; 8, external iliac

node; 9, prefemoral node; 10, supramammary lymph node;

11, mammary lymphatics.

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Figure 9 Diagram of the nerves supplying the udder.

336 Mammary Gland | Anatomy

Nervous System

The major nerves of the udder are the sensory nerves

that carry impulses from the four quarters to the brain.

Other nerves are sympathetic and are composed of

motor fibers to the smooth muscles of arterial walls

and those of the teat sphincter. These fibers control

the rates of blood flow through the udder by regulating

the diameter of arteries and are involved in inhibiting

the milk ejection reflex.The main spinal nerves are the first, second, third, and

fourth lumbar nerves and the external spermatic nerves,

which become inguinal nerves as they pass through the

inguinal canal; these nerves are distributed to the glands

and skin via anterior and posterior fibers. The first lumbar

nerve supplies the anterior portion of the udder but does

not innervate the parenchyma. The second lumbar nerve

joins the third lumbar nerve, which fuses with the second

and fourth lumbar nerves, composing the inguinal nerve.

The perineal nerve, derived from the second, third, and

fourth sacral nerves, feeds the caudal portion of the udder.

Afferent fibers of the inguinal nerve send signals from the

udder to the spinal cord and brain, while the efferent

fibers send signals from the brain and spinal cord to the

udder via the ventral root ganglia.Each quarter is supplied with nerves terminating in the

dermis of the udder skin and teats, which lead to the

spinal column and brain. Innervation of the udder is

greatest in the dermis of the teats where pressure-sensi-

tive receptors have been identified. These terminal

endings are sensitive to physical stimuli such as pressure,

touch, and stretching, and they tend to be more numerous

at the proximal end of the teat and close to the surface.

The precise nature of the nerve endings has not been

established, but the highly specialized sensory nerve end-

ings present in the teats of some species have not been

documented in the cow. Impulses travel via afferent fibers

through the mammary nerves to the inguinal nerve,

which courses through the inguinal canal to the second,

third, and fourth lumbar nerves, and the dorsal roots of

these nerves carry the afferent signal along the spinal cord

to the brain (Figure 9).Nerves also arise from the spinal column and termi-

nate in the muscles of the teat and arteries. The circular

smooth muscle bundles surrounding the teat canal

undergo continuous rhythmic contractions between milk-

ings via impulses from the sympathetic nervous system.

When these nerves are severed or blocked, the cow tends

to leak milk. During milking, impulses from the brain and

spinal cord cause the muscle bundles to relax, allowing

the teat canal to dilate for the flow of milk.The nervous system has no direct involvement in the

synthesis and secretion of milk or in milk removal (ejec-

tion) from the udder. These processes are controlled

Encyclopedia of Dairy Sciences, Seco

directly by hormones circulating in blood. However, thenervous system is essential to the milking process itselfbecause it triggers the mechanisms of hormone releasefrom the brain to the mammary tissue.

See also: Mammary Gland: Growth, Development and

involution. Mammary Gland, Milk Biosynthesis and

Secretion: Secretion of Milk Constituents. Milk Quality

and Udder Health: Test Methods and Standards.

Further Reading

Akers RM (ed.) (2002) Mammary development, anatomy, andphysiology. In: Lactation and the Mammary Gland, pp. 45–65, Ames,IA: Iowa State Press.

Akers RM and Denbow DM (eds.) (2007) Lactation and animal agriculture.In: Anatomy and Physiology of Domestic Animals, pp. 475–500. Ames,IA: Blackwell Publishing.

Anderson R (1985) Mammary gland. In: Larson BL (ed.) Lactation,pp. 3–38. Ames, IA: Iowa State University Press.

Blowey R and Edmonson P (eds.) (1995) Structure of the teats andudder and mechanisms of milk synthesis. In: Mastitis Control in DairyHerds, pp. 5–16. Ipswich: Farming Press Books.

Bruckmaier RM and Blum JW (1998) Oxytocin release and milk removalin ruminants. Journal of Dairy Science 81: 939–949.

Caja G, Such X, and Rovai M (2000) Udder morphology and machinemilking ability in dairy sheep. In: Proceedings of the 6th Great LakesDairy Sheep Symposium, pp. 17–40. Guelph, Ontario, Canada.

Cowie AT and Tindal JS (1971) The Physiology of Lactation,pp.185–282. Baltimore, MD: Williams & Wilkins Baltimore, MD:Williams & Wilkins. pp. 185–282.

Giesecke WH, du Preez JH, and Petzer IM (eds.) (1994) Mammarystructure and function. In: Practical Mastitis Control in Dairy Herds,pp. 9–20. Durban: Butterworths

Larson BL (ed.) (1985) Lactation, pp. 39–79. Ames, IA: Iowa StateUniversity Press.

National Mastitis Council (ed.) (1996) Development of mastitis. In:Current Concepts of Bovine Mastitis, pp. 15–19. Madison, WI:National Mastitis Council, Inc.

Nickerson SC (1992) Anatomy and physiology of the udder.In: Bramley AJ, Dodd FH, Mein GA, and Bramley JA (eds.) MilkingMachine and Lactation, pp. 37–68. Burlington, VT: Insight Books.

Nickerson SC (1995) Milk production: Factors affecting milkcomposition. Harding F (ed.) Milk Quality, pp. 3–24. London: BlackieAcademic & Professional.

Schalm OW, Carroll EJ, and Jain NC (eds.) (1971) Gross andmicroscopic structure of the bovine mammary glands. In: BovineMastitis, pp. 36–47. Philadelphia, PA: Lea & Febiger.

Smith VR (ed.) (1964) Anatomy of the udder. In: Physiology of Lactation,pp. 16–49. Ames, IA: Iowa State University Press.

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Swett WW (1942) Arrangement of the tissues by which the cow’s udderis suspended. Journal of Agricultural Research 65: 19–22.

Tanhuanpaa E (1995) Anatomy and physiology. In: Sandholm M,Honkanen-Buzalski T, Kaartinem L, and Pyorala S (eds.) The BovineUdder and Mastitis, pp. 7–13. Jyvaskyla, Finland: GummerusKirjapaino Oy.

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Tucker HA (1985) Endocrine and neural control of the mammary gland.In: Larson BL (ed.) Lactation, pp. 39–79. Ames, IA: Iowa StateUniversity Press.

Turner CW (ed.) (1973) Construction of the udder. In: Harvesting YourMilk Crop, pp. 3–27. Oak Brook, IL: Babson Bros. Dairy Researchand Educational Service.

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