AAP/mammary gland/sem2 1 Mammary Gland Anatomy External ...

Mammary Gland Anatomy External Anatomy The udder of the cow consists of 4 separate glands located in the inguinal region of the ventral or underside of the cow. Each gland has one teat and each teat has one opening. The glands are covered with hair, but the teats do not have hair. The right and left halves are entirely separate (externally this is indicated by the intermammary groove seen at the underside of the udder). The rear quarters account for 55-60% of the milk produced and 55-60% of udder weight. Rear teats are usually shorter than the front teats. Teats - (papilla mammae) The teat functions as the only exit for the mammary secretion and the only means for the calf to receive milk. Usually, only one teat drains one gland. No hair, sweat glands or sebacious glands are found on the teats of the cow. Teat size and shape is independent of the size, shape or milk production of the udder. Average size for the fore teats is about 6.6 cm (2.6 in.) long and 2.9 cm (1.1 in.) in diameter, and for the rear teats is 5.2 cm (2.1 in.) long and 2.6 cm (1.0 in.) in diameter. Supernumerary Teats About 50% of all cows have extra teats, referred to as supernumerary teats. Some of these extra teats open into a "normal" gland, but many do not. Generally they are removed before 1 yr of age. A pseudo-teat has no streak canal, and therefore, no connection to the internal structures of the gland.

Streak canal (ductus papillaris) Functions as the only orifice of the gland between the internal milk secretory system and the external environment. The streak canal is the main barrier against infection. It is lined with a skin-like epidermis. The streak canal is kept closed by sphincter muscles around the streak canal. Canal patency decreases and streak canal length increases with increasing lactation number. Furstenburg's Rosette - These are mucosal folds of the streak canal lining at the internal end of the canal. It may fold over the canal opening due to pressure when the udder is

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full. It may be a major point of entry for leukocytes leaving the teat lining and entering into the teat cistern. Cricoid rings - (Annular folds) Marks the region at the proximal end of the teat cistern which marks the boundary between the teat cistern and the gland cistern. Not always recognizable in the dissected gland. Teat cistern (Sinus papillaris) The cavity within the teat. It is continuous with the gland cistern. The teat cistern is lined with numerous longitudinal and circular folds in the mucosa, which form pockets on the inner lining of the teat.

Parts of udder

Interior Anatomy The interior of the gland is made up of: • Connective tissue - fibrous tissue (collagen) and fatty tissue (adipose cells). • Secretory tissue - secretory epithelial cells; these are the cells which produce the milk. The relative amount of connective vs. secretory tissue varies from animal to animal, by stage of mammary development, and even by location within the gland. Gland Cistern - (sinus lactiferus) Also called the udder cistern or milk cistern opens directly into the teat cistern. Occasionally a septum forms between the teat and the gland cisterns resulting in a AAP/mammary gland/sem2

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quarter, which may be blind (no milk can be removed). This can be corrected surgically. The cisterns function for milk storage (holds ~100-400 ml). The gland cistern varies greatly in size and shape. There are often pockets formed in the cistern at the end of the larger ducts. The major ducts which empty into the gland cistern sometimes are called cisternal ducts. Organization of Secretory Tissue Secretory tissue in the udder is organized into lobes, with each lobe made up of many lobules. Each lobule contains 150-220 microscopic alveoli.

Organisation of mammary gland Alveoli - (acini) Are sack-like structures where milk is synthesized and secreted. An alveolus is the discrete milk producing unit. A single layer of secretory epithelial cells lines the lumen of the alveolus. Contractile myoepithelial cells surround the epithelial lining. Myoepithelial cells contract in response to the hormone oxytocin, resulting in milk being squeezed out of the alveolar lumen and into the small ducts. Outside of the myoepithelial cells the alveolus is surrounded by a connective tissue basement membrane. The capillary bed on the outside of the alveolus is part of the stromal connective tissue between alveoli. A group of alveoli can be visualized as a clump of grapes, with the stems acting as the small ducts leading from the alveoli.


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Parts making up the alveolus Lobules - Clusters of 150-220 alveoli are encapsulated by a connective tissue sheath and are organized as a lobule (~.7-.8 mm dia.). Lobes - Groups of lobules are surrounded by a connective tissue sheath and comprise a lobe. Each mammary gland is made of numerous lobes. Duct System Ducts are the tubules by which milk drains from the alveoli down to the gland cistern. Interlobar or primary ducts drain multiple lobes. These are generally lined with two layers of non-secretory cells and have many myoepithelial cells. Intralobar ducts are within a lobe and drain several regions of the lobe. Interlobular or secondary ducts drain multiple lobules. They are lined with one layer of secretory cells and surrounded by myoepithelial cells, and so participate in the oxytocin-induced milk ejection. Intralobular ducts are small ducts within a lobule. Intercalary or tertiary ducts are the small ducts which exit from the alveolus. While this organized classification of ducts provides a basis for understanding the duct system of the gland, there is no uniformity in the system of duct branching. Figure below shows the duct system in udder.

Intecalary duct


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Suspensory System A strong udder suspensory system is required to maintain proper attachments of the gland to the body. For example, a high producing Holstein cow may have 22kg of empty udder weight plus 27kg of milk in the gland, or 49kg suspended from the body just prior to milking. Remember that the mammary gland is a skin gland, and is therefore external to the body cavity. The system of ligaments and other tissues which attach the udder to the cow are critical for successful lactation. The tissues, which provide some degree of support for the udder: 1. Skin is of minor support, but is considered one of tissues. 2. Superficial fascia or Areolar subcutaneous tissue -attaches the skin to the underlying tissuee. 3. Coarse areolar or cordlike tissue - forms a loose bond between the dorsal surface of the front quarters and abdominal wall. Weakening of these causes the udder to break away from abdominal wall. This is part of what is referred to as the forequarter attachments when evaluating dairy cattle conformation. 4. Superficial lateral suspensory ligament - mostly composed of fibrous tissue (with some elastic tissue), arising from the subpelvic tendon; extends downward and forward from the pubic area; when it reaches the udder it spreads out, continuing downward over the external udder surface beneath the skin and attaching to the areolar tissue. 5. Deep lateral suspensory ligament - thicker than the superficial layer, mostly fibrous tissue, arises from the subpelvic tendon; extends down over the udder and almost enveloping it. The ligament attaches to the convex lateral surfaces of the udder by numerous lamellae, which pass into the gland and become continuous with the interstitial framework of the udder. Collectively, the lateral suspensory ligaments provide substantial support for the udder. The left and right lateral suspensory ligaments do not join under the bottom of the udder, and the fibrous nature of these ligaments means that they do not stretch as the gland fills with milk. So, the center of the udder tends to pull away from the body as the gland fills. 6. Median Suspensory Ligament - This is the most important part of the suspensory system in cattle. It is composed of two adjacent heavy yellow elastic sheets of tissue which arise from the abdominal wall and which attach to the medial flat surfaces of the two udder halves. The median suspensory ligament has great tensile strength. It is able to stretch somewhat as the gland fills with milk to allow for the increased weight of the gland. It is located at the center of gravity of the udder to give a balanced suspension, so that even if the rest of the layers are cut away, except for the median suspensory ligament, the gland should stay balanced under the animal. Median Suspensory ligament partially separates the left and right halves of the udder. A thin membrane separates front and rear quarters. There is NO internal crossover of the milk duct system of the quarters (glands). If a dye is injected into the teat and duct system of one quarter, it will be seen only in that quarter.


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Figure showing suspensory apparatus Vascular System The blood supply to the mammary gland is extremely important for mammary function. All of the milk precursors come from blood. On average 400 - 500 units of blood passes through the udder for each unit of milk synthesized by a high producing dairy cow; that is ~280 ml per sec. High producing dairy goats have a lower (460:1) ratio of blood flow through the gland : milk produced, compared with low producers (1000:1). This means that the amount of blood flow through the mammary gland may be similar for the high and low producing goats, but the efficiency of extraction of the components from the blood while it passes through the udder is very important. This principle is similar for cows. Total udder blood volume for lactating cows is about 8% of total body blood volume, while for a non-lactating cow it is about 7.4%. There is a 2-6 fold increase in blood flow in the mammary gland starting 2-3 days prepartum. The decrease in production with advancing lactation is not due to decreased blood flow. Arterial System Blood leaves the heart and flows towards the rear of the cow by the abdominal aorta. When it reaches the pubic area of the animal the vessels are called the common iliac arteries. These divide into the internal and external iliac arteries. The external iliac artery becomes the femoral artery (supplies the leg muscles). A branch off of the femoral artery forms the prepubic artery from which branches the posterior abdominal artery and the external pudic (or external pudental) artery. The external pudic artery passes through the inguinal canal and out of the body cavity. The inguinal canal is the orifice in the body cavity in the inguinal region where blood vessels, lymph vessels and nerves enter and leave the body wall to supply the skin in the posterior part of the animal (again, the mammary gland is a skin gland and is external to the body cavity). As the external pudic artery passes out of the body cavity it becomes the mammary artery. Once it enters the gland, the mammary artery then divides into the anterior (or cranial) and posterior (or caudal) mammary arteries, which then it further branches as it descends down into the gland. A small amount of blood also reaches the mammary gland by the perineal artery (from the internal iliac artery), but this only supplies the upper rear portion of gland. Sigmoid flexure - Just below the inquinal canal, the pudic artery forms an S-shaped flexure. This allows for downward distention of the udder as it fills with milk, without AAP/mammary gland/sem2

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stressing the blood vessels. There is essentially no cross over of blood supply between left and right udder halves (although there sometimes are a few minor exceptions)

Arteries supplying blood to udder Venous System Veins leave the mammary gland anti-parallel to the arteries. There are three veins on each side that carry blood away from the gland: 1. External pudic vein leaves the udder anti-parallel to the external pudic artery 2. Subcutaneous abdominal vein (milk vein) exits the gland at the anterior end of the front quarters and passes along the abdominal wall. This is the large vein that is visible under the skin on the belly of the cow. It enters the body cavity at the xiphoid process via "milk wells", and eventually empties into cranial vena cava. 3. Perineal vein leaves the rear of the gland anti-parallel to the perineal artery. This vein carries less than 10% of blood leaving the udder. Venous circle - Formed by anastomoses between anterior and posterior mammary veins. Prevents pinching off of areas of venous outflow when the cow is lying down.

Veins draining blood from udder.

Nervous System This is a summary of several important points about the nervous system of the mammary gland: • Innervation of the udder is sparse compared with other tissues. • Sympathetic nerves in the tissue associate with the arteries, but not with alveoli.


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Sensory nerves are found in the teats and skin; these are involved in the afferent pathway of the milk ejection reflex. There is no parasympathetic innervation to the gland; this is similar to other skin glands. There is no innervation of the secretory system: myoepithelial cells are not innervated; they do not contract in response to direct innervation, but rather they contract in response to the blood-borne hormone, oxytocin. Few nerves go to the interior of the udder; consequently, biopsy of the gland can be done with only local anesthesia to the skin.

Lymph and Lymphatics Many molecules of all sizes leave the capillaries but not all return to the venous drainage at the tissue level, especially the larger molecules like proteins. These, along with cellular metabolites and some secretory products are in the interstitial (extracellular) spaces. If they stayed there, they would disrupt with the normal balance of osmotic pressure in the tissue, upsetting transcapillary fluid exchange. Excess fluid (called extracellular fluid) would accumulate in the interstitial spaces, resulting in tissue edema. Functions of lymphatics: • The extracellular fluids are drained from the tissue and conducted back to the circulatory system via the lymphatic network. • The lymphatics contain concentrated areas of leukocytes (particularly lymphocytes and macrophages) in the lymph nodes; these leukocytes can mount an immune response to bacteria and foreign material. • The lymphatic network serves to transport some components absorbed in the intestine (lipids). The lymphatic network originates in tissue spaces as very thin walled, endothelial tubes (lymphatic capillaries). These are analogous to blood capillaries, but are much more permeable, with little resistance to fluid passage. They have no basement membrane. Lymph capillaries converge to form larger vessels. Lymph flow is undirectional from the udder through lymphatic vessels, eventually dumping lymph into the vena cava. Lymph is a clear, colorless liquid with a composition similar to blood plasma. Changes in plasma composition will change lymph composition. Protein concentration of lymph is lower than in plasma, 1.5% vs. 6% for plasma. The protein concentration in lymph varies inversely with the rate of lymph fluid formation. Rate of filtration varies with the tissue. Lymph flow rate is usually low. It is influenced primarily by the rate of lymph formation. For example, if blood capillary pressure is increased by arterial vasodilation or venous constriction, the flow rate of lymph increases. Also, the flow rate is affected by compression of lymphatics by contraction of neighboring musculature and by negative intrathoracic pressure (breathing). Valves in the lymphatic vessels prevent retrograde flow. In the udder, the lymph system nearly all flows through the supramammary lymph nodes. There are usually one or two per udder half, but sometimes up to 7 nodes per udder half. They are about 7 x 5 x 2 cm on average. Frequently there are accessory lymph nodes inside the gland and often there are superficial lymph nodes just beneath skin. The lymph vessel leaves the mammary gland via the inquinal canal (similar to the blood vessels). There is extensive lymph drainage from the teats.


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Lymphstic system draining out of udder Clinical importance Edema is the excess accumulation of fluids in extracellular tissue spaces. This can retard normal exchange of nutrients and metabolites. Filtration of the extracellular fluid exceeds drainage via the venous and lymph systems. Anything that causes increased capillary pressure, such as decreased plasma protein concentration, increased capillary permeability or lymphatic blockage, can result in swelling and congestion of the extravascular compartment. Udder Edema is swelling of the udder and occurs particularly in first calf heifers at calving time. Fluid accumulates between skin and glandular tissue, as well as in the gland. The skin is usually ~1/4" thick including subcutaneous layers, but during edema it can increase in thickness to 2". Severe edema can strain supportive structures of the udder. Udder edema is often caused by an imbalance of hydrostatic and osmotic pressures, increasing fluid flow out of the capillaries into the interstitial spaces. This may occur because of damage to the capillary walls or obstruction of the lymphatic system. It is not known exactly why it happens, but from human medicine, increased salt intake, increased fluid intake, increased environmental temperature and damaged innervation may contribute to edema. Lymph flow through mammary gland: • Goat (lactating) - 6.5-35 ml/hr or 150-840 ml/day • Cow (dry) - 14-240 ml/hr or 68-5760 ml/day • Cow (lactating) -1300 ml/hr or 31,200 ml/day or 31 kg/day About 1.6 units lymph leave udder for every unit milk produced. Mastitis Inflammation of udder due to bacterial or fungal infection. In this condition the physical and chemical composition of the milk will be changed. In our condition one of the reasons for poor production from local cattle could be due to subclinical mastitis that go on undetected in the field.


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