The Mammary Glands

These modified sweat glands are discussed in this chapter because they are highly vas­cular and the distribution of their veins is more extensive than their arteries. Collectively called mammae, their function is to provide a source of nutrients for the newborn.

A mammary gland consists of glandular tissue arranged in lobules and consisting of secretory epithelial cells arranged in spherical alveoli. The secretion (milk) collects in the lumen of an alveolus and then enters a duct system that conducts it to a teat, or nip­ple, on the surface of the mammary gland. The mammary secretions correspond to gestation so that they are timed to commence with parturition. The first secretions are of colostrum, a milk precursor containing antibodies, immunoglobulins and growth factors. It is very important that the newborn animal receives this protective cocktail of antibodies. This is particularly important in farm animals because the epitheliochorial placenta is a barrier to the passage of antibodies from maternal to foetal blood circulation.

The number of ducts opening on a teat varies by species and individual. The gland sinus receives secretion, which then drains into a teat sinus and then via the teat canal to a teat orifice.

The mammary glands are extremely vascular. In the bitch they receive their blood supply from the internal thoracic artery, a branch of the subclavian artery in the thorax and the pudendoepigastric artery, a branch of the external iliac artery.

9.3.1 Species variations

Horse: There are only two mammary glands in the mare. They are located at the junc­tion between the abdomen and pelvis; they are relatively small and are concealed from view by the thighs. Each gland has a single teat that is small except when lactating. The secretory tissues of the two glands are combined and difficult to separate.

Ox: There are four mammary glands in the cow. Commonly called the udder, the four glands, each with a teat, are combined as a single unit, but with a midline groove sepa­rating left from right. The udder of a lactating dairy cow can weigh at least 50 kg. Accessory teats can be present, each with glandular tissue but often interfering with milking. The cranial and caudal glands on the same side are usually combined. The skin of the udder is thin and mobile with a covering of fine hair.

Following ovulation there is some swelling of the glands. If pregnancy occurs there is marked hyperplasia of the glandular tissue.

The number of ducts opening on a teat varies by species and individual. The gland sinus receives secretion, which then drains into a teat sinus and then via the teat canal to a teat orifice. The skin of the udder is thin and mobile with a covering of fine hair.

The symphyseal tendon originates from the pelvic symphysis and provides a common origin for the gracilis and adductor muscles as well as the suspensory apparatus of the udder. The collagenous lateral laminae of the suspensory apparatus attach to the right and left aspects of the udder. The cranial parts of the lateral laminae originate from the yellow elastic tunic on the lateral side of the superficial inguinal ring (see Section 1.3.2).

The main arterial supply to the udder is provided by the external pudendal artery, a branch of the deep femoral artery. On entry to the udder the artery forms a sigmoid flexure to allow extension when the udder is full of milk. This artery then divides into a large cranial mammary artery and a small caudal mammary artery. There are many anastomoses between the many branches entwined with the lactiferous sinuses. The arrangement of the venous drainage is more complex. There is a venous ring at the base of the udder that collects the blood from the glands; it then leaves the ring via a large subcutaneous abdominal vein to enter the internal thoracic vein followed by the cranial vena cava. Many large superficial lymphatic channels conduct lymph to the mammary lymph nodes.

Sheep: The udder of the sheep and goat is located in the inguinal region and com­prises two mammary glands. There is variation in the shape of the teats and the amount of hair cover. With regard to the topography and vascularity the udder is similar to that of the cow.

Pig: The sow usually has seven or eight mammary glands distributed over the ventral aspect of the thoracic, abdominal and inguinal body wall. In other respects the mam­mary glands of the pig are similar to those of the ruminants.

Dog and cat: The number of mammary glands in the bitch varies from 8 to 12, but there is commonly a reduced number in the smaller breeds. In the cat there are six to eight glands. The secretory tissue is prominent only during pregnancy, lactation, pseu­dopregnancy and for a period about 45 days while the bitch or queen is nursing. The structure of the mammary glands is basically the same as in the other domestic mam­mals. The highly vascular mammary glands derive their vascularity from the epigastric arteries, branches of the internal thoracic arteries. There is a rich network of lymphatic vessels and lymph nodes associated with the mammary tissues.

Clinical disease of the mammary glands is common in the bitch, especially neoplasia and mastitis. Malignant neoplasia usually requires radical excision and even so involve­ment of the lymphatic tissues conveys a poor prognosis.