Ferenc Husvéth (2011)
Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem
Table of Contents
The mammary gland, like sebaceous and sweat glands, is a cutaneous gland. Histologically, in the more advanced mammals it is a compound tubuloalveolar type that originates from the ectoderm. Although the mammary gland is basically similar in all animals, there are wide species variations in the appearance of the gland and in the relative amounts of the components secreted.
The mammary glands of cattle, sheep, goats, horses, and camel are located in the inguinal region; those of primates and elephants, in the thoracic region; and those of pigs, rodents and carnivores, along the ventral structure of both the thorax and the abdomen (Fig.14.1.)
Figure 14.1. Figure 14.1.: Diversity in anatomic position, number and teat morphology among mammals (Singer (2003)
Normally, cattle have four functional teats and glands, whereas sheep and goats have two; each teat has one streak canal and drains a separate gland. The glands and teats of domestic animals are collectively known as udder. Pigs and horses usually have two streak canal per tea, with each canal serving a separate secretory area. Because more is known about bovine mammary glands than about those of other mammals, more attention will be given here to that species. A cow’s udder is composed of two halves, each of which has two teats and each teat drains a separate gland (quarter). The quarters are separated by connective tissue and each has separate milk collecting system (Fig.14. 2.).
Figure 14.2. Figure 14.2.: Duct system of the bovine mammary gland: 1.,: lobule; 2., intralobular duct; 2’., interlobular duct; 3., lactiferous ducts of various diameter; 4., lactiferous sinus; 5., gland sinus; 6., teat sinus; 7., papillary duct; 8., teat sphincter; 9., teat orifice; 10., parenchyma of gland; 11., skin. Dyce et al. (1996).
In addition to the four normal teats, there may be supernumerary teats associated with a small gland, with a normal gland, or with no secretory area. About 40 per cent of all cows have supernumerary teats. Supernumerary teats are also found in sheep, goats, pigs and horses. In these species, with the exception of the horse, rudimentary teats are usually found in the male.
The two halves of the bovine udder are separated by the median suspensory ligament, which is formed by two lamellae of elastic connective tissue originated from the abdominal tunic (Fig. 14.3.). The posterior extremity of its ligament is attached to the prepubic tendon. The lateral suspensory ligaments are composed largely of fibrous, nonelastic strands given rise to numerous lamellae that penetrate the gland and become continuous with the interstitial tissue of the udder. The lateral suspensory ligaments are attached to the prepubic and subpubic tendons, which in turn are attached to the pelvic symphysis. The lateral and median suspensory ligaments are the primary structure supporting the bovine udder.
Figure 14.3. Figure 14.3.: Transverse section of the pelvic floor and caudal quarters of the cow’s udder 1., Pelvic symphysis; 2., symphysial tendon; 3., lateral suspensory laminae; 4., mammary lymph node; 5. medial suspensory laminae; 6., branch of external pudendal vein. Dyce (1996)
The bovine teat has a small cistern terminating at its distal extremity in the streak canal, which is the opening to the exterior of the teat (Fig. 14.2.). Radiating downward from its internal opening into the streak canal is a structure known as Fürstenberg’s rosette, which is composed of about seven or eight loose folds of double layered epithelium and underlying connective tissue; each folds has a number of secondary folds. In cattle, the primary structure responsible for the retention of milk is a sphincter muscle surrounding the streak canal. Large ducts empty into a gland cistern located above each teat. These ducts branch profusely, ultimately ending in secretory units called alveoli or acini (Fig. 14.4.).
Alveoli are generally recognised as the basic functional units of the lactating mammary gland. Milk is formed in the epithelial cells of the alveolus. The alveolus, when filled with milk, is approximately 100 to 300 µm in diameter. The size of the alveolus is affected by many factors, primarily the amount of milk in the lumen. The alveoli are grouped together in units known as lobules, which are surrounded by more extensive connective septa. The alveoli are surrounded by contractile myoepithelial cells that are involved in the milk-ejection (or milkletdown reflex; Fig. 14.14). Myoepithelial cells also are located along the ducts. Apparently these cells are widely distributed among mammals, since they have been identified in the cat, dog, goat, pig, rabbit, rat, sheep, and human.
Figure 14.4. Figure 14.4.: An alveolus surrounded by blood vessels and myoepithelial cells in the mammary gland (Reece, 2004).
The blood supply to the udder is primarily through the external pudendal artery (Fig. 14.5.), a branch of the pudendoepigastric trunk. The external pudendal artery passes downward through the inguinal canal in more or less tortuous manner and divides into cranial and caudal branches that supply the front- and hindquarters on the same side as the artery. A small artery that may be single or paired (as determined by chance) is the ventral perineal artery, which continues from the internal pudendal artery and passes downward from the vulva just deep to the skin on the median line. The perineal artery usually supplies a small amount of blood to the caudal part of both halves the udder.
The venous drainage from the udder is largely by way of a venous circle at the base of the abdominal wall. The venous circle is formed from the main veins that drain the udder. The external pudendal vein of each side receives blood from both the cranial and caudal quarters of the same side. Cranially, each external pudendal vein is continuous with the subcutaneous abdominal veins (cranial and caudal superficial epigastric veins) and caudally with the perineal vein. An anastomosis between the subcutaneous abdominal veins just at or in front of the udder completes the venous circle.
Figure 14.5. Figure 14.5.: Blood circulation to and from the udder. RA=right atrium; LV=left ventricle; CA= caudal aorta; CVC=caudal vena cava; EIV=external iliac vein; EIA=external iliac artery; EPA=external pudendal artery; EPV=external pudendal vein; CMA&V=caudal mammary artery and vein; CrMA&V=cranial mammary artery and vein; SAV=subcutaneous abdominal vein. (Reece et al., 2004
The subcutaneous abdominal veins provide a potential route for passage of blood from the udder to the heart by way of the cranial vena cava. Before the heifer comes into milk, the connection between cranial and caudal superficial epigastrics is poorly developed. During first pregnancy, when the udder undergoes a marked increase in size and, consequently, blood supply, the two veins develop a functional anastomosis, after which they collectively constitute the subcutaneous abdominal vein or the milk vein. In high-producing dairy cows the subcutaneous abdominal vein is large and tortuous. It passes through a foramen in the rectus abdominis muscle (the milk well), joins the internal thoracic vein, and ultimately drains to the cranial vena cava.
The lymphatic vessels draining the udder show up rather well superficially just under the skin, particularly in high-producing cattle. They drain from the entire udder, including the teat, to the superficial inguinal (mammary or supramammary) lymph nodes near the superficial (external) inguinal ring above the caudal part of the base of the udder.