X Международная студенческая научная конференция Студенческий научный форум - 2018

Animals that belong to the class Mammalia are characterized as having bodies that are basically covered with hair, delivering live young instead of eggs (the monotremes are an exception) and nurturing their young through the use of structures called mammary glands.

The development of the mammary gland starts early in the fetal life. Already in the second month of gestation teat formation starts and the development continues up to the sixth month of gestation. When the calf fetus is six months, the udder is almost fully developed with four separate glands and a median ligament, teat and gland cisterns.

An ectodermal thickening develops along the ventral body wall extending from the thoracic to inguinal region - this is the mammary ridge. Cells aggregate, multiply and differentiate to form a chain of condensed mammary buds. Most mammary buds regress, but each of those, that remain, develops into a mammary gland. A mammary gland is the secretory and duct system associated with one teat. Mammary buds grow into overlying mesenchyme, and primary epidermal sprouts grow out of the bud apex. The epidermal sprout branches extensively and develops a complete duct system. Mammary adipose tissue is derived from mesoderm. This is required for complete mammary development and is absent in the male. As a result, mammary development in the male is halted at the epidermal sprout stage.

The developments of milk ducts and the milk secreting tissue take place between puberty and parturition. The udder continues to increase in cell size and cell numbers throughout the first five lactations, and the milk producing capacity increases correspondingly. This is not always fully utilized, since the productive life time of many cows today is as short as 2.5 lactations.

The mammary gland of the dairy cow consists of four separate glands each with a teat. Milk which is synthesized in one gland cannot pass over to any of the other glands. The right and left side of the udder are also separated by a median ligament, while the front and the hind quarters are more diffusely separated.

Macrostructure of the mammary gland

The udder is a term designating all the mammary gland in the ruminant and the mare (sometimes also used for the sow). The udder of a matured cow is a very big organ weighing around 50 kg (including milk and blood). However, weights up to 100 kg have been reported. Therefore, the udder has to be very well attached to the skeleton and muscles. The median ligaments are composed of elastic fibrous tissue, while the lateral ligaments are composed of connective tissue with less elasticity. If the ligaments weaken the udder will become unsuitable for machine milking since the teats then will often point outward.

The mammary gland may contain one or more duct systems. The lobes are the internal compartments of the udder, separated by adipose tissue. The lobes are divided into lobules, consisting of connective tissue containing alveoli, which are clusters of milk secreting cells. The lactiferous ducts are large ducts conveying milk from the alveoli to the lactiferous sinus. The openings of the lactiferous ducts convey milk formed in the alveolus to the gland sinus.

The lactiferous sinus (milk sinus) is the milk storage cavity within the teat and glandular body. The gland sinus is part of the milk sinus within the glandular body and the teat sinus is part of the milk sinus within the teat.

The teat is the projecting part of the mammary gland containing part of the milk sinus. The papillary duct (teat canal) is the canal leading from the teat sinus to the teat opening and may be single or multiple. The ostium (teat opening) is the opening of the papillary duct and the exit point for milk or entrance point for bacteria. The sphincter consists of muscular fibres surrounding the teat opening that prevent milk flow except during suckling or milking.

In species with large udders, especially in dairy cattle, there is a suspensory apparatus, which is organized into the lateral and medial laminae suspending the mammary gland from the ventral aspect of the trunk by their attachment to the pubic symphysis. The lateral lamina consists of collagen fibres from the fascia of the pubic symphysis and the edge of the superficial inguinal ring. The medial lamina consists of elastic fibres from the tunica flava ventral to the pubic symphysis. The intermammary groove divides the left and right rows of mammary gland.

Microstructure of the mammary gland

Secretory tissue is arranged into lobes, each consisting of many lobules. Each lobule contains groups of alveoli (secretory compound tubule-alveolar cells) surrounded by a network of blood vessels and connective tissue stroma. The alveolar lumen is filled with milk during lactation. Myoepithelial cells lie between alveolar epithelial cells and the basement membrane. These contract under the influence of oxytocin to release milk to the exterior. Lobes and lobules are drained by lactiferous ducts into the gland sinus, which is continuous with the teat sinus. The epithelium lining the lactiferous ducts and the sinus is two-layered cuboidal. A teat canal connects the teat sinus to the exterior. The lining is stratified squamous epithelium. Circular smooth muscle in the wall of the canal forms a sphincter. Between milkings, the narrow lumen of the teat canal is filled with a soft keratin plug to prevent bacteria entering the teat sinus and prevent milk leakage.

The mammary gland consists of secreting tissue and connective tissue. The amount of secreting tissue, or the number of secreting cells is the limiting factor for the milk producing capacity of the udder. It is a common belief that a big udder is related to a high milk production capacity. This is, however, not true in general, since a big udder might include a lot of connective and adipose tissue. The milk which is continuously synthesized in the alveolar area, is stored in the alveoli, milk ducts, udder and teat cistern between milkings.

60-80% of the milk is stored in the alveoli and small milk ducts, while the cistern only contains 20-40%. However, there are relatively big differences between dairy cows when it comes to the cistern capacity. This is of importance for the milking routines to be applied.

The knowledge of anatomic, histologic and physiologic features of the mammary gland allows better understanding of mammary gland diseases causes, mechanisms and, consequently, their treatment.

References:

1. Wikivet.net [Электронный ресурс]. – Режим доступа: https://en.wikivet.net/Mammary_Gland_-_Anatomy_%26_Physiology . - Загл. с экрана;

2. Nydairyadmin [Электронный ресурс]. – Режим доступа: http://nydairyadmin.cce.cornell.edu/uploads/doc_113.pdf. - Загл. с экрана.

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