Anatomy of the mammary gland
The gross anatomy of the mammary gland differs a lot among different species. The number of glands and teats are not the same for the cow, the sow or the horse. However, the microscopic anatomy is very similar among species.
The development of the mammary gland begins in the fetal period. During the second month of gestation, teat formation starts and 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.
The development of milk ducts and the milk secreting tissue takes 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 lifetime 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 that is synthesized in one gland cannot pass over to any of the other glands. The right and left sides of the udder are also separated by a median ligament, while the front and the hindquarters are more diffusely separated.
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The udder is a very big organ weighing around 50 kg (110 lbs) (including milk and blood). However, weights up to 100 kg (220 lbs) 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 consists of secretory tissue and connective tissue. The amount of secretory tissue, or the number of secreting cells is one of the limiting factors 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.
Milk is synthesized in the secretory cells, which are arranged as a single layer on a basal membrane in a spherical structure called alveoli. The diameter of each alveoli is about 50-250 mm. Several alveoli together form a lobule. The structure of this udder tissue is very similar to the structure of the lung. 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 individuals when it comes to the cistern capacity. This is of importance when developing or evaluating milking routines.
The teat consists of a teat cistern and a teat canal. Where the teat cistern and teat canal meet, 6-10 longitudinal folds form the so-called Furstenbergs rosette, which is involved in the local defense against mastitis. The teat canal is surrounded by bundles of smooth muscle fibers, longitudinal as well as circular. Between milkings the smooth muscles function to keep the teat canal closed. The teat canal is also provided with keratin or keratin like substances which between milkings act as a barrier for the pathogenic bacteria.
The mammary gland is densely innervated especially in the teat. The skin of the teat is provided with sensory nerves that are sensitive to the calf’s suckling or massaging by milker and/or milking machine and thus influenced by pressure, heat and frequency of suckling. The udder also contains nerves connected to the smooth muscles in the circulatory system and the smooth muscles in the milk ducts. However, there is no innervation directly controlling the milk producing tissue.
The mammary gland is very well supported with blood vessels, arteries and veins. Right and left udder halves have separate arterial supply, although there are some small arterial connections that pass from one half to the other. The primary function of the arterial system is to provide a continuous supply of nutrients to the milk synthesizing cells.
To produce 1 liter of milk, about 500 liters of blood have to pass through the udder. When the cow is producing 60 liters of milk per day, 30,000 liters of blood are circulating through the mammary gland. This illustrates the extreme demands placed on the high producing dairy cow.
The udder is also served by the lymphatic system. The lymphatic system carries waste products away from the udder. The lymph nodes serve as filters to destroy foreign substances and also provide a source of lymphocytes to fight infections. Sometimes, around parturition first calf heifers suffer from edema, partly caused by the presence of milk in the udder which compresses the lymphatic system.
Milk secretion and milk composition
Milk synthesis takes place in the alveoli where the milk secreting cells in the mammary gland are provided with a continuous supply of nutrients from the blood.
Milk fat consists mainly of triglycerides, which are synthesized from glycerol and fatty acids. Long-chained fatty acids are absorbed from the blood; these fatty acids generally originate from either feed or body fat mobilization. Short-chained fatty acids are synthesized in the mammary gland from acetate and beta-hydroxybutyrate. Acetate Butyrate (which is converted to BHB in the liver) is produced by the fermentation of fiber in the rumen (first stomach) and transported by the blood to the mammary gland. Milk protein is synthesized from amino acids also originating from the blood, and consist mainly of caseins and to a smaller extent whey proteins. Lactose is synthesized from glucose and galactose within the milk-secreting cell. Vitamins, minerals, salts and antibodies are transferred from the blood across the cell cytoplasm into the alveolar lumen.
The composition of the milk varies between different breeds but also during lactation within breed.
| Breed |
Total solids % |
Fat % |
Casein % |
Whey protein % |
Lactose % |
Ash %
|
| Brown Swiss |
12.69 |
3.8 |
2.63 |
0.55 |
4.80 |
0.72
|
| Holstein |
11.91 |
3.56 |
2.49 |
0.53 |
4.61 |
0.73 |
| Jersey |
14.15 |
4.97 |
3.02 |
0.63 |
4.70 |
0.77 |
In the beginning and at the end of lactation, the fat and protein contents are higher compared to mid lactation.
The higher concentration of components in milk at early lactation is due to the special needs of the young calf. As an example, the higher content of milk protein during the first days after parturition is due to the high level of immunoglobulins. On average, milk from dairy cows has a fat content between 3.0 and 5.5%, protein content between 3.0 and 3.8%, and lactose in the range of 4.0 and 4.8%.
Related Links:
An introduction to the mammary gland from Oregon State University |