Mammary Glands and The Milk Let-Down

Updated: Apr 23, 2020

Team Agasthya (Students of Dairy Technology)

why is it so important?

Answer: The entire Dairy science starts from here.

1. If there is no mammary gland, there is no milk. and

2. Even if there is milk and if there is no milk letdown, the milk won't come out of the animal.


The first three months of the newborn calf depends upon the milk for normal growth and development. We human beings also depend on the bovine or cattle milk to meet our daily dietary requirements. In this scenario, it is essential to know about the structure and function of the mammary gland for better feeding and survival of the calves and also for better yield of milk for supplying. The size and shape of the mammary gland vary from species to species, however, the anatomical microscopic structure of the mammary gland is the same in all the domestic animals. The mammary gland begins its development during fetal life. After parturition, mammary tissues continue to proliferate and milk secretion accelerates. The secretary cells reach their peak a few weeks after parturition. Milk secretion is under the control of several hormones which cause milk let down. Finally, milk is obtained.

check this video below for a detailed explanation for milk production inside the cow.

Structure and function of the mammary system:

The udder is located at and in between hind limbs in the inguinal region supported by various ligaments and connective tissue. The mammary system or udder of the cow should be capacious, leveled and strongly attached with tortuous milk vein. The texture should be soft, pliable and elastic and collapsible after milking.

It can be divided into two halves separated by inter mammary groove. The half is again divided into two separate units (called as quarters) by thin membranes. The front portion is called the fore udder and the rear portion is called the rear udder. The four quarters are independent with no communication between them. The two rear quarters are larger and produce 60 percent of the milk whereas the fore-quarters produce about 40 percent of the milk. The size, shape, and placement of the udder are done by the median and lateral suspensory ligaments.

Each quarter of the udder is composed of the secretory tissue and the connective tissue. The alveolus is a microscopic structure almost spherical in shape lined by a single layer of epithetical cells or the milk secretory cells.

Structure of udder

The alveoli are surrounded by a capillary network that provides nutrients. The myoepithelial cells surrounding the alveolus contract during milking causing milk let down. The alveolus is grouped together into lobules. A group of lobules forms a lobe. The secretary tissue contains the terminal ducts. Intralobular ducts unite to form interlobular ducts. Their ducts communicate into gland cistern and teat cistern. The teat cistern is joined with the streak canal. It is surrounded by teat sphincter responsible for preventing the entry of the pathogens onto a teat.

Hormones Involved in Milk Synthesis and Let Down

1. Oxytocin - Milk let down

2. Prolactin - Anterior pituitary gland secretion helping in the synthesis and maintenance of milk secretion.

Milk let down:

Milk Let Down is the process by which a cow will release the milk from her mammary tissues. This process occurs after the mammary gland is stimulated.

check this video for a detailed explanation on milk let-down👇

During the milk secretion, the alveoli, ducts and gland and teat cisterns are gradually filled with milk. Milk in the cisterns and larger ducts can be removed readily, but the milk in the smaller ducts and alveoli does not flow out easily. Milk ejection is an involuntary act on the part of the cow. It is a neuro-hormonal reflex that is very important if maximum milk production is to be obtained. Stimulation of the central nervous system by something associated with the milking process is necessary to initiate the reaction. Stimulation of nerve endings in the teats that are sensitive to touch, pressure, or warmth is the usual mechanism. Milk ejection is initiated by a stimulus such as the washing of udder, manipulating the teats, suckling of a calf. The sucking action of the ‘calf is ideal for this milk let down.

Stimulation is carried by the nerves to the brain which is connected with the pituitary gland located at its base. The brain causes the release of hormone Oxytocin from the posterior pituitary gland into the blood system, which carries into the mammary gland.

Mechanism of milk let down:

Oxytocin acts on the myoepithelial cells that surround the alveoli and ducts and contracts them. This creates pressure forcing the milk out of the alveoli and smaller ducts as fast as it can be removed through the teat resulting in milk ejection. The process occurs in about 45 to 60 seconds after stimulation. The maximum effect starts only 7-8 minutes. Prompt initiation of milking and rapid milking is important in obtaining maximum milk yield.