Lesson 8. MAMMARY SYSTEM, MILK SECRETION AND MILK LETDOWN
Module 5. Mammary system
Lesson 8 MAMMARY SYSTEM, MILK SECRETION AND MILK LETDOWN
The first three months of the new born calf depends upon the milk for normal growth and development. 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. The size and shape of the mammary gland varies from species to species, wherever the anatomical microscopic structure of the mammary gland is same in all the domestic animals. The mammary gland begins its development during the foetal life. Once the animals attain puberty, the surge of hormone secretions results in increase in the rate of growth of mammary gland is relation to whole body. After parturition, mammary tissues continue to proliferate and milk secretion accelerates. The secretary cells reach their peak few weeks after parturition. Milk secretion is under the control of several hormones.
8.2 Structure and Function of Mammary System
Udder is located at and in between hind limbs in 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 is done by the median and lateral suspensory ligaments.
Each quarter of the udder is composed of the secretary tissue and the connective tissue. The alveolus is a microscopic structure almost spherical in shape lined by single layer of epithetical cells or the milk secretary cells.
Fig. 8.1 Structure of udder
The alveoli are surrounded by a capillary network which provides nutrients. The myoepithetical cells surrounding the alveolus contract during milking causing milk let down. The alveolus is grouped together into lobules. A group of lobules form a lobe. The secretary tissue contains the terminal ducts. Intralobular ducts unite to form interlobular ducts. There ducts communicate into gland cistern and teat cisternae. The teat cisterna is joined with streak canal. It is surrounded by teat sphincter responsible for preventing the entry of the pathogens onto teat
8.3 Hormones Involved in Milk Synthesis and Let Down
1. Oxytocin - Milk let down
2. Prolactin - Anterior pitutary gland secretion helping in synthesis and maintenance of milk secretion.
8.4 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 cow. It is a neurohormonal 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 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 pitutary gland located at its base. The brain causes the release of hormone Oxytocin from posterior pituitary gland into blood system, which carries into mammary gland.
Fig. 8.2 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 are important in obtaining maximum milk yield.