Lactation
Lactation, the production of milk by the mammary gland, is a distinguishing
characteristic of mammals, whose young at first feed solely on milk from their mothers.
Mammary gland growth and development occur rapidly as the female reaches puberty.
The ovarian hormones have a large effect on development. Estrogen is primarily
responsible for duct and cistern growth, whereas progesterone stimulates growth of the alveoli.
Growth hormone, adrenal corticoids, and prolactin are primarily responsible for the initiation of lactation.
Lactation is maintained primarily through hormonal influence. Prolactin, thyroid hormones, adrenal hormones, and growth hormone are all important in the
maintenance of lactation.
General structure
The udder is composed of two principal types of tissue, secreting and connective. A limited amount of
connective tissue is necessary for support of the glands. The desirable udder is one which contains a minimum amount of connective and fatty tissue and a maximum amount of secretory tissue.
The Cow’s Udder
External appearance; the four mammary glands of the dairy cow are grouped together in a structure called the udder. The udder should be reasonably large, possess a level floor, and be neatly attached both front and rear.
The teats should be squarely placed, hang perpendicularly, and be of good size (teat length; mean 6-7 cm).
•The mammary gland serves two functions: 1) it provides nutrition to calf, and 2) it is a source of passive immunity to the calf.
•The mammary gland is an exocrine gland and located in the groin area.
Internal structure
The udder is divided into right and left halves by a heavy membrane which extends lengthwise of the body and
helps to support the udder by its attachment to the abdominal wall. Front and rear quarters are separated by a
very thin connective tissue membrane. The milk from each quarter can be removed only from the teat of that
quarter. At the end of the teat is the opening through which the milk is removed. This opening is called the
streak canal and is surrounded by a sphincter or circular muscle. This muscle normally keeps the stored milk
from running out.
The
importance of the
suspensory attachments for the
udder is
indicated by the weight they have to carry.
The secretory tissue of
the mammary gland is
composed of millions
of grapelike structures
called alveoli.
• The teat widens out into the cistern of the teat, which opens into the larger cistern of the gland. Leading into the cistern of the gland are 8 to 12 large milk ducts which branch into all parts of the gland and carry the milk secreted down to the cistern. Each gland is divided into lobes, separated by connective tissue membranes which appear as white glistening bands
between the orange-colored glandular tissues. The lobes are further divided into lobules, each made up of a large number of hollow spherical structures called alveoli. The cells lining the
alveoli, called epithelial cells, first manufacture the milk from the constituents of the blood, then discharge the milk into a hollow cavity called the lumen.
• It is estimated that 300 to 400 volumes of blood pass through the udder for each one volume of milk secreted.
• On the average each of the front quarters produces about 20 per cent of the milk and each of the rear quarters, about 30 per cent. There is almost no difference in production between the right and left halves of the udder.
• Secreting tissue is soft, pliable, and spongy to the touch, with no firm, fatty tissue or edema in
the udder. A ‘’meaty’’ udder is a relatively poor producer.
Circulatory System;
The arterial blood from the heart reaches the base of each half of the udder through an external pubic artery which comes through the inguinal canal. Each of these branches into two mammary arteries, the cranial, which supplies the fore-quarter, and the caudal the rear. These arteries branch to supply blood to the mammary glands. Blood from the capillaries collects in two large veins, one at the base of each half of the udder, which are joined together by a smaller vein in the rear.There are three routes by which blood can return to the heart. One is the so-called ‘’milk vein’’ or subcutaneous abdominal vein. The second is the external pubic vein which parallels the external pubic artery. The third route is the perineal vein which carries the blood through the pelvic arch.
•The lymph vessels of the udder carry lymph upward and toward the rear of each half where it passes through the supramammary lymph glands. When the udder is congested at calving time, it is simply the accumulation of large quantities of lymph.
•Milk let-down; as soon as one begins the milking process, either by hand or by machine, a small quantity of milk can be removed from the cistern and the larger ducts. The stimulus of milking causes the posterior lobe of the pituitary to discharge a hormone called ‘’oxytocin’’ into the blood stream. In about one minute it reaches the udder and causes a contraction of the smooth muscles in the udder forcing the milk from the lumina of the alveoli down the duct system into the cistern (great inflow of milk into the cistern). While the contractions continue, milk can be removed very rapidly from the udder. After about 8 minutes the gradual disappearance of the hormone permits the muscles to relax and, if milking has not been completed, the last milk cannot be removed. When cows are disturbed by barking dogs, shouting, or ill-treatment, they frequently will not ’’let down’’ their milk. This situation is believed to be due to the discharge of adrenalin from the adrenal glands when animals are excited. Adrenalin causes a relaxation of the smooth muscles of the udder and prevents the normal action of oxytocin upon the muscles. Later, when the adrenalin has been eliminated from the blood, the cow again can ’’let down’’ her milk upon the stimulus of milking.