Dairy Cattle Production (95314) Topic 4-1: Mammary glands and milk secretion

Mammary gland Exocrine gland Two front quarters (40%) and two rear quarters (60%) About 400 to 500 kg of blood pass through the udder to form 1 kg of milk Front and rear quarters are separated by connective membranes Median suspensory ligaments separate right and left quarters

Teats Usually one per quarter Supernumerary teats (extra teats)  - inherited congenital defect - may be functional or nonfunctional - include three types: 1. Caudal teats, are teats located at the rear of the normal ones. 2. intercalary teats, are those found between the normal teats. 3. Ramd teats, supernumeraries that are a ramification of the normal ones

Median Suspensory Ligament -The most important support for the udder - Divides the udder into the right and left halves - Elastic tissue that stretches to allow the udder to expand as it fills with milk.

Lateral Suspensory Ligaments - Fibrous and non-elastic (don't stretch). - extend along both sides of the udder and at intervals send sheets of tissue into the gland to provide support to the inside contents of the udder

Pendulous Udder: results when the Medial and Lateral suspensory ligaments weaken and the cordlike structure that attaches the fore udder to the body wall stretches, Disadvantages of a pendulous udder include: Cleaning difficulty Milking difficulty Risk of injury Risk of mastitis

Streak Canal - approximately ¼ to ½ inch in length, -closed by sphincter muscles. Serves two very important purposes: 1. Prevents the escape of milk between milkings 2. It acts as a barrier to the entry of bacteria and other foreign material. -Milking speed is related to the size of the canal and also the tightness of the sphincter muscles.

Diagram of Duct System

Keratin - The cells that line the streak canal contain keratin. - Keratin is a waxy substance similar to ear wax. - This substance helps to seal the teat end between milkings. - Keratin also has properties that inhibit the growth of bacteria (disease organisms). - The physiological process of the teat canal forming a keratin plug after drying off appears to be a major front-line defensive mechanism.

Fürstenburg's Rosette - Located directly above the streak canal. Made up of loose folds of membrane that smooth out as milk accumulates in the udder. -This aids in blocking the escape of milk between milkings. -This can be damaged by improper milking or by improper use of mastitis infusion nozzles.

Teat Cistern - Is the holding chamber where milk accumulates before it is removed through the teat end during milking. - Is the cavity inside the teat that holds from 1/2 to 1 1/2 ounces of milk, depending on the size of the teat. -It refills continuously during milking. -This is where the first milk to be removed accumulates between milkings.

Gland Cistern The gland cistern joins to the teat cistern at the base of the udder. The gland cistern, which can vary greatly in capacity, functions as a collecting vessel for milk from the major milk ducts that flow into it. The gland cistern fills rapidly during milk letdown.

Secretory System Milk is released by alveoli into ducts which carry milk to the gland cistern. Between milkings approximately 40% of the milk is stored in the teat and gland cisterns and the major ducts while the remaining 60% is stored in the alveoli.

Alveoli -grape-like clusters -Each alveolus is composed of a single layer of epithelial cells surrounding a central cavity. -These cells absorb nutrients from the blood, transform them into milk and discharge the milk into the cavity of the alveolus. -Each alveolus is also surrounded by specialized muscle cells known as myoepithelial cells that are responsible for milk ejection during letdown.

Alveolus

Around the time of calving, the secretory cells of the alveoli start to produce milk. During parturition, there is a large surge of prolactin and this is the signal to start lactation. The important hormones for milk production and the maintenance of lactation are prolactin, insulin-like growth factor (IGF)-1 and growth hormone.

Milk Secretion in the udder Lactose: synthesized in the alveoli from glucose and galactose uder control of the enzyme lactose synthetase. Proteins: - Caseins found in the milk are synthesized from the amino acids taken up from the blood under genetic control (packed in micelles before they are released in the lumen of the alveolus). - The immunoglobulins are synthesized by the immune system, and these usually large proteins are drawn from the blood into the milk.

Milk Secretion in the udder Synthesis of Fats: Acetate and butyrate produced in the rumen are used, in part, as the building blocks of the short-chain fatty acids found in milk. The glycerol needed to “unite” three fatty acids into a triglyceride comes from glucose. About 17-45% of the fat in the milk is built from acetate and 8-25% from butyrate Lipids mobilized from body reserves in early lactation are another building block for milk fat synthesis. In general, only half the amount of fatty acids in milk fat is synthesized in the udder, the other half comes from the predominantly long chain fatty acids found in the diet.

Milk Let-Down Neuro-hormonal reflex The suckling stimulus or massaging of the udder stimulates somatic nerves in the teat, which send a signal to the posterior pituitary gland and causes the release of the hormone oxytocin. Oxytocin causes the myoepithelial (muscle) cells around the alveoli to contract. For efficient milking, there are several important factors to remember. Stimulate 1 min before milk let-down The maximal effect of oxytocin occurs during the first 2 to 3 minutes of milk let-down. Stress during cow preparation or during milking will inhibit release of oxytocin. Inhibition of oxytocin release

Lactation Cycle 4 phases of mammary gland 1. Dry period: development 2. Around calving (-4 to +4 days): Differentiation 3. Lactation: All cell activity directed towards milk synthesis and no further mammary growth. 4. Involution of the mammary gland: this is the gradual but irreversible regression of the gland (i.e. a reduction in the numbers of active alveoli). This starts after the peak of lactation, but is more pronounced during late lactation.

Physiological function during the dry period 1) The period of active involution: 0-30 days after drying-off 2) The period of steady state involution: the period of time the gland is maintained in the fully involuted state. 3) The period of lactogenesis and colostrogenesis: 15-20 days prepartum Regeneration and differentiation of secretory epithelial cells Selective transport and accumulation of immunoglobulin The onset of copious secretion