The mammary gland develops early in fetal life, with teat formation starting in the second month of gestation. Development continues through the sixth month of gestation, resulting in four separate glands by the time the calf is six months. Between puberty and parturition, the development of milk ducts and secreting tissue takes place. Milk is produced by alveolar epithelial cells clustered in lobules. Prolactin and corticosteroids are required for the initiation of lactation around birth, while galactopoiesis and continued milk production depends on milk removal through milking. Oxytocin released during milking causes contraction of myoepithelial cells, forcing milk from the alveoli into duct

1. Lactation physiology
By Dr Iqra Bano chandio

2. • The development of the mammary gland starts early in the fetal life.
Already in the second month of gestation teat formation starts and
the 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 medial ligament, teat and gland
cisterns.

3. • The development of milk ducts and the milk secreting tissue take place
between puberty and parturition.
• The udder continues to increase in cell size and cell numbers throughout the
first five lactations of the cow, and the milk production capacity increases
correspondingly.

4. • The cow has four mammary glands grouped into a structure called an
udder, which is located in the inguinal region of the cow's body.
• Milk is secreted by alveolar epithelial cells, which are grouped into
small clusters called lobules.

8. Endocrine Glands Affect Mammary Function

9. Diagram of the arteries supplying the
udder with blood.

10. Initiation of Lactation (Lactogenesis)
• Colostrum or first milk accumulates in the epithelial cells before
parturition.
• It is concentrated with antibodies and passively immunizes the young to a
variety of antigenic factors.
• The initiation of lactation, called lactogenesis, is characterized by increases
in the RNA/DNA ratio, the number of ribosomes, the endoplasmic
reticulum, and the number of mitochondria per cell.
• The anterior pituitary produces hormones essential for lactogenesis.
Prolactin causes localized initiation of milk secretion when injected into
the rabbit mammary gland, but corticoids are required for lactogenesis in
most animals.

11. Maintenance of Lactation (Galactopoiesis)
• Galactopoiesis is dependent on removal of milking stimulus in all animals.
• Milk synthesis will not continue if the product is not effectively removed.
• Prolactin is released at milking along with adrenocorticotrophic hormone
(ACTH) and oxytocin.

12. Mammary Cell Function
• Alveolar milk component synthesis:
• RER > lipid, caseins
• Golgi apparatus > lactose
(also packages lactose,
caseins, minerals, water)

13. Control of Prolactin Production
• Prolactin secretion, release by the pituitary gland, or both are under
control of the hypothalamus.
• The hypothalamus produces a chemical compound called prolactin
inhibiting factor (PIF) that normally inhibits prolactin secretion or
release from the pituitary.
• Compounds such as epinephrine, and other biogenic amines and
acetylcholine decrease PIF and therefore increase blood
concentrations of prolactin.
• Thyrotropin releasing hormone, estradiol, triiodothyronine (T3), and
tetraiodothyronine (T4) stimulate prolactin release.

14. Milk Ejection
• Removal of milk from the mammary gland is dependent on a
functional milk ejection reflex. This is a neurohormonal-dependent
process.
• The ejection of milk results from a nervous stimulus that an animal
associates with milking or suckling activity, such as manual massage
of the udder teats, suckling, or sight and smell of the calf
• The milking machine itself can also stimulate the reflex .The neural
stimulus reaches the central nervous system and causes the posterior
lobe to release oxytocin.

15. Milk Ejection
• Oxytocin reaches the mammary gland and myoepithelial cells, and
contraction of these cells forces milk from the alveoli into the duct
system.
• Milk then flows freely into the larger ducts and cisterns.
• Adrenalin inhibits milk ejection primarily by reducing blood flow to
the gland so that sufficient concentrations of oxytocin cannot reach
the receptors on the myoepithelial cells.

17. Milk Protein Synthesis
• Most of the protein synthesized by the epithelial cells is synthesized from
amino acids that are absorbed from the bloodstream.
• Plasma proteins may provide a small portion of the essential amino acids
of milk protein synthesized in the mammary gland.
• The essential amino acids are absorbed from the bloodstream, and most
of the nonessential amino acids are also absorbed from blood.
• The mammary gland is capable of producing nonessential amino acids
from other amino acids.
• For example, ornithine can be converted to proline by the secretory
epithelial cell.

18. Lactose, Minerals, and Vitamins
• The major sugar of milk is lactose. Lactose is a disaccharide made up of a
glucose and a galactose molecule. The primary precursor of lactose is
glucose .
• The glucose molecule is phosphorylated to form glucose-6-phosphate,
which is then converted into glucose-1-phosphate. The glucose-l-
phosphate, in the presence of uridine triphosphate, forms uridine
diphosphate (UDP) glucose, which is converted to UDP galactose.
• The UDP galactose is united with free glucose to form lactose with the
liberation of UDP.
• The last step is catalyzed by the enzyme lactose synthetase.
• This is a unique enzyme that is composed of two subunits: the common
galactosyl transferase and the milk protein alpha-lactalbumin.

19. Minerals in milk
• The major mineral constituents of milk are calcium, phosphorus,
potassium, chlorine, sodium, and magnesium.
• Potassium, chlorine, and sodium are in soluble form. Phosphates, citrates,
and casein bind various minerals such as calcium and sodium.
• The buffering capacity of milk is due to citrates, phosphates, and
bicarbonates, along with the proteins in the milk.
• Serum calcium is in equilibrium with bone calcium, making it difficult to
increase the calcium content of the milk by increasing the calcium content
of the feed.
• Inorganic phosphate of blood serum is the precursor of milk phosphates.

20. Vitamins in milk
• Vitamins are absorbed by the mammary gland from the bloodstream.
Generally, the vitamin content of milk can be increased by increasing
the vitamin content of blood supplying the mammary gland.
• The ruminant animal depends on feed supply and exposure to
sunlight for its fat-soluble vitamins A, D, and E. Beta-carotene is
converted to vitamin A in the intestinal mucosa of the ruminant.
• Vitamin D in the milk of cows comes from activation of ergosterol in
feed or from the animal's exposure to sunlight. This activates 7-
dehydrocholesterol in the skin of the animal. Milk contains vitamins E
and K.

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