Chapter 10 Lactation
10-1 Structure of mammary glands
Mammary gland: ŕßÓ═ ; Udder: ŕß█«
* Anatomical and morphological differences of mammary glands by species:
See TABLE 10-1
* Structure of mammary glannd of cow and mare: See FIGUREs 10-1 and 10-2
10-1-2 Morphology of cow mammary gland
* Components of mammary gland:
a. Supporting tissues
Figure 10-1 Diagram of the duct system in one quarter of the mammary gland of the cow with a single lobe illustrated. Four quarters are fused into a single gland complex.
Figure 10-2 Diagram of the gland complex found in the mare.
2) Ligaments: See FIGURE 10-3
Figure 10-3 Diagram of a cross section of the supporting structures of the mammary glands of the cow as viewed from the rear.
Lateral suspensory ligaments: not elastic, and sends lamellae into udderíŠ continuous with interstitial framework
Median suspensory ligament: elastic, and stretch as udder fills with milk
3) Connective tissue
Form lobe and lobules
a. lobule: contain 150 to 225 alveoli
b. Tissues involved in milk synthesis
Alveoli: See FIGURE 10-4
Figure 10-4 Diagram of alveolus showing lumen, epithelial cells. myoepithelial, and capillaries.
Epithelial cells: synthesis and secretion of milk into lumen of alveoli
c. Tissues involved in milk transport
Milk flows from alveoli lumen íŠ ducts íŠ major ducts íŠ gland cistern
íŠ teat cistern íŠ teat meatus
d. Myoepithelial cells: Covers surface of alveoli and small ducts down
within a lobule: contraction for milk ejection
e. Blood supply to mammary gland
Volume ratio of blood passed to milk synthesized: 400: 1
In cow, ewe, doe, and mare: External pudic artery; External pudic and
subcutaneous veins: See Ref. Fig. 17.2 , Bath et al., 1985
In sow: External pudic and thoracic arteries; External pudic and external
10-2 Hormonal regulation of the development and function of the mammary gland
* Embryonic formation of mammary bud: Under any endocrine control?
Mammary band: at 30 days in cattle: thickened area of epithelial cells íŠ mammary line íŠ mammary crest íŠ mammary hillock íŠ mammary bud
10-2-1 Mammary development
A. During fetal period: Mummary bud íŠ primary sprout: first milk secretion tissue at 3 months íŠ secondary sprout íŠ tertiary sprout Hormonal regulation: Not comopletely understood; but by PRL synergizing with insulin, adrenocortical steroid hormones and progesterone
B. From birth to pregnancy:
Replacement of fatty tissues with ductal tissue : From 3 months before puberty to several months after puberty: By cyclic surges in estrogen
C. During pregnancy:
a. Mammary ducts: By estrogen
b. Alveolar development: By progesterone synergizing with estrogen
c. Preparation of mammary tissue for milk synthesis: By synergistic action of progesterone, estrogen, PRL, GH, insulin, thyroxine and cortisol.
d. Other hormones: Placental lactogen: For development of mammary tissue
e. Actual milk secretion: By high concentration of progesterone during
10-2-2 Milk secretion
a. Initiation of milk secretion: By PRL dominantly íš released by suckling
stimulus and its synergistic action with cortisol, GH íšreleased by
suckling , thyroxine and insulin
b. Maintenance of lactation:
1) By suckling and milk removal : By GH rather than PRL in lactating cows after peak milk production at 2 months into lactation
2) Adequate level of nutrition is necessary for maintenance of lactation.
Lactation proceeds at expense of body reserves of mother.
3) Involution of mammary tissue: From termination of suckling and milk removal íŠ Degenration and loss of alveolar epithelial cells, and then connective tissue and fat cells become more prominent, and only duct system remains
* Milking twice a day at 12 hrs. interval: an important practice in dairy farming
d. Agalactia (complete lactation failure) or hypogalactia (partial lactation) in sow: Loss of milk in first 3 days after farrowing:
1) A part of complex condition of MMA ( mastitis, metritis, and agalactia)
2) Promptly inj. antibiotics and oxytocin, if body temperature is elevated.
If not effective, prompt fostering and artificial rearing must be made.
3) No effective preventive measures, but keep lactating sows from high ambient temperature.
10-2-3 Milk ejection (= Let-down of milk)
a. Suckling, teat massage, presence of young,and sounds or ordors associated with milking íŠ sensory nerves íŠ hypothalamus íŠ posterior pituitary:
release of oxytocin íŠ arterial circulation: stimulates contraction of myoepithelial cells of alveoli and small ducts íŠ forces milk dowm: See FIGURE 10-5
Figure 10-5 Effect of exogenous bovine somatotropin on milk yield.
b. Inhibition of milk ejcetion: Excitement íŠ release of epinephrine íŠ
vasoconstrictions of small arteries and veins íŠ 1) preventing oxytocin
from reaching myoepithelial cells or 2) inhibiting oxytocin release
from posterior pituitary íŠ inhibition of milk ejction
10-3 Composition of milk
A. Composition of milk : different by species: See TABLE 10-3
Figure 10-6 The neurohormonal reflex of milk ejection. (A) that a cow associates with milking causes a nerve impulse (B) to travel via the inguinal nerve(1) to the spinal cord(2)and the brain(3) The brain causes the release of oxytocin (D) from the posterior pituitary (C). Oxytoxin is released into a branch of the jugular vein(4) and travels to the heart(5) and is then transported to all parts of the vody by the arterial blood. The oxytocin reaching the udder leaves the heart by the aorta(6) and enters the udder through the external pudic arteries (7).Un the udder. it causes the myoepothelial cells to contract, resulting in milk ejection from the alveioli.
Milk fat %: Donkey : 1.3% Sow : 5.4%
Mare : 2.0 Ewe : 7.5
Holstein cow : 3.6 Rabbit :12.2
Human : 4.5 Bear :31.5
Jersey cow : 4.9 Seal :53.2
Some data: From Schmidt, 1978, pp. 2
B. Colostrum: high protein and fat, vit. A and minerals than normal milk
C. During late lactation: Milk volume dec. and fat % inc.
D. During milk removal: First milk: low fat; last milk: high fat