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Lactation
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Role of Hormones in Galactopoiesis


Role of
Hormones

Cows walking to the barn.

In most species, the mammary gland is not static during most of the lactation period. Although it is thought that any given cell can not divide and lactate at the same time (this is still controversial), it should be remembered that the gland is made up of many epithelial cells and alveoli, and different alveoli may be in different functional states at any given time.

The lactating goat offers an example of the dynamic changes which occur in the mammary gland during a lactation (see Table below). The increase in milk yield in early lactation is caused by increasing mammary cell numbers (mammary growth during lactation) and some increase in milk secreted per cell. Nutrient availability, metabolic state of the animal, and environmental factors will affect all stages of lactation, but particularly the period of peak and declining lactation. The nanny is often pregnant in very late lactation and concurrent pregnancy with lactation will affect milk yield. In very late pregnancy, there is cell loss (via programmed cell death, called apoptosis), as well as a decline in the milk secreted per cell. A similar scenario can be assumed for the dairy cow.

Qualitative changes in milk yield, mammary secretory cell number, and milk yield per cell occurduring lactation in goats. For example, mammary DNA in lactating goats increases by over 25% during the initial three weeks of lactation, but milk production does not peak until about the eight week (Knight and Peaker, 1984). Activity of lactogenic enzymes continue to increase beyond the third week of lactation, indicating an increase in differentiation of secretory cells. The table below is a summary of work by Knight, C . and M. Peaker (1984, Quarterly J. Exper. Physiol. 69:331).

Weeks of Lactation

Phase of the Lactation Curve

Milk Yield

Cell Numbers

Yield per Cell

1

Ascending

Increasing

Increasing

2

Ascending

Increasing

Increasing

3

Ascending

Increasing

Increasing

Increasing

4

Ascending

Increasing

Static

Increasing

5

Ascending

Increasing

Static

Increasing

6

Ascending

Static

7

Peak

Static

8

Peak

Static

Declining

8 - 11

Peak

Declining

Declining

11 - 23

Declining

Declining

Declining

23 - 36

Late lactation, pregnant

Declining

Static

Declining

In dairy cattle, mammary DNA also increases in early lactation (Akers et al., 1981), but the precise timing and extent of this postpartum mammary growth has not been defined. Few other estimates of mammary gland changes in DNA content in cattle are available, but postpartum mammary growth may be expected for only a short period in early lactation.

The changes in mammary cell numbers (by growth or by cell death) and in milk yield per cell are regulated in part by galactopoietic hormones and in part by local mammary factors. The other sections of this Module discuss specific hormones and their role in regulating established lactation, includinhg Prolactin, Growth Hormone, Placental Lactogen, Adrenal Corticoids, Thyroid Hormone, and Ovarian Steroids.


 
Lactation
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