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Mammary Gland Development

Mammary Gland
Development During

Cross section of a beef heifer's udder.

For more on mammary growth during lactation see Tucker (1987) in the general reference list below for review.

The number of mammary cells in the lactating gland are critical for milk production. Mammary cell numbers continue to increase even after parturition. Mammary wet weights and total DNA contents continue to increase in early lactation. The impact of this increased mass of mammary tissue on milk production can be substantial in some species. For example, total DNA amount in mammary glands of rats during lactation is highly correlated (r-squared = 0.85) with litter weight gain (Tucker, 1966). Mammary growth during lactation has been described in a number of species (see above reference list).

In rats, total mammary DNA can increase by over 100% during lactation depending upon litter size (Tucker, 1969). Litter size in a species like the rat or pig is directly related to nursing intensity (a combination of number of nursing young and the intensity with which they nurse). Nursing intensity has a major role in the extent of mammary growth during lactation.

In the sow, mammary wet weight can increase by 55% and total mammary DNA can increase by 100% between day 5 and day 21 of lactation when the sow nurses 9 or 10 pigs (Kim et al., 1999).

In cows, mammary DNA increased by 65% from 10 days prepartum to 10 days postpartum (Akers et al., 1981, Endocrinology 109:23), although how much of this increase occurred pre- and postpartum was not determined. Cell numbers in the cow mammary gland have not been determined throughout the lactating period.

The role of nursing intensity in mammary growth during lactation is seen in sows. From the discussion above on the role of relaxin in mammary development in the pig during pregnancy, the removal of the source of relaxin during late pregnancy results in substantial reduction of mammary development at the time of parturition. If such animals undergo cesarian section to remove piglets (the mothers can not give birth normally in the absence of relaxin) and allowed to nurse normal day-old piglets, they are still able to maintain litter weight gain at nearly normal levels (Zaleski et al., 1996). Mammary growth seems to be stimulated by nursing. The mammary tissue of the lactating sow has substantial growth potential. This growth is stimulated by nursing of the young.

Mammary Development
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