The number and activity of mammary epithelial cells, determining factors for milk production.

The ability of ruminant mammary glands to produce milk is determined by the number of cells secreting milk and their level of activity. Changes in the number of cells in the udder occur during lactation. It has been shown that mammary cells proliferate during this process, while other cells die through apoptosis. The decline in milk production after peak lactation appears to be due to a gradual reduction in the number of milk-secreting cells, either through cell death or by the abrasion of epithelial cells during milk ejection. Other factors are also known to modify cell turnover in the udder, such as reproductive status, growth hormone treatment or milking frequency and nutrition. A description of the effects of husbandry practices makes it possible to envisage different processes for mammary tissue regeneration during lactation. Indeed, changes in milking frequency are capable of modifying the number of epithelial cells in an alveolus, while GH treatment acts on the total number of alveoli. Thus recent studies have demonstrated an heterogeneity of the processes of proliferation and cell death within the mammary gland. However, unanswered questions still remain concerning the presence of stem cells in ruminants, the lifespan of mammary epithelial cells or the relative rate of loss of mammary cells due to apoptosis and epithelial abrasion.