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

Reproduction Nutrition Development, Jul 2018

Marion Boutinaud, Jocelyne Guinard-Flament, HélèneJammes

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

Reprod. Nutr. Dev. The number and activity of mammary epithelial cells, determining factors for milk production Marion BOUTINAUD 1 Jocelyne GUINARD-FLAMENT 1 Hélène JAMMES 0 0 INRA, Neurobiologie de l'olfaction et prise alimentaire , 78352 Jouy-en-Josas , France 1 INRA-Agrocampus, UMR sur la Production du Lait , 35590 St-Gilles , France - 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. - Review 1. INTRODUCTION The mammary gland is made up of a tubulo-alveolar epithelium. Even though this epithelium comprises different types of accessory cells, such as adipocytes, fibroblasts, myoepithelial cells and endothelial cells, it is mainly composed of epithelial cells organised into alveoli. Milk is synthesised within these epithelial cells. It is then secreted into the alveolar lumen and transported via a network of ducts towards the gland cistern. A mammary gland’s ability to produce milk is determined by the number of secreting cells and their level of activity. The mammary gland is an organ whose secreting tissue develops as a function of gestation-lactation cycles. Variations in the number of mammary cells mainly occur during gestation, but also during lactation. Changes in cell number during lactation may be responsible for variations in milk yield. Indeed, the decline in milk production following peak lactation appears to be due to a gradual reduction in the number of milk-secreting cells. Furthermore, changes in cell activity may be involved in variations in milk yield, as a function of milking frequency. However, the phenomena involved in changes in the number and activity of mammary cells are not yet fully understood. It seems necessary to take stock of the knowledge available on how changes occur to the number and activity of mammary cells. The current challenge is thus to better understand the mechanisms controlling cell turnover in the mammary gland, since this determines the quantity of secreting tissue for the production of milk, as well as evaluating the activity of this tissue. However, it is also interesting to define the effects of husbandry practices on these parameters, in order to determine the point at which such practices have irreversible effects on milk production. This review will mainly present data concerning ruminants, although in order to describe novel processes, studies involving other species are also reported. 2. THE NUMBER AND ACTIVITY OF MAMMARY EPITHELIAL CELLS VARY 2.1. Importance of the mammary gland size For many years, a positive relationship was demonstrated between mammary gland size and milk yield [ 1 ]. More recently, a study showed that cows of high genetic merit produced 1.3 times more milk than cows of low genetic merit with 1.3 times more mammary tissue [ 2 ]. Rather than udder size, it appears that the “number of mammary cells” is a better determinant factor for milk production. Indeed, it has been shown in the female rat that milk production is better correlated with the number of mammary cells than with mammary gland size [ 3 ]. 2.2. Changes in mammary cell number during lactation 2.2.1. Acquisition of a production potential at the onset of lactation It has long been accepted that in ruminants, after the development of finished tissue prior to parturition, the acquisition of a milk synthesis potential expressed at peak lactation is largely due to an increase in cellular activity. However, mammary growth can occur at the beginning of lactation in some ruminants. Indeed, it has been shown in the goat and cow that the number of mammary cells, estimated by the total quantity of DNA in the udder, rises markedly between the final days of gestation and the first (...truncated)


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Marion Boutinaud, Jocelyne Guinard-Flament, HélèneJammes. The number and activity of mammary epithelial cells, determining factors for milk production, Reproduction Nutrition Development, pp. 499-508, Volume 44, Issue 5, DOI: doi:10.1051/rnd:2004054