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)