C .H. Knight Livestock Production Science 66 2000 169 –176
173
not have a shortened life expectancy. As yet, very few data have been generated in this way.
4. Lactation rescue: a model system for investigating relationships between proliferation
and apoptosis
For many years cell proliferation was intensively studied in the mammary gland and elsewhere, where-
as for the last few years it has almost been forgotten in favour of apoptosis. However, it must be re-
emphasised that changes in cell number result from the balance between the two processes assuming
necrotic cell death is minimal, and so it is very important to understand the relationship between the
Fig. 2. Litter weight gain during lactation rescue in mice. Pups
two. As we have just seen, simply recording rates of
were removed to a foster mother for 2 days on day 6 of lactation
proliferation and apoptosis in normal mammary
and then reunited with their mother. During weight loss day 1 of resuckling pups spent part of their time with the foster mother to
tissue can lead to erroneous results, so a model
ensure they were fed; the data shown is for the period with their
system is required in which the two events can be
own mother. Control mice are shown by the continuous line.
turned on and off such that their time courses and
Values are means6standard error S.E., n 5 6.
spatial relationships can be studied. The majority of apoptosis occurs during post-lactational involution,
so a logical starting point would be to arrest lactation We then repeated the 48-h separation, but this time
so as to turn on apoptosis and, having done so, with administration of bromodeoxyuridine to mea-
attempt to reverse the process. If lactation were to sure cell proliferation during either the separation or
restart, would it require renewed cell proliferation? the resuckling period. Apoptosis was measured using
We have done exactly this in mice Sorensen and a commercial kit Apoptag as before. Compared
Knight, 1997,1998. with control mice with normal lactation, apoptosis
The effects of removing pups from a day 6 was greatly increased during litter removal and then
lactating mouse, placing them with a foster mother quickly returned to normal levels during resuckling
and then replacing them with their own mother 48 h Table 1. This shows that the absence of a suckling
later are shown in Fig. 2. For the first day after stimulus results in involution and cell loss which can
replacement, weight gain was negative during the be reversed if the stimulus is reapplied. The highest
period when the pups were with their own mother level of bromodeoxyuridine incorporation was ob-
they spent 12 h with the foster mother to ensure served during the resuckling period, indicating that,
they were fed. From the second day onwards the
Table 1
pups spent all their time with their own mother and
Cell proliferation and apoptosis in mammary glands of mice either
did gain weight and, by the fourth day, their weight
lactating normally control, separated from their young for 2 days
gain had recovered to normal levels. At this age 11
Separated or separated for 2 days and then reunited for 3 days
a
resuckled
days mouse pups are still entirely dependent on maternal milk, so it was evident that lactation had
Proliferation Apoptosis
been successfully restored. Shorter and longer up to
Control 4.6060.53
0.2360.05
72 h periods of separation resulted in faster and
Separated 9.5060.81
4.6060.55
slower restoration, respectively, and lactation was
Resuckled 13.7662.50
0.6960.24
also recoverable but with greater difficulty follow-
a
Values are means6S.E. for groups of six mice.
ing separation later in lactation Sorensen and
: P , 0.05, : P , 0.01, : P , 0.001, all with respect to
Knight, 1997.
the respective control values.
174 C
.H. Knight Livestock Production Science 66 2000 169 –176
not only was apoptosis turned off, but cell prolifer- tricably linked and that induction of apoptosis re-
ation was also turned on in order to recover the quires not only an initial signal causing cells to enter
lactation. This very neat picture is, however, incom- the cell cycle but also the subsequent absence of a
plete. Whilst bromodeoxyuridine incorporation was survival signal which would otherwise ‘‘rescue’’ the
highest during resuckling, it was also elevated above cell. This reduces the likelihood of inappropriate
control levels during the separation period, i.e., when proliferation i.e., cancer since two mutations would
apoptosis was up-regulated and, we would have have to occur to achieve the desired combination of
anticipated, proliferation was firmly turned off signals, but it also provides the necessary means by
Table 1. This raises the intriguing possibility that, which mammary involution can be rapidly reversed.
faced with the absence of her pups, the lactating mouse decides to turn off lactation but does so in
such a way that a fresh population of secretory cells
5. Cell survival and lactation length
