Agriculture Reference
In-Depth Information
mammary development of gestating gilts. A recent report indicated that nutrition of sows
in gestation and lactation can affect mammary development of their offspring. Indeed,
dietary supplementation with 10% flaxseed from day 63 of gestation until the end of
lactation increased mammary parenchymal mass of the offspring at puberty (Farmer and
Palin, 2008). This is a first demonstration of such an in utero effect in swine and it leads
to new avenues in terms of development of feeding strategies to enhance mammogenesis.
Nutrition during the last phase of rapid mammary accretion, namely lactation, also
affects mammary development, yet there is very little information on the subject. Kim
et
al.
(1999b) fed lactating primiparous sows four diets that were a combination of different
protein (32 or 65 g lysine/d) and energy (12 or 17.5 Mcal ME/d) levels. Wet and dry
weights of suckled mammary glands were positively affected by both energy and protein
intakes. Results suggested that wet and dry mammary weights were maximized when
sows consumed an average of 16.5 Mcal of ME and 950 g of crude protein per day, the
latter being equivalent to 52.3 g of lysine daily. It is therefore apparent that nutrient intake
during lactation is important for mammary development during that period.
4.6
Mammary involution
4.6.1
At weaning
The mammary gland is particularly interesting because it undergoes repeated cycles of
growth, lactation and involution. Indeed, when piglets are weaned, there is an abrupt
cessation of milk removal which leads to involution of the mammary glands. This process
is characterized by rapid regression of the mammary parenchyma during at least the first
7 days post-weaning (Ford
et al.
, 2003). Changes in mammary tissue are quite dramatic
(Figure 4.4). Parenchymal tissue wet weight and parenchymal DNA decrease by 68.8
and 66.8%, respectively, in those first 7 days. Significant changes are seen as early as
in the first 2 days post-weaning, characterized by drastic decreases in cross-sectional
area, wet weight per gland and parenchymal DNA. From days 2 to 4, regression of the
mammary gland is minimal followed by additional significant declines until day 7 post-
weaning (Ford
et al.
, 2003). The proportion of DNA per tissue mass was found not to
be altered after weaning suggesting that the number of cells per milligram of wet tissue
did not change during the involution process. However, other compositional changes
were observed in mammary parenchyma after weaning, namely, a reduction in protein
percentage and an increase in fat percentage. This is likely due to an increase of lipid
within the tissue, which could very well reflect a transitory accumulation of milk lipid
(Ford
et al.,
2003). An earlier histological study also showed that weaned mammary
glands become engorged in the first few days after weaning and that milk in the lumen
then seems to be reabsorbed (Cross
et al.
, 1958).
Ford
et al.
(2003) suggested that mammary gland involution in weaned sows is achieved
in three phases, namely: (1) from weaning until day 2; (2) from day 2 until day 4 or 5
post-weaning; and (3) from day 4 or 5 until at least day 7 post-weaning. In the initial
phase, the sudden absence of milk removal leads to milk stasis and to an inhibition
