Mammary development - The Gestating and Lactating Sow - page 73

Figure 4.4. Mammary regression in a first-parity sow. Images are taken at weaning (d 0), and days 2, 3, and 7
post-weaning. Note the apparent engorgement of the glands on day 2 and the dramatic decrease in underline
on day 3.
of milk secretion via accumulation of autocrine feedback inhibition of lactation in the
alveolar lumen (Wilde et al. , 1995). A drastic loss of tissue fluid occurs alongside with an
increase in tissue fat content and a considerable cell loss due to apoptosis. Cell loss during
the involution process occurs at least in part through the expression of a set of genes
that control apoptosis but also involves phagocytosis of apoptotic cells by macrophages
(Motyl et al. , 2001). Mammary blood flow decreases by 40% within the initial 16 hours
after weaning the litter (Renaudeau et al. , 2002). The second phase is characterized by
more limited changes in mammary gland component mass, being consistent with the
limited alterations in milk metabolites (Atwood and Hartmann, 1995). In the last phase
of involution, only limited mammary secretions can be collected and they are very viscous
(Atwood and Hartmann, 1995). There are very few alveolar structures remaining at this
stage (Cross et al. , 1958) and there are final reductions in mammary parenchymal tissue
and DNA (Ford et al. , 2003).
The process of mammary involution is affected by the duration of lactation. When
comparing lactation lengths of 22 and 44 days, the Na/K ratio in milk was greater at 44
days indicating that mammary epithelial cell tight junctions became leaky as lactation
advanced (Farmer et al., 2007a). The disruption in mammary epithelial integrity coupled
with an increase in plasma lactose concentrations suggests that the process of involution
started before weaning on day 44 of lactation. The onset of involution could be related
to a reduced suckling intensity because piglets that were weaned at 44 days had access to