Mammary development - The Gestating and Lactating Sow - page 63

In pregnant gilts, quantitative development of the mammary glands is slow in the first
two-thirds of gestation, while almost all accumulation of mammary tissue and DNA
takes place in the last third (Hacker and Hill, 1972; Kensinger et al. , 1982; Sorensen et al. ,
2002). Concentrations of DNA in mammary tissue increase dramatically during the last
third of gestation (King et al. , 1996). Ji et al. (2006) also reported a significant increase
in weight of mammary glands between days 45 and 112 of gestation, with accelerated
mammary accretion occurring after day 75. Histologically, between days 45 and 75, the
mammary tissue is primarily composed of adipose and stromal tissue, with elongating
ducts and limited branching of ducts to form lobular structures (Figure 4.1; Ji et al. ,
2006) that are similar to the terminal ductal lobular units identified in gilts in response
to mammogenic hormones (Horigan et al. , 2009). During the period between days 75
and 112 glands undergo major histological changes as the adipose and stromal tissues
are extensively replaced by lobuloalveolar tissue (Figure 4.1; Hacker and Hill, 1972; Ji
et al. , 2006; Kensinger et al. , 1982). Ji et al. (2006) also reported a shift in mammary
gland composition going from a high lipid content, reflective of the extensive adipose in
the tissue, to a high protein content during the last third of gestation. Both histological
changes and differences in DNA concentrations in mammary tissues from gilts indicate
increased epithelial cell division between days 75 and 90 of gestation, with maximum cell
concentrations present by day 90. Then, between days 90 and 105, there is an increase
in cellular organelles associated with functional differentiation of the epithelia and
Figure 4.1. Stained histological sections of mammary tissue from pregnant gilts representing days 45, 75, 90
and 112 of gestation. Note terminal ductal lobular units in day 45 and day 75 images. Bar = 50 μm. Images
are from gilts from the Ji et al . (2006) study.