Agriculture Reference
In-Depth Information
Mammary gland development does not stop at the end of gestation but it continues during
lactation. In the nulliparous sow, the mammary gland consists of cell buds distributed
among fat and connective tissue, whereas in the lactating gland, the connective tissue is
largely displaced by glandular parenchyma. Mammary glands of the lactating sow are
composed of a compound tubuloalveolar tissue with the secretory units arranged in
lobules, which are lined by epithelial cells (lactocytes) that synthesize milk. The average
weight of suckled mammary glands increases linearly from 381 g on day 5 of lactation up
to 593 g on day 21 (57% increase). In first-parity sows, the increase in mammary volume
during lactation is the consequence of both cellular hyperplasia and hypertrophy (Kim
et al.
, 1999a), whereas in multiparous sows, this appears to be mainly due to hypertrophy
(Manjarin
et al.
, 2011). Mammary growth in lactation is related to the position of the
gland on the udder, being greater for the five more anterior teat pairs than for more
posterior teat pairs (Kim
et al.,
2000), and there are indications that it may be related to
the intensity of the post-ejection massage (Thodberg and Sorensen, 2006). Mammary
development is also affected by parity because mammary gland wet weight increased by
63, 21 and 39% between day 113 of gestation and day 26 of lactation, for sows of parity
1, 2 and 4, respectively (Beyer
et al.
, 1994). Both cell division and cell differentiation
contribute to milk production in early lactation in other species such as the goat (Knight
and Peaker, 1984). While it is clear that the mammary gland of the sow grows during
lactation, the extent to which an increase in the differentiation state of porcine mammary
cells contributes to greater milk production has not been fully explored.
4.3
Control of mammogenesis
As noted above, mammary development occurs throughout many stages of growth
and reproduction in swine. Various hormones are involved in the control of mammary
development in swine during the post-pubertal period and pregnancy, the most important
ones being estrogens, relaxin and prolactin. The essential role of estrogens is evidenced
by the drastic effect of puberty onset on mammogenesis (Farmer
et al.
, 2004; Sorensen
et al.
, 2006). Farmer
et al.
(2004) reported a 51% increase in parenchymal tissue mass of
cycling compared with non-cycling gilts of similar ages, which in turn led to increases in
total parenchymal fat, protein and DNA.
During gestation, total plasma estrogen concentrations increase dramatically after
day 75 in gilts (DeHoff
et al.
, 1986). Kensinger
et al.
(1986b,c) demonstrated that the
drastic increase in metabolic activity of the mammary gland occurring in late gestation
is associated with the increase in estrogens of fetal origin; indeed, mammary DNA was
related to circulating concentrations of estrogen in sows on day 110 of gestation. An
earlier study also showed that zearalenone, a mycotoxin with estrogen-like activities,
affects mammary development. An increase in mammary glandular elements due to
ductal hyperplasia was observed in sows receiving zearalenone (Chang
et al.
, 1979).
Mammary development was even observed in some of the 7-day old piglets sucking
the zearalenone treated sows (Chang
et al.
, 1979). Recently, an attempt was made to
specifically stimulate mammary development in gilts by providing a dietary source of
estrogen. When 2.3 g/d of the phytoestrogen genistein was added to the diet of growing
