Agriculture Reference
In-Depth Information
by piglets was exacerbated with the use of hyperprolific sows. It is therefore imperative
to develop management strategies that will increase sow milk yield. One crucial factor
determining sow milking potential is the number of mammary cells that are present at the
onset of lactation (Head and Williams, 1991) and this should receive more attention in
terms of developing the best management practices to optimize mammary development
in growing gilts and in gestating and lactating sows. Rapid mammary development
occurs at three distinctive periods in the life of pigs and it is during these periods that
it is possible to attempt to stimulate mammogenesis via management, nutritional and
hormonal strategies. The present chapter summarizes what we know on the process of
mammogenesis in swine and on the various factors that can affect it. More specifically, the
impacts of nutrition, hormonal status and suckling of a teat on mammary development
in swine will be covered as well as the process of mammary involution.
4.2
Ontogeny of mammary development
The mammary glands of swine are located in two parallel rows along the ventral body
wall from the thoracic region to the inguinal area. The glands (thoracic, abdominal, or
inguinal) are attached to the ventral body wall by adipose and connective tissue. Each
gland is separate and distinct from adjoining glands (Turner, 1952) and it normally has
one teat with two separate teat canals. Each of these canals leads to a small dilation
of the sinus and eventually ramifies into its own section of alveolar-lobular tissue so
that each teat opening has its own self-contained duct and glandular system (Hughes
and Varley, 1980). Mammary tissue is derived from the ectoderm in the embryo and
differentiation of the eventual udder first becomes apparent in the very early embryonic
stage when two parallel ridges of ectoderm appear, these are known as 'milk lines.
Nodules along these milk lines form themselves into mammary buds, each of which
being the progenitor of a teat. At birth, there is relatively little development of the duct
system and mammary glands consist mainly of subcutaneous stromal tissue (Hughes and
Varley, 1980). Accumulation of mammary tissue and mammary DNA, which is indicative
of cell number, is slow until 90 days of age. The rate of accretion of mammary tissue and
DNA then increases four- to sixfold (Sorensen
et al.
, 2002) so that by the time the gilt is
mated, mammary glands are still very small but contain an extensive duct system with
numerous budlike outgrowths (Turner, 1952). More recent data shows that puberty has
a stimulatory effect on mammogenesis as parenchymal tissue mass (which contains the
epithelial cell component of the gland) increases by 51% and extraparenchymal tissue
mass (containing mainly adipose tissue) decreases by 16% in gilts that have reached
puberty compared with gilts of a similar age that have not started cycling (Farmer
et al.
,
2004). Mammary stem cells provide the source of progenitor cells during all stages of
mammary development (Borena
et al.
, 2013). Mammary adipose tissue, including that
within the parenchymal and extraparenhymal tissues, is also important for development
of the epithelial components of the gland (Hovey and Aimo, 2010). The role of mammary
stem cells and adipose-derived stem cells in mammary development in swine has received
only limited attention to date.
