Agriculture Reference
In-Depth Information
responses. The PP consist of large B cell follicles at germinal centres with adjacent areas of
T cells surrounded by follicular dendritic cells. The PP are overlaid by a follicle-associated
epithelium containing the microfold cells which are interdigitated within the epithelium.
This epithelium expresses Toll-like receptors (TLR) that contribute to ligand-specific
transcytosis. In pigs, lymphocyte production rate, number of total germinal follicles and
mean follicle diameter are greater in ileal PP compared with jejunal PP (Pabst
et al.
,
1988). Moreover, ileal PP may act as a primary lymphoid organ involved in the generation
of B cells (Butler
et al.
, 2009).
After immune induction, the lamina propria (composed of smooth muscle cells, fibroblasts,
blood and lymphatic vessels) functions as a regulator of gut immune responses. Effector
functions reside in macrophages, dendritic cells, neutrophils, mast cells and lymphocytes
(Bailey
et al.
, 2001). In pigs, the gut mucosa has a greater degree of organization compared
with that of rodents and humans. Indeed, plasma cells are preferentially localized into the
intestinal crypt, CD4+ T cells are localized into the core of the intestinal villi, and CD8+
lymphocytes constitute the majority of intraepithelial lymphocytes.
15.2.2
Intestinal exchange surface
The intestine changes its surface by growing in length, in circumference, and in villous
size. These changes have spatial and temporal patterns (Figure 15.2). The increase in
weight of the small intestine during the first weeks of postnatal life is initiated by the first
meals (Butler
et al.
, 2009). It results from the increase in intestinal mucosal mass and
protein accumulation (Xu
et al.
, 1992). Cells produced in the crypt regions differentiate
and mature as they migrate along the crypt to the villous axis. Therefore, the increased
crypt cell hyperplasia observed after birth allows the continuous increase in villous height
which occurs up to weaning. These modifications result in an increase in the surface
of exchange of more than 14-fold in both jejunal and ileal parts of the small intestine
from birth to the end of the suckling period (i.e. during the first three-four weeks of
postnatal life), as estimated by the combined increase in small intestinal length, mucosa
density, dry matter content and villous sizes. In contrast, postnatal growth of the colon is
moderate during the suckling phase, while it accelerates after weaning when pigs are fed
more complex and cereal-rich diets (Montagne
et al.
, 2007). It is noteworthy that crypt
sizes in the colon are more than 3-fold higher than in the small intestine. This may reflect
the necessity of having a large exchange surface in the absence of villi in the colon for
maintaining its main function of water and electrolyte absorption and providing crypt-
associated bacterial reservoirs (Lee
et al.
, 2013).
Colostrum stimulates intensive growth of the small intestine within the first two days
after birth. Zhang
et al.
(1997) reported that 6 h of colostrum ingestion is sufficient to
cause significant changes in intestinal structure. This effect mainly concerns the weight
of the small intestine, which increased by 80% during the first two days of lactation when
the weight of the colon only increased by 30% (Figure 15.2). Accordingly, jejunal villi and
crypt areas increased by 120% and 40%, respectively. The trophic property of colostrum
relies on the presence of growth factors and immunoglobulins (Ig) (see
Chapter 9 on
composition of colostrum and milk; Hurley, 2015). The major trophic role of colostral
