Page 238 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

220 Veterinary Histology of Domestic Mammals and Birds

Smooth muscle cells (myocyti villi) originating from gut flora frequently prevents colonisation of the mucosa
VetBooks.ir the lamina muscularis mucosae pass through the lamina by pathogens.
propria along the longitudinal axis of the villus. These cells
In its entirety, the mucosal barrier protects against
undergo rhythmic contraction, resulting in shortening and absorption of antigens, modifies antigens (antigen han-
lengthening of the villus. Through these contractions, dling) and hinders trans-epithelial uptake of antigens via
the cells act as a pumping station that aids in transport the formation of complexes with secretory immunoglob-
of blood and lymph into deeper vessels (Figure 10.50). ulin A (IgA) (see below).
Myofibroblasts lying perpendicular to the smooth muscle Adaptive mechanisms comprise cellular responses that
cells extend to the sides of the villus. These may also play result in localised production of immunoglobulins. These
a role in contraction of the villus. antibodies, predominantly IgA, are synthesised by plasma
The lamina propria contains dense aggregates of cells in the lamina propria. IgA is taken up by epithelial
lymphatic tissue rich in T and B lymphocytes, plasma cells, coupled with transport peptides and delivered by exo-
cells, monocytes, macrophages, mast cells, neutrophils cytosis from the intestinal glands to the intestinal lumen.
and eosinophils. This is a manifestation of the immune Secretory IgA is resistant to proteolytic secretions
responses that occur continuously within the gastrointes- and thus has a long duration of action. It prevents bac-
tinal mucosa. The lymphatic tissue of the intestinal mucosa teria from binding to the mucosa, agglutinates bacteria
is collectively termed GALT (gut-associated lymphoid and neutralises viruses and toxins. IgA forms the first line
tissue) and resembles the BALT (bronchus-associated of immune defence in protecting the surface epithelium
lymphoid tissue) of the respiratory tract (see Chapter 8, against agents of disease. Other immunoglobulins (IgM,
‘Immune system and lymphatic organs’). occasionally IgG) are of lesser importance in the local
In carnivores, and to a limited extent in horses, a stra- immune response. As the immune system of neonates is
tum compactum is present at the base of the lamina not fully developed, early development of localised immu-
propria near the lamina muscularis. This is an extension nity in the gastrointestinal tract is particularly important.
of the stratum compactum of the gastric mucosa, which
it resembles morphologically. Tela submucosa
The structure of the collagen fibre-rich connective tissue
Species variation of the tela submucosa is typical for the tubular digestive
Birds: Particularly in the chicken, numerous mucosa- tract. In addition to adipose tissue, vessels and neural
associated lymphatic follicles may be present near networks (plexus nervorum submucosus), the submucosa
Meckel’s diverticulum, in the distal jejunum and at the contains solitary lymphatic nodules (noduli lymphatici
entry to the caecum. These tonsil-like lymphatic organs solitarii) and aggregated lymphatic nodules (noduli lym-
(Peyer’s patches) are part of the immune system of the phatici aggregati, Peyer’s patches). These are described in
avian gastrointestinal tract, in which discrete lymph more detail below (see ‘Ileum’). Mucous glands are pres-
nodes are absent in most species. ent in the tela submucosa of the duodenum (glandulae
submucosae) (see below).
LAMINA MUSCULARIS MUCOSAE
The layers of the lamina muscularis mucosae resemble Tunica muscularis and tunica serosa
those of the gastric mucosa. Species variation in muscle The structure of the tunica muscularis and tunica serosa
layer thickness is observed. corresponds to the earlier description applying generally to
the gastrointestinal tract. Macroscopically, the thickness of
INNATE AND ADAPTIVE PROTECTIVE MECHANISMS the layers varies with species, in some cases to a consider-
OF THE INTESTINAL MUCOSA able degree (see anatomy textbooks).
Throughout the life of the organism, the intestinal
mucosa is exposed to a multitude of foreign materials Distinguishing features of segments
and microorganisms (antigens within foodstuffs, viruses, of the small intestine
bacteria, fungi, parasites). To prevent these from enter-
ing the body, the tunica mucosa incorporates innate and DUODENUM
adaptive mechanisms that form a mucosal barrier. The The duodenal mucosa is extensively folded with marked
bactericidal effect of mucus and the acid pH of the gastric plicae circulares. The intestinal villi are uniform, dense and
juice have been described above. Other innate phenomena relatively broad. Abundant intestinal glands are present,
include inactivation of enteropathogenic viruses by bile and the epithelium contains isolated goblet cells.
and the bacteriostatic properties of lactoferrin secreted by A defining histological feature of the duodenum is the
the intestinal mucosa. Interferon also contributes to local presence of glandulae submucosae (Brunner’s glands)
immune defence. In addition, competition from normal (Figures 10.48 and 10.53). Brunner’s glands lie in the tela

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