Page 239 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Digestive system (apparatus digestorius) 221

submucosa and may sometimes protrude through the The activity of intestinal and pancreatic secretions
VetBooks.ir lamina muscularis into the lamina propria. They are lim- gradually reduces in the large intestine. Ongoing digestion
ited to the first 1.5–2 cm of the duodenum in the dog and of residual foodstuffs, particularly hydrolysis of cellulose,
are more widely distributed in other species, extending is taken over by carbohydrate- and protein-cleaving intesti-
over 20–25cm in the goat, 60–70 cm in the sheep, 4–5 m in nal bacteria and protozoa. In the caecum and colon of the
the ox, 3–5 m in the pig and 5–6 m in the horse. horse and pig, short-chain fatty acids are formed anaero-
Brunner’s glands are usually densely packed, gradually bically by microbial digestive processes similar to those
thinning out to individual glands more distally. In most occurring in the forestomach of ruminants. These are
domestic species, they are branched tubulo-acinar glands. subsequently absorbed; vitamins B and K are also formed
In carnivores the tubular form is dominant, while in rumi- by bacteria in the large intestine.
nants the glands tend to be acinar. Brunner’s glands are Other important functions of the large intestinal
mucous in dogs and ruminants. In pigs and horses the mucosa include absorption of water and electrolytes, in
secretion may be mixed. The glands produce a viscous, exchange for calcium and bicarbonate, associated thick-
slippery secretion rich in neutral glycoproteins (carni- ening of the intestinal contents and secretion of mucus
vores). The alkaline mucus (pH 8–9) buffers the acidic from goblet cells for lubrication of the faeces. Absorption
gastric juice, protects the intestinal mucosa and creates an of nutrients and vitamins also takes place.
optimal environment for the action of pancreatic enzymes.
In contrast, the mucus produced by the Brunner’s glands Tunica mucosa
of ruminants is typically acidic. A feature common to all segments of the large intestine of
domestic mammals – in contrast to birds – is the absence of
JEJUNUM intestinal villi (Figures 10.58 to 10.60). The internal surface
The basic structure of the jejunum is generally typical for of the intestine is thus predominantly smooth, with some
the musculomembranous tubular digestive tract (Figures longitudinal folds formed by protrusions of the tela submu-
10.49 and 10.54). The finger-like intestinal villi are longer cosa. Throughout the large intestine, elongated, relatively
and finer than in the duodenum. Isolated small lymphatic straight and unbranched intestinal glands extend into the
follicles occur deep in the mucosa and in the submucosa, lamina propria. The glands are tightly packed, filling the lam-
particularly in the pig. Goblet cells are more numerous ina propria to a large extent. The mucosal surface is lined by
than in the duodenum. simple columnar epithelium. Uniformly arranged microvilli
form a brush border on the apical surface of the absorptive
ILEUM epithelial cells. This significantly increases the surface area of
Mucosal folding is notably reduced in the ileum. The the cells and the total surface area available for absorption of
intestinal villi are shorter, less dense and broader than in water and electrolytes from the intestinal lumen.
the jejunum and goblet cells are more plentiful (Figures The surface enterocytes continue into the intesti-
10.56 and 10.57). The ileum is characterised by the pres- nal glands as a simple columnar glandular epithelium
ence in the tela submucosa of aggregated lymphatic (exocrinocyti columnares). Distally these epithelial cells
nodules (Peyer’s patches) that usually protrude far into become less frequent and are replaced by an increasing
the tunica mucosa to reach the surface of the intestine. number of goblet cells.
They frequently bulge into the intestinal lumen, displac- Goblet cells (exocrinocyti caliciformes) are the domi-
ing the intestinal villi. In these regions, the tunica mucosa nant cell type in the intestinal glands of the middle and
(particularly the epithelium) is extensively infiltrated with distal segments of the large intestine (Figures 10.60 and
lymphoid cells. Peyer’s patches are tonsil-like structures 10.61). The epithelial cells are constantly replaced from
that provide localised immune surveillance and defence a population of undifferentiated cells at the base of the
(see Chapter 8, ‘Immune system and lymphatic organs’). gland. The structure of the lamina propria is typical of
musculomembranous tubular organs. A continuous lam-
Large intestine (intestinum crassum) ina muscularis mucosae is present.
The large intestine comprises the caecum, colon and rec-
tum. Macroscopically, the structure of certain segments Tela submucosa
varies dramatically with species (e.g. the caecum in the pig Isolated lymphatic nodules (noduli lymphatici solitarii) and
and horse; refer to anatomy textbooks). These differences aggregated nodules (noduli lymphatici aggregati) are scat-
are reflected only to a certain extent in the histological tered throughout the tela submucosa.
structure of the intestinal wall, particularly in the tunica
muscularis. The layers of the wall of the large intestine Tunica muscularis
essentially conform to the basic structure of a musculo- In the pig and horse, the inner circular layer of the tunica
membranous tube (Table 10.5). muscularis of the caecum and colon forms a continuous

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