Page 227 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Digestive system (apparatus digestorius) 209

Table 10.3 Comparative structure of the compartments of the ruminant forestomach and abomasum.
VetBooks.ir Compartment Epithelium Glands Muscles Special features

Lamina muscularis mucosae Conical or elongated
No glands
Keratinised
Rumen
stratified absent, circular and finger-like papillae,
squamous longitudinal layers of varying in length and
smooth muscle in tunica morphology
muscularis
Reticulum Keratinised No glands Lamina muscularis Presence of papillae on
stratified mucosae present in upper surface of crests
squamous portion of the primary
reticular crests,
tunica muscularis is
weaker than in the
rumen
Omasum Keratinised No glands Lamina muscularis Prominent tunica
stratified extends to varying degree muscularis, fibres of
squamous into the higher order (I– circular layer extend into
III) omasal laminae higher-order omasal
laminae
Abomasum Simple Cardiac, proper Inner circular layer, outer
columnar gastric and longitudinal layer
epithelium, pyloric glands
gastric pits

RETICULUM The smooth muscle comprises an inner circular and
The reticulum is characterised morphologically by reticu- outer longitudinal layer. A tunica serosa forms the out-
lar crests (cristae reticuli) that form the four- to six-sided ermost layer in the region facing the peritoneal cavity.
boundaries of reticular cells (cellulae reticuli) (Figures Between the rumen and the spleen, a tunica adventitia
10.39 and 10.42). Secondary and tertiary crests further is present.
divide the reticular cell into smaller chambers. Stratified The mucosa of the rumen has important functions and
squamous epithelium lines the reticular crests. The sides of is responsive to dietary factors. Within 4–5 weeks, a high-
the crests are studded with small conical papillae. energy, low-cellulose diet induces growth of the papillae,
Originating from the oesophagus, a lamina muscularis is proliferation of a thickened, non-keratinised epithelium
present in the upper portion of the primary crests. The mus- and elaboration of the subepithelial capillary network. In
cle bundles surround each reticular cell and form networks response to an energy-poor, high-fibre feed, the papillae
with those of adjacent cells. The lamina propria blends with- become shorter and plump keratinised cells appear in the
out clear distinction with the tela submucosa. The tunica epithelium. These changes reflect the adaptability of the
muscularis is composed of an inner circular and outer lon- structural elements of the papillae and represent normal
gitudinal layer. Additional striated muscle fibres, remnants processes occurring in wild ruminants due to seasonal
of the oesophageal musculature, radiate into the reticulum. changes in diet. Comparable cycles of proliferation and
The outer layer of the reticulum comprises a tunica serosa. regression are also observed in high-performance animals
during the dry and subsequent lactation period.
RUMEN The key determinant of functionally induced mor-
The rumen is distinguished by the presence of papil- phological adaptations of the ruminal papillae is the
lae (papillae ruminis). These tongue-shaped structures concentration of short-chain fatty acids, particularly
(Figures 10.40, 10.42 and 10.43) vary considerably in form, β-hydroxybutyric acid, in the forestomach. Fatty acids are
length, distribution and density. The surface of the rumen formed in the rumen and in the ruminal epithelium by
is lined by non-glandular mucosa. The connective tissue microbial (bacterial and protozoal) digestion of feedstuffs
of the lamina propria incorporates condensed layers of containing carbohydrate. Numerous other substances are
collagen fibres and smooth muscle cells; a delicate subepi- also produced by anaerobic fermentation in the rumen and
thelial capillary network is present. The tunica muscularis reticulum, most of which are absorbed across the tunica
consists of smooth muscle, with some oesophageal skel- mucosa. These include water, methane, carbon dioxide,
etal fibres extending into the ruminal atrium. lactic acid, non-protein nitrogen compounds (ammonium

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