Let’s Ruminate on it

Ruminants are artiodactyl characterised by their possession of stomachs with four compartments. They are said to have four “stomachs” which is technically untrue as they have only one stomach but divided into four parts: the rumen, reticulum, omasum and abomasum. Nearly 200 wild ruminant species exist including cows, goats, deer, giraffes, moose and elk.

Origin of the Ruminants Stomach

When the first ruminant groups emerged, they were rabbit sized (< 5kg; Hackmann & Spain, 2010). Their skull and dental morphology were optimal for consuming fruits and insects. This shows that the first ruminants were small, forest welling omnivores (Webb, 1998b). The first ruminants did not have a functional rumen until about 40ma as indicated by dental morphology and molecular techniques (Hackmann & Spain, 2010). At about 18 to 23 ma, new families of ruminants evolved with these families having masses of 20kg to 40kg. The new families ate primarily leaves as evidenced by dental wear (Hackmann & Spain, 2010). By about 5 to 11 ma, grasslands had expanded and some species began including more grass in their diets leading to the development of more complex stomach to help in the digestion of the grasses.

Structure and Histology of the Ruminant’s Stomach

The ruminant’s stomach has four compartments: the rumen, reticulum, omasum and abomasum.

Rumen: this is the first and largest compartment of the stomach. After a ruminant feeds, food initially collects in the rumen. The rumen is saclike, thin walled and lined and lined with numerous projecting papillae that increases its absorptive surface area. The rumen has highly vascularized connective tissue covered by stratified squamous epithelium. The epithelium is involved in the absorption of short-chain fatty acids (i.e. sodium salts, the microbial breakdown products of cellulose). This transepithelial transport is highest in areas where the cells appear to be swollen (Kwan, P.). Muscularis mucosae are absent in this part of the forestomach. The muscularis external contains two layers of smooth muscle fibers. Myenteric plexus can be found between

these layers. (Kwan, P.). The rumen serves as a large holding and fermentation vat (Kardong, 2002). Later, food in the rumen is regurgitated back into the mouth, remasticated and swallowed again. The process is repeated until there has been thorough break down of plant material (mastication) and chemical attack on cellulose (fermentation; Kardong, 2002).

Reticulum: this is the most cranial part of the stomach and food and sometimes objects drops in here first. The reticulum consists of mucosa, submucosa, tunica muscularis and serosa. The reticular walls are lined with mucous membranes that are organized in folds (laminae) to form a honeycomb pattern. On the laminae are short conical projections called papillae. The lamina epithelial is keratinized stratified squamous epithelim. Contractions of the honeycomb cells, with the purse-string action of the smooth muscle strands, help the mechanical digestion of the ingesta. The reticulum contracts to slosh the ingesta between itself and the rumen (Kardong, 2002).

Omasum: this is the third portion of the system that ingesta pass through in the ruminant. It is spherical in shape and located to the right of the rumen and reticulum, as well as being the smallest of the four compartments. The inside walls are covered in muscular laminae that are studded with short papillae. The mucosa and portions of muscularis mucosae form parallel folds that resemble pages of an open book. The mucosal surface is covered by stratified squamous epithelium. The core of the laminae are quite characteristic with three layers of smooth muscle. Extending upward from the muscularis mucosae are fibers that form the two outer layers. The orientation of the muscle cells is parallel to the free edge of the laminae. Sandwiched between these two layers is a single inner layer of smooth muscle. This is derived from the muscularis externa and the fibers are perpendicular to those of the outer layers. The omasum operates like a two phase machine. First, there is relaxation of the muscular walls which aspirates fluids and fine particles from reticulum into the omasum (Kardong, 2002). Second, the omasum contracts to force the digesta into the abomasum. The omasum absorbs volatile fatty acids, ammonia and water and at the same time separates the fermenting content of the rumen and reticulum from the highly acidic content of the abomasum (Kardong, 2002).

Abomasum: this is the fundic part of the stomach in which further digestion occurs before digesta pass to the intestine. It is the glandular portion of the ruminant stomach and its histology is highly similar to that of monogastric animals. The lumen of the glandular stomach is lined with simple columnar mucus-secreting epithelium. The mucosa is thrown up into longitudinal folds called rugae which stretch flat as the stomach distends. The lamina propria is a loose connective tissue layer rich in capillaries and lymphoid cells and is entirely occupied by glands, the gastric glands (Kardong, 2002). The abomasum produces hydrochloric acid and digestive enzymes, such as pepsin (breaks down proteins), and receives digestive enzymes secreted from the pancreas, such as pancreatic lipase (breaks down fats). These secretions help prepare proteins for absorption in the intestines. The pH in the abomasum generally ranges from 3.5 to 4.0. The chief cells in the abomasum secrete mucous to protect the abomasal wall from acid damage (Kardong, 2002).

Development of the Ruminant Stomach