Histology of the complex ruminant stomach (Day 162)

Vet School Diary Histology Bovine Rumen Section under the Microscope

Today’s Diary Entry is sponsored by Find Pet Boarding

Start of my second week of my second semester, last week vanished in a blur and this week I have tried to plan my time better to get a lot more of the research for my Emergency First Aid for Animals book completed.

Now I think I have previously said before how the stomach with ruminants is divided into four different compartments; the rumen and reticulum, omasum and the abomasum. Each of these has a specific role to play in digestion and specific structure to match. This is known as the complex stomach and the esophagus enters this between the reticulum and rumen. The rumen is the largest stomach compartment and is connected to the reticulum with a large opening allowing food to be passed back and forth between the two. Remembering that ruminants eat plant matter the rumen is where fermentation takes place with a large amount of bacteria and protozoa act on the contents breaking it down. The surface of this is covered in small papillae (low conical projections) and looks like this.

Vet School Diary Histology Bovine Rumen Section under the Microscope
Bovine Rumen Histology

The reticulum has a honeycomb like structure and sits next to the rumen and starts the mechanical breakdown of food into fine particles. This compartment is usually involved in “hardware disease” where cows swallow screws, nails wire or other hardware and it embeds into the lining. Occasionally this will completely penetrate the reticulum lining causing disease and as the reticulum sits next to the diaphragm will head into the thorax towards the heart.

Vet School Histology Section of Bovine Reticulum with Honeycombe structure
Histology of Reticulum showing a cell from the honeycomb structure

The last structure unique to compound stomach is the omasum which is where the final stage of mechanical digestion takes place before the plant matter enters to Abomasum which is basically the same as a simple stomach. The lining of the omasum consists of really large muscular folds which help the mechanical breakdown of food into tiny particles suitable for digestion in the stomach and intestine. Below you can see the size of these folds in relation to the Rumen and Reticulum (its so big I’ve had to put multiple different pictures together to make it!)…

Histology section of the bovine omasum showing the folds in the mucus tissue
Histology section of the folds in the omasum of the cow

After this the food then enters the abomasum which is the same as the simple glandular stomach and enzymes and gastric juices start to digest the food here before it then enters the small intestine.

This afternoon in Physiology we finished up looking at the endocrine system which is responsible for controlling the body systems using chemical messengers (aka hormones). There are several different hormones and I do have plans to do a diary entry in depth on each hormone in the near future so will leave this here until then.

What makes a Herbivore, the science behind Ruminants!

Herbivore white horse eating grass
Herbivore white horse eating grass

So continuing on the nutrition topic, tonight I am writing a little bit on herbivores which include a lot of farm animals, horses, and rabbits. When talking about herbivores the easiest way to define it is non-meat eaters, or eaters of vegetation.

Whilst its common knowledge that many animals eat grass because they are herbivores, whats not so common is an understanding of the differences internally to allow the digestion of plant matter. I know personally when I was younger I was aware that cows had 4 stomachs, and used to picture this as 4 different containers connected by tubes. In fact a cow has a compound stomach, with one organ that is split into 4 different chambers carrying out different digestive functions. Anyways, before I get off topic and turn this entire article into cows (I think they’re cute in the day, but have you ever met a cow in the dark?!?!)…

Herbivores can further be broken down into Ruminants (like the cow, sheep, deer and goats)  and non-ruminants such as the horse and rabbit. To quickly define the difference ruminants have a compound stomach, and non-ruminants have a single chamber (or monogastric) stomach. In addition ruminants will also regurgiate and re-chew (the scientific name for chewing is mastication) their food. This is because they do not chew much when eating as large ruminants have the space to store up to 95 litres of undigested food in the rumen!

Ok, so revising my original title a little, tonight I am just going look at Ruminants as its such a big topic, lets start the tour of the ruminant digestive system from the mouth!

Dentition (or teeth)

All ruminants apart from the llama and camel have no upper front teeth, instead they have a hardened gum area. Generally ruminants eat fast, and then regurgitate their food for mastication with saliva which breaks it down to allow easier digestion. Most cows produce between 66 – 88 litres of saliva a day depending on the water content of their food, and in some extreme cases up to 300 litres a day! Saliva is important as digestion of plant matter occurs best in a neutral pH environment because this is where the enzymes and microbes work best to break down the tough cell walls (I will write more on this another day).


Is the muscular tube which connects the mouth to the compound stomach. The muscles contract along it to push food into the reticulum (the scientific name for this process is peristalsis) and in ruminants this process can also occur in reverse to allow rumination.


The reticulum is the first chamber of the compound stomach in ruminants, the lining of this chamber has epithelial (skin) folds forming polygonal cells which give it a honeycomb like appearance. Small papillae cover this area.


The rumen is the next chamber which as previously stated can hold up to 95 litres in large animals. Vertebrates lack the enzyme cellulase which is needed to break down cellulose which forms much of the sugars within plant material, instead ruminants use symbiotic anaerobic which do contain this enzyme. The rumen becomes colonised with these symbiotic anaerobic bacteria in the first week after birth which helps to break down food into sugars and release nutrients by fermentation. In fact each ml of rumen fluid contains over 10 billion bacteria. The epithelium is covered with numerous papillae which vary in shape and shape.


After being broken down enough food passes into the omasum. The epithelium of the omasum is heavility folded and it has been estimated that the omasum contains over a third of the surface area of the compound stomach. This allows for rapid absorption of water and salts before the movement of food into the abomasum.


Also called the “true stomach” this chamber is different to the previous chambers in that it is the closest that resembles a monogastric stomach and secretes enzymes and hydrochloric acid which hydrolyse protein within the food and bacteria. The hydrolysis breaks the protein into small subunits such as amino-acids and dipetides ready for absorbtion in the small intestine.

Small Intestine

The small intestine in ruminants is around 20 times the length of the animal and is where most of the absorption of water and nutrients with further enzyme breakdown. It is split into 3 parts the Duodenum, Jejunum and Ileum.


Is where the common bile duct and the main pancreatic duct terminate. The common bile duct carries a greenish fluid from the liver which helps with the breakdown of fats. The pancreatic duct carries digestive fluids including enzymes and bicarbonate which deactives the hydrochloric acid from the Abomasum to prevent enzymes being denatured.


The lining of the Jejunum contains projections called villi which increase its surface area and which are in turn covered with microvilli making it even large. It uses these to absorb nutrients with a specialisation for carbohydrates and proteins. There are different ways that nutrients can cross this wall and be absorbed which I will look at another day.


The main function of the ileum is the absorption of nutrients that have not yet been absorbed included vitamin B12 and bile salts. Where it joins the large intestine there is a one way valve which stops contents from flowing back into the small intestine.


This is a large pouch with a single opening where the ileum and large intestine meet, generally in herbivores it is larger to allow further breakdown of cellulose that has not yet been broken down. With ruminants it is not as important as it is for monogastric herbivores.

Large Intestine

This comprises the ascending colon, transverse colon, sigmoid colon, rectum, and anus. Much of the large intestine comprises the colon which is shorter but wider than the small intestine. This is responsbile for the absorption of water and sodium as well as providing a environment for bacteria to grow and reproduce. Many important nutrients are produced by this bacteria including vitamin K, thiamine, riboflavin and more. The final function is the excretion of waste through the rectum and anus.

Wow now that was an adventure!!! And now you should have a overview of the digestive tract of ruminants which we can compare to non-ruminants later!