Rumen
[2][3] The rumen is known for containing unique microbial networks within its multiple sac compartments to break down nutrients into usable energy and fatty acids.
[1][4] The outer lining of the rumen, known as the epithelium, serves as a protective layer and contributes to the metabolic processing of fermentation products.
[3] The inner lining of the rumen wall is covered in small fingerlike projections called papillae, which aid in nutrient absorption.
[1] These features increase the surface area of the reticulorumen wall, facilitating the absorption of volatile fatty acids and capture of smaller digesta particles.
[1] The rumen and the reticulum differ with regard to the makeup of the lining but account for approximately 80% of total ruminant stomach volume.
[1] Digestion in the rumen and reticulorumen occurs through fermentation by diverse microbe communities to optimize resources from nutrient dense feed.
Peptides, amino acids, ammonia, and other sources of nitrogen originally present in the feed can also be used directly by microbes with little to no hydrolysis.
[1] This creates a symbiotic relationship between the ruminant and the microbial communities, as the microbes can be used as a protein source when washed into the abomasum section of the digestive tract.
[1] Digested food (digesta) in the rumen is not uniform, but rather stratified into gas, liquid, and particles of different sizes, densities, and other physical characteristics.
Though they may seem trivial at first, these complicated stratification, mixing, and flow patterns of digesta are a key aspect of digestive activity in the ruminant and thus warrant detailed discussion.
The uppermost area of the rumen, the headspace, is filled with gases (such as methane, carbon dioxide, and, to a much lower degree, hydrogen) released from fermentation and anaerobic respiration of food.
The different sacs of the rumen allow for varying ecological niches for microbes in the reticulorumen, including bacteria, protozoa, fungi, archaea, and viruses.
Protozoa (40-60% of microbial mass) derive their nutrients through phagocytosis of other microbes, and degrade and digest feed carbohydrates, especially starch and sugars, and protein.
[9] Fungi occupy an important niche in the rumen because they hydrolyse some ester linkages between lignin and hemicellulose or cellulose, and help break down digesta particles.
[1] Subsequently, microbial biomass is digested in the small intestine and smaller molecules (mainly amino acids) are absorbed and transported in the portal vein to the liver.
[3] Additionally, there must be an adequate amount of short chain fatty acids, produced during fermentation, to properly develop the papillae.
[3][12] Due to ruminants being born with a sterile gastrointestinal tract, the developing rumen must be exposed to an array of microflora at an early stage.
[3] This is due to the nutrients ingested by the animal not entering into the rumen stomach compartment, as it is instead bypassed by the reflexive closure of the esophageal groove.
Other bacteria, such as Lachnospira multiparus, Prevotella ruminicola, and Butyrivibrio fibrisolvens, play essential roles in the creation of volatile fatty acids (VFAs).
[14] Previous research identified the significant impact of volatile fatty acids on rumen development through the effects of the inter-ruminal insertion of acetate, propionate, and butyrate.
[15] The most visually notable and impactful of these volatile fatty acids was butyrate, which is synthesized naturally in ruminants through multiple anaerobic fermentation pathways of dietary substrates.
