Author: Dr. Thomas Caceci
Here are two views of the rumen papillae. The one at the left is a scanning electron micrograph, and the three-dimensionality of this method of visualization shows the papillae to be long, slender finger-like projections. The conventional image below demonstrates that they're projections of the tunica mucosa only. They have a core of CT (the lamina propria) and, as is true of all the divisions of the ruminant forestomach, are covered with stratified squamous epithelium. There is no muscularis mucosae in the rumen.
You will still see the statement in some texts that the parts of the forestomach are derived from the embryonic esophagus, and that's the reason why (like the esophagus) they're lined with stratified squamous epithelium. This is open to doubt. While there is no doubt that the rumen, reticulum, and omasum are all derived from the embryonic foregut, there is evidence to indicate that all three come from that an area of the foregut homologous to the part that develops into the single stomach of non-ruminants. If this is so, then the forestomach can be considered diverticula (or derivatives) of the stomach proper, not the esophagus, despite the nature of the lining. The image at right is of a single papilla. It has a core of lamina propria, and no muscularis mucosae. At this magnification you can see the entire length of this papilla, and the absence of the muscularis mucosae is more obvious.
Picture Credit: I am indebted to Dr. Mohammed Khalil of Purdue University's College of Veterinary Medicine for the scanning EM image here.
A higher magnification of this area shows more detail, as seen at right: the nature of the epithelium becomes clear. Note that the keratinization is fairly heavy in this part of the forestomach; it becomes less so in the deeper parts, but nevertheless keratinization is a feature of the reticulum and the omasum as well.
This image shows some black flecks close to the epithelium. These aren't artifact. They are the preserved remains of the ciliated microorganisms that live in the forestomach and are the actual means by which the energy contained in cellulose is utilized.
Bovine rumen; H&E stain, paraffin sections, 20x, 40x, and 200x
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