VM8054 Veterinary Histology


Author: Dr. Thomas Caceci

The omasum is sometimes called the "butcher's bible" because its mucosa is formed into leaf-like folds, or foliae, somewhat resembling the leaves of a book. In histological sections like this one, they appear similar to the papillae of the rumen, but they're actually flattened in three dimensional view.




Picture Credit: I am indebted to Dr. Mohammed Khalil of Purdue University's College of Veterinary Medicine for this striking scanning EM photo.

The omasum's mucosal epithelium, as is the case in other regions of the forestomach, is a keratinized stratified squamous type. The omasum, however, unlike the rumen and reticulum, has a true muscularis mucosae. As you can see here, there's a distinct "core" of muscle in each of the folds. Only part of this is muscularis mucosae, however; some of it is the tunica muscularis.

Even in this low-magnification image you can see that there are three distinct bands of smooth muscle in the core of each fold. The central one is not muscularis mucosae. It's part of the tunica muscularis. This really is a flat sheet of smooth muscle extending up all the way to the tip of the omasal fold, and it's sandwiched between two parts of the true muscularis mucosae. It's easier to see this in the high-magnification image below.





In this image it becomes more obvious that the innermost band of muscle fibers in each of these folds is really part of the tunica muscularis. Specifically, the inner layer of the tunica muscularis sends strands of smooth muscle up into the fold. These fibers are separated from the muscularis mucosae by a very scanty—but nevertheless present—submucosa.



Bovine omasum; H&E stain, paraffin sections, 20x and 40x

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