The source for this specimen should be obvious (and confidently
so), provided you are familiar with the organ it represents.
You should easily see three distinct layers, 1 mucosa, 2
submucosa, and 3 two-layered muscularis. Only one organ
system has such distinct layers.
The mucosa is unique. Note that the mucosa contains numerous
tubular invaginations, which extend from the surface all the way to
the muscularis mucosae. Also note that lamina propria is conspicuous
between these invaginations.
Note that one type of columnar epithelium lines both the surface and
the invaginations. Many of these epithelial cells contain a large,
unstained "bubble" (maybe a mucus droplet?).
Only one organ has features like this.
No more hints.
