This image shows some of the submucosal glands, or esophageal
glands proper (SG).
Note the location of these glands and compare them to those of the stomach. They lie outside the muscularis mucosae and the lamina propria (LP/MM), hence they're submucosal. This is one of only two locations in the mammalian digestive tract where true glands are in the submucosa. At this level of magnification the muscularis mucosae can't easily be made out, but its nature is revealed below.
This image shows the extent to which these glands underlie the mucosal layer. The association of submucosal glands and a lining of stratified squamous epithelium is unique to the esophagus. The tunica muscularis is clearly defined as two layers, and inner (ITM) and outer (OTM) at right angles to each other.
The image at the right makes the relationship of the lamina propria, the muscularis mucosae, and the submucosa easier to see. The lamina propria (LP) underlies the epithelium, and it's set off from the submucosa (S) by the muscularis mucosae (MM).
The muscularis mucosae of the esophagus is composed of smooth muscle fibers, running parallel to the long axis of the organ. In some animals—such as the pig, from which this slide was made—the muscularis mucosae is discontinuous. In at least some sections, it is incomplete, the muscle being gathered into bundles. When cut in cross section (as they are here) they look like "islands" of smooth muscle. In other animals, such as humans, the muscularis mucosae is pretty much completely continuous, though it's still arranged along the long axis.
The esophagus is one of two places in the digestive
tract of mammals in which there are distinct glandular elements in
the submucosa (the duodenum is the other place). The rest of
the tract has glandular elements but they're in the lamina propria.
Pig esophagus; H&E stain, paraffin section, 20x and 100x
