The image to the left is of the esophageal-stomach junction (E-S Jct). On the right side
you see the stratified squamous epithelium of the esophagus (E) which
terminates abruptly at the point of entry into the stomach (S). The
lining of the stomach is a simple columnar type without goblet
cells.
There is no particularly thickened region of the tunica muscularis at this location, however. Although you will hear the term "esophageal sphincter" there isn't much in the way of a true anatomical sphincter at the entrance to the stomach, as there is at the outlet. The "esophageal sphincter" is physiological: intra-esophageal pressures are generally higher than those of the stomach, and hence reflux of the stomach contents is prevented.
Occasionally there is a drop in
intra-esophageal pressure and reflux occurs. It's perceived
symptomatically as "heartburn" in humans. If it persists the
erosive effect of the acid stomach contents can cause ulceration of
the lower esophagus.
In most species the epithelial transition from esophagus to stomach is right about at the
point where the anatomic esophagus and the anatomic stomach meet. But
in others (such as the horse and rat) it occurs actually inside the
anatomic stomach. Thus the gross anatomist will speak of the "non-glandular" and "glandular" regions of the stomach. In suchsituations the point of change is easily
visible, and the gross anatomist's term for it is the margo
plicatus.
Here is an example of the margo plicatus from a rat. This
particular transition occurs very deep and the region of the stomach
next to the "esophageal" epithelium actually contains fundic
glands!
Rat stomach; H&E stain, paraffin section, 100x