Very busy secretory epithelia tend to be simple columnar.
Here is epithelium from the lower stomach. It has to produce mucin to keep things moving. Think about the alternative.
Mucin-producing cells often present easy-to-see borders, since the carbohydrate-rich pale-staining mucin contrasts with the protein-rich pink-staining cell surfaces.
This is stomach.
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Kansas City University of Medicine and Biosciences