The appendix is histologically composed of four layers:

Mucosa: Mucosal membrane, lining the inside of the tube.
Submucosa: Connective tissue layer
Muscularis externa: Tubular wall of a bilayer of smooth muscle, interspersed with loose connective tissue and blood vessels.
Serosa: Outer serosal coat, lined with mesothelial cells.

The duodenum is histologically composed of three main structural layers:
• Mucosa: Mucosal membrane, lining the inside of the intestine.
• Submucosa: Connective tissue layer with Brunner’s glands.
• Muscularis externa: A bilayer of smooth muscle.

On microscopic examination, the esophagus is divided into four layers:
• the mucosa, located closest to the lumen.
• the submucosa, located right beneath the mucosa.
• the muscularis propria, the thickest layer.
• the adventitia, the outer layer.

Learn more at https://www.proteinatlas.org/