A histological specimen from the fistula. The specimen represents a cross section through the mucosa, submucosa and muscularis externa. The slit-like lumen (L) is lined by squamous epithelium. The submucosa contains inactive glandular structures (arrows), and the muscularis externa is comprised of the inner circular (C) and the outer longitudinal (L) layer, and contains plexus structures comprised of unaltered neurons (arrowhead). A. Hematoxylin-eosin stained section. B. Immunohistology for desmin highlights the striated muscle layers of the muscularis externa and the absence of a lamina muscularis mucosae. Streptavidin peroxidase method, Mayer’s hematoxylin counterstain. Scale bars = 100 μm. (× 50).