The gut of the juvenile African lungfish Protopterus annectens: a light and scanning electron microscope study.

We describe the microstructure of the alimentary canal of the juvenile lungfish Protopterus annectens. Following the oesophagus, the gut is formed by a long segment that extends down to the pyloric valve. This segment, classically named stomach, is lined by a transitional epithelium but lacks all characteristics of the vertebrate stomach. It has been defined here as the intestinal vestibule. The spiral valve is divided into a first large chamber, which contains mucosal ridges, and a second smooth portion. The entire spiral valve is lined with a pseudostratified columnar epithelium that contains approximately six cell types: enterocytes, goblet cells, ciliated cells, leukocytes, dark pigment cells, and vascular cells. Enterocytes and goblet cells show a high number of cytoplasmic vacuoles. The number and size of the vacuoles, and the number of ciliated cells, decreases from the anterior toward the posterior end, suggesting that most of the digestive processes take place in the anterior part of the spiral valve. The epithelium overlies a lamina propria in the first large chamber and a vascular plexus in the smooth portion. The cloaca has a thick muscular wall covered by a transitional epithelium. An extensive lymphatic system formed by capillaries and lymphatic micropumps is present along the entire wall of the alimentary canal.