The anatomy of the gastrointestinal tract of fishes follows the same basic plan as in other vertebrates with a degree of variation reflected in phylogeny and ontogeny, diet, and environment. Morphological studies provide us with a context for understanding the spatial organization and relationship of physiological and biochemical data, and the molecular machinery that is rapidly being elucidated through molecular techniques directed at the genome, transcriptome, and proteome. Morphological data are also key to understanding fish nutrition in ecology and aquaculture, and during development as well as mechanisms for physiological adaptations to a changing environment. A number of the multifunctional roles of the fish gut that are discussed in the following chapters (e.g. respiration and ion regulation) of this volume incorporate distinctive morphological features that will be highlighted in this chapter. The stomach represents a significant vertebrate innovation and is also the most highly diversified region of the gut, yet it has undergone a number of independent secondary losses with stomachless fishes accounting for approximately 20% of species. The diversity of fishes makes the identification of the causes of stomach loss elusive. An updated survey is given.

Morphological diversity of the gastrointestinal tract in fishes

The histology of the digestive tract of the amberjack (Seriola dumerili, Risso) was studied using light and scanning electron microscopy. The anterior oesophagus mucosa displays primary and secondary folds lined with a stratified squamous epithelium with fingerprint-like microridges which is substituted, on the top of the oesogaster folds, by a simple columnar epithelium with short microvilli. Only primary folds are present in the stomach. The anterior portion is rich in simple tubular glands, whereas the oesogaster and the pyloric region are devoid of them. Pyloric caeca and anterior and middle intestine mucosa display the same pattern of folding. The dominant cell type is the enterocyte, which exhibits larger and thinner microvilli in the caeca than in the intestine. The columnar epithelium of the rectum is replaced, in the anal sphincter, by a stratified flattened epithelium. Goblet cells are numerous throughout the whole length of the tract with the exception of the initial part of the oesophagus, the oesogaster, the stomach and the anal sphincter. Mucosubstances have been shown to vary in the different regions of the gut: acid mucines are found in the oesophagus, pyloric stomach, caeca, intestine and rectum, whereas neutral mucosubstances dominate in the anterior portion of the stomach. The muscularis is well developed throughout the length of the tract: two layers of striated muscle at the oesophageal level; two layers of smooth muscle in the stomach wall and three at the intestinal level.

The digestive tract of the amberjack Seriola dumerili, Risso: a light and scanning electron microscope study

The anatomy and the histology of the digestive tract of young and adult sea-bream is described from studies using light and scanning electron microscopy. The dentition in the juvenile (25–30 mm long) comprises all canine-like teeth, to which plate teeth and transition elements are added in the adult.

The oesophagus shows a multi-layered mucosa in the upper part, and single-layered regions in the lower part. The multi-layered regions are formed by epithelial cells, mucus-secreting cells and by cells rich with eosinophilic granules.

The Y-shaped stomach, clearly distinguishable, has a single-layered columnar epithelium under which, in the cardiac and fundic portion, gastric glands, comprised of all similar cells, are present. The pyloric region is characterized by four caeca, to the base of which the ductus pancreaticus and the ductus hepaticus discharge.

The pancreas is composed of small masses spread along the upper intestine; in the adult, pancreatic infiltrations can be seen in the liver.

The intestine is short (relative length 0.5–0.6). The intestine epithelium consists of columnar cells intercalated with mucus-secreting cells. A funnel-like valve marks the passage to the intestine terminal region, characterized by a mucosa of cells with an abundance of vacuoles full of eosinophilic granules.

A study of the histology and morphology of the digestive tract of the sea-bream, Sparus aurata

Development of the digestive tract of milkfish, Chanos chanos (Forsskal): Histology and histochemistry

Dentex dentex has a short esophagus, a large caecal type stomach, three to six pyloric caeca and a short intestine. Light and electron microscope studies reveal that the esophageal mucosa displays primary and secondary folds, a stratified squamous epithelium with fingerprint-like microridges alternating with a few zones formed by a single layer of columnar cells with apical microvilli. Only primary folds are present in the stomach, which is rich in simple tubular glands, these being absent in the pyloric valve. Two cell types occur in the gastric glands, one with a well developed apical intracytoplasmic membrane system consisting of a vesicular network of smooth membranes, and the other with a supranuclear tubulovesicular system. Pyloric caeca and anterior and posterior intestine mucosae display the same pattern of folding, with primary and secondary folds, without following a definite pattern in their orientation. In the rectum, the folds are oriented longitudinally. Small dense particles containing chylomicrons appear in groups in the intercellular spaces of the caecal and anterior intestinal epithelia. Eosinophilic granular cells (mast cells) appear along the digestive tract mainly within the stratum compactum. Histochemical studies reveal no differences in the composition of goblet cell mucus along the digestive tract. No histochemical differences were detected between enterocytes of the intestine, pyloric caeca and rectum. Neutral mucosubstances dominate in the stomach epithelium and in the goblet cells of the esophagus, pyloric caeca and anterior intestine. Results of the present study are discussed in relation to descriptions of the digestive tract in other sparids.

https://digitum.um.es/digitum/bitstream/10201/22664/1/A%20histological%20histochemical%20and%20ultrastructural%20study.pdf

A histological, histochemical and ultrastructural study of the digestive tract of Dentex dentex (Pisces, Sparidae)

The structure of the mucous layer covering the oesophageal epithelium was analysed by scanning electron microscopy in the eel, Anguilla anguilla. Different fixation procedures were used to conserve the mucus in situ. The mucous layer changes progressively down the oesophagus from a thick dense layer to a very thin fibrous network. The possible roles of these mucous structures in ion absorption are discussed.

Scanning electron microscopic study of the oesophageal mucous layer in the eel, Anguilla anguilla L

The oesophageal epithelium of eleven sea-water and four fresh-water teleosts was studied by light and electron microscopy. In sea-water species, the three main regions from pharynx to stomach are: a thick stratified pharyngeal epithelium with complex foldings and numerous mucous cells, and a simple microvillous columnar epithelium in the middle and posterior part. In fresh-water species, foldings are simple, the first two regions occupy the whole length, and no microvillous epithelium is observed. The adaptation from fresh-water to sea-water is characterized by the development of the third, microvillous epithelium. Different microridge cells appear in the mucous region of fresh-water and sea-water Anguilla anguilla oesophagus. The ultrastructure of the microvillous epithelium suggests an active transport of ions along the lateral membranes with water recycling in frequently dilated intercellular spaces.

R. Kirsch

Papers

Scanning electron microscopic study of the oesophageal mucous layer in the eel, Anguilla anguilla L

Structure and ultrastructure of the oesophagus in sea-water and fresh-water teleosts (Pisces)