The fine structure of the intestinal epithelium in the swell fish (
Spheroides stictonotus) was studied by electron microscopy in materials fixed in buffered osmic acid and embedded in Epon epoxy resin. Three types of cells can be distinguished in the epithelium.
1. Columnar cells with a striated border are most numerous and common throughout the entire midgut. The luminal surface of the columnar epithelial cell is tightly packed with regular microvilli. In a few places the intermicrovillous plasma membranes are invaginated into the terminal web, giving rise to numerous furrows or a variety of vesicles in the apical cytoplasm. The lateral plasma membranes do not show complex interdigitations between neighboring cells, but desmosomes occur here sporadically. Particular concentration of mitochondria is observed both in the apical and basal cytoplasm of the columnar epithelial cell.
2. The most striking feature in the columnar epithelial cells is the occurrence of endoplasmic lamellar structures which are found mainly infra-nuclear regions. These structures are composed of two very regular, parallel membranes which are separted by a distance of about 250Å. From this characteristic appearance, they are readily distinguished from the usual endoplasmic reticulum. In cross sections, the cavity of these lamellae often shows continuity with the intercellular space. These membranes are never seen associated with RNP particles. These lamellae are independent of each other, being never anastmosed.
3. Typical goblet cells, which are basically the same in structure as those found in other vertebrates, are encountered among columnar epithelial cells.
4. A few of granular cells are occasionally observed singly in the epithelium. These cells contain large granules bounded by a single membrane, aboundant glycogen particles, and a variety of vesicles and tubules. Since these cells do not show any desmosomes between adjacent epithelia, and a similar type of cell is recognized in the lamina propria mucosae, they may be migratory in nature.
5. The present study demonstrates the fine structure of the intestinal epithelium in a marine teleost. Of particular interest is the existence of lamellar structures which have been reported so far only in fresh water fishes. From the observations in fresh water teleosts and in the present study, it is strongly suggested that these lamellar structures are in general characteristic of the fish intestine, even though not in all species of fishes. These structures are assumed to be involved in the mechanism of concentration of salts or of water transport, probably the latter.
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