In addition to lung and gill respirations, there is a specific type of respiration in the Misgurnus anguillicaudatus which is generally called ‘Loach's intestinal respiration’ and is considered to be of as large as one third of the total respiration volume.
Many experimental and physiological studies about the intestinal respiration have been made (SUYEHIRO 1933, KOYAMA 1958 etc.), yet no one has made any morphological study up to the present.
After our cytological and electronmicroscopical studies of matured loach's intestines it has been confirmed that the respiration is carried out by straight intestine (lower intestine) and especially that columnar cells each are to play an active part in it. Furthermore comparative studies have been made between lung epithelium (LOW 1952, BARGMANN and KNOOP 1956, KARRER 1956 and GRONIOWSKI and DJACZENKO 1961) as well as gill epithelium (HAYEK 1960, MIZUHIRA et al. 1962) and loach's lower intestine epithelium from the respiratory functional view point.
The results obtained are summarized as follows;
Material and methods: Matured loachs (Misgurnus anguillicaudatus) were sacrificed after one day's starvation, and upper intestine (coiled intestine) and lower intestine (straight intestine) were extracted and fixed in LEVI'S solution, ZENKER-formol and 10% formalin. For the electron microscopy, small pieces of both intestines were fixed in 1% osmium tetroxide adjusted to pH7.4 with phosphate buffer, dehydrated through a series of alcohol, and then embedded in styrene and n-butyl methacryrate (1:1), epon, and new epoxy resine ‘epok 533’ recommended by KUSHIDA's method. Thin sections were cut on a PORTER-BLUM microtome with glass knives, stained with uranyl acetate, and examined in a HITACHI's HU-10 electron microscope and a NIPPON DENSHI's JEMT-6 electron microscope at magnification of 1, 000-10, 000.
1. The epithelium of the intestinal mucous membrane is composed of one layer of columnar cells. Villi are well developped in the upper intestine where columnar cells form a great majority of the intestinal epithelial cells, and goblet cells are recognized scatteringly. These columnar cells take slender columnar form and have striated border. The nuclei are round or oval in shape and are found at the basal part of the cell. Mitochondoria are of a short thread shape or a short bacilli-form and mostly gather at the apical portion of the cell. Small arteries and capillaries are observed in the lamina propria of the mucous membrane.
2. In the lower intestine (straight intestine), no villi are observed and yet goblet cells are found in comparatively great numbers. What especially differs from the upper intestine (coiled intestine) is the fact that blood capillaries get into intercellular spaces of the columnar cells from the lamina propria, and widen their space and swell toward the intestinal lumen. As a result the surrounding columnar cells remarkablly transform themselves because of the pressure of the capillaries, turn into a thin layer and lose their striated border.
These phenomena are recognized in almost all parts of the lower intestine and are particularly remarkable about the anus.
However, as far as goblet cells are concerned, their form differs according to the mass of the mucous contents as in the upper intestine, and the above said insertion of the capillaries has practically no deforming effect upon their form. Furthermore, they open into the intestinal lumen with a small stoma, and do not cover the intestinal side of capillaries.
Consequently it is the columnar cells that perform the respiration, while goblet cells only assume charge of producing mucus for the smooth discharge of feces through the intestinal cavity.
No PANETH's cells and no chromaffin cells are found.
The intestinal muscle layers consist of inner circular and outer longitudinal layers