Endodermal secretion of chitin in the 'cuticle' of the earthworm gizzard.
The gizzard of earthworms is definitely of endodermal origin. Contrary to the opinion that tissue of endodermal origin is unable to synthesize chitin, the gizzard epithelium secretes large amounts of chitin-containing material which has special properties. 28.7 +/- 5.7% of the dry weight proved to be chitin; the microfibres form a random felt-like texture. They are not arranged in layers comparable to those found in arthropod cuticle. The protein content amounts to 44.9 +/- 7.1% of the dry weight; the remaining material contains at least uronic acids. In the protein matrix large amounts of the enzymes amylase and protease have been demonstrated. As the so-called cuticle is sloughed off at the lumen side, these enzymes are mingled with the gut contents. It might be called a gastric shield, as it closely resembles this structure, widespread among Mollusca. The thickness of the gizzard cuticle varies between 10 and 90 mum in Lumbricus terrestris and 10-50 mum in L. rubellus. Autoradiography has shown that it is replaced at a high rate. In Lumbricus terrestris it is renewed completely in less than 60 hr, and in smaller Lumbricus rubellus, this takes place in about 48 hr. These results are in agreement with the biochemical findings.
Peters
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Institut für Zoologie II, Universität Düsseldorf, Universitätsstr. 1, D-4000 Düsseldorf, Federal Republic of Germany.
Walldorf
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