Histochemical and biochemical aspects of iodine binding in the tunic of the ascidian,Dendrodoa grossularia(van Beneden)

Abstract

The tunic ofDendrodoahas a very tough consistency, and its matrix contains an abundance of protein-rich fibres. The mantle epithelium makes a major contribution to the formation of the tunic, probably secreting protein as well as carbohydrate. Glycoprotein-rich wandering cells (morula-like cells) come to underlie the epithelium, and these may also provide material for the tunic. It is suggested that secretory activity is supplemented by a quinone-tanning system, the polyphenols being contributed by a type of granular wandering cell (polyphenol cell). Autoradiography shows that bound iodine is distributed throughout the tunic, down to the level of the mantle epithelium, but that it is especially concentrated in the surface cuticle, which is mainly formed from a concentration of the fibres. There is a tendency for the iodine to become clumped around certain wandering cells and accumulations of granules in the tunic; this could be a result of adsorption, or it may indicate some metabolic activity at these points. The association is sporadic, however, and the iodine is certainly not exclusively associated with any particular type of cell. Its widespread distribution appears to reflect the corresponding distribution of tyrosine-rich proteins in the tunic. Preliminary results of radiochromatography of tunic extracts show that monoiodotyrosine and diiodotyrosine are readily identifiable. Unknown iodinated compounds are present, but thyroxine and triiodothyronine have not yet been found. The chromatograms closely resemble those previously obtained by radiochromatography of extracts of the tunic ofCiona. The possibility that thyroxine and triiodothyronine may be present in trace amounts cannot be excluded, but the results as a whole give no support to the view that iodine binding by the ascidian tunic is a form of thyroidal biosynthesis. It seems rather to resemble the iodination that occurs in the surface skeletal tissues of many invertebrates.