Endostilo

BULLETIN OF MARINE SCIENCE, 45(2): 228-242,

]989

FUNCTIONS OF THE ENDOSTYLE IN THE TUNICATES Jean E. A. Godeaux
ABSTRACT
For more than two decades, electron microscope observations on the endostyle of different species of tunicates have revealed the presence of a densely packed rough endoplasmic reticulum in the glandular cells of the organ. The presence of such a developed ergastoplasmfully supports the hypothesis that these cells are capable of synthesizing digestive enzymes. Comparative analyses of extracts of the endostyle and the stomach provide evidence that several kinds of enzymes are secreted and mixed with the food cords. The glycosidases are well represented. Other cells of the endostyle, able to bind iodine, are involved in its incorporation into tyrosine andproteins, a biochemical proof that the endostyle can be considered as the forerunner of the thyroid gland of vertebrates. The endostyle is also an important constituent of the stolon of Pyrosomas and salps (Thaliacea), as it gives these to the whole digestive apparatus of the blastozooids. The unique role played by the endostyle strictly opposes the Thaliacea to the other Tunicata. Another distinctivetrait of taxonomic importance is their single proto stigma while the tadpoles of ascidians bear at least two. Probably Pyrosomas and salps branched out very early from the common stock.

The endostyle is a ciliated and glandular groove extending along the floor of the branchial cavity of the Tunicata (Urochordata) and of the Cephalochordata (Amphioxus). A similar organ, the subpharyngeal gland, ispresent in the ammocoete larvae of the Cyclostomata (lampreys), but transforms into the thyroid gland at metamorphosis (Mueller, 1873). The presence of the endostyle in the lower chordates was early related to their microphagous diet (filter-feeders). As long ago as 1876, Fo1 considered the endo style a mucus secreting organ (Schleimdriise). The endostyle of tunicates displays a complexhistological structure. Its U-shaped cross-section reveals (Fig. 1) that the organ consists of severallongitudina1, one cell thick, cellular tracts located symmetrically on both sides of a mid-ventral zone bearing long flagella. These flagella often extend the whole depth of the groove. The lateral cellular tracts are either glandular or ciliated: two ciliated bands separate three glandular zones. Theepithelium is lined by a basal membrane and the mesenchym lacunae. The structure ofthe endostyle is remarkably constant as variations only concern either the number of cells of the tracts or the number of tracts constituting the organ (e.g., Doliolids, Godeaux, 1971; 1981). For a long time the ideas about the role of the endostyle were quite simple: the mucus secreted by the glandular cells ispropelled by the cilia and flagella onto the internal pharyngeal walls and allows the trapping of the food particles (microalgae and particulate organic matter) entering the buccal siphon as water is drawn in. The investigations during the last several decades showed that the endostyle is far from a simple mucus forming system. Histochemical investigations revealed that the cytoplasm of the largeglandular cells is basophilic attesting the presence of ribonucleic acid (RNA) as the reaction becomes negative after a pretreatment with ribonuclease (Olsson, 1963; Barrington, 1957). These observations strongly suggest that proteins are synthesized in the glandular cells. Secretion products are visible at the top of most of the cells, even those bearing cilia, as claimed by
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Figure I. Cross-section through the endostyle of Molgula manhattensis. The different epithelial zones are numbered from I to 8 (modified after Godeaux and Firket, 1968).

Ghiani et al. (1965), Ghiani and Orsi (1966), and Orsi and Relini (1966) in their exhaustive investigations on several species of ascidians. These investigations have been completed...