THE FORMATION OF THE SPICULES OF DIDEMNID AE (ASCIDIACEA)

The spicules of didemnids ascidians are concretions made of calcium ca rbonate in the form of aragonite. Their origin and composition have be en analyzed by electron microscopy, electron probe microanalysis, and secondary ion mass spectroscopy. Our results, both morphologic and mic roanalytic, confirm clearly the intracellular origin of the spicules. In the cells of the lateral thoracic organs (LTO), which are different iations of the epidermal mantle, Golgi secretory vesicles are formed a nd calcium and strontium are trapped on the organic matrix that is ric h in sulfur. A crystalline precipitation occurs in the cell before or during its migration into the cavity of the LTO. Then, the new spicule s, always surrounded by an organic envelope, are liberated in the LTO cavity and migrate into the tunic; the cells also migrate in the tunic . The spicules continue to grow extracellularly in the tunic. They hav e the same morphology and chemical composition as the young spicules, which demonstrates a control of the ionic balance by the cells, both i ntra- and even extra-cellularly where the mechanism of spicule growth remains to be determined.