Formation of Nervous Systems and Neural Stem Cells in Ascidians

Abstract

A review. Urochordate ascidians share some morphol. and developmental characteristics with those of vertebrates. About 33% of the larval central nervous system is functional neurons and the rest is glial cells, called the ependymal cells. At metamorphosis, most of the functional neurons and glial cells in the tail region disappear. Adult central nervous system is generated from the rearranged cells (undifferentiated neural cells) of the larval central nervous system after the onset of metamorphosis. On the other hand, the peripheral nervous system is later generated from the dorsal strand which is formed by repeated invaginations of the thickened dorsal epithelium of the neural gland. The olfactory placode in vertebrates and the dorsal epithelium in ascidians are both derived from the anterior region of the embryonic neural plate and generate LH-RH and some other neurons. In some solitary ascidians, LH-RH and prolactin-like neurons are continuously generated in the dorsal strand throughout life. Therefore ascidians are very useful for neural stem cell studies in providing important informations about fundamental processes of neural stem cell formation. [on SciFinder(R)]