Formation of Nervous Systems and Neural Stem Cells in Ascidians

Abstract

The central nervous system of the ascidian embryo is formed from the neural plate by its rolling into a hollow tube on the dorsal surface. This feature is unique to and common in chordates (synapomorphy). Cell number of mature tadpole remains nearly constant until metamorphosis begins. From earlier, two histological features are noticed in the tissues of the swimming larva; the first is the cessation of cell division in all the larval tissues and also in rudiments of adult organs, and the second is that functional cells are restricted to the larval organs. These cells differentiate almost synchronously in the larva. Larvae swim first for distribution and then for search the settlement places for metamorphosis without feeding. After onset of metamorphosis, the cells of larval organ disintegrate and/or rearrangement and those of adult rudiments begin to divide and differentiate into functional organs. Body wall muscle begins to contract intermittently, concomitant with the beginning of the feeding. That is, the cells of adult organs inhibited by some factors during swimming stage. The adult ascidian neural complex comprises the cerebral ganglion and the neural gland/its derived organ. The former is formed from the primodium of the cerebral ganglion constructed by rearrangement of the larval central nervous system during metamorphosis, and the latter is formed from a thin tube called the neurohypophyseal duct, respectively. The larval central nervous system contains functional neurons and glial cells, called the ependymal cells. Most of functional neurons and the glial cells in the tail region are lost during metamorphosis. Neurohypophyseal duct cells, located in the anterior left side of the sensory vesicle of swimming larvae, are derived from the anterior embryonic neural plate, which expresses common transcription factors in vertebrates and urochordates. After metamorphosis begins, the duct elongates anteriorly and fuses with the stomodeal ectoderm, where the dorsal tubercle, a large ciliated structure that opens into the upper part of the pharynx, later develops. The rudiment of the cerebral ganglion and the duct elongate posteriorly. The duct also differentiates into the neural gland. The dorsal wall of the neural gland in more developed ascidians has a thick epithelium (placode), the central part of which forms the dorsal strand by repeated invaginations along the visceral nerve. Both