https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1096-0031.1986.tb00462.x

Heads and tails: a chordate phylogeny

At 19â€”20 h of age, the Ciona intestinalis ascidian tadpole has six developed organ systems (tunic, epidermis, notochord, tail musculature, adhesive organ, and nervous system) and approximately four organ system rudiments (including: pn mordial pharynx, atrial primordia, gut primordium, and mesodermal pockets). Con temporarylight and electron microscopic descriptions of the anatomy and the his tology of these tissues support the old contention, originated by Kowalevsky in 1866, that the body plans of tunicate tadpole and of vertebrate larvae are homologues.

Comparative anatomy of the tunicate tadpole, ciona intestinalis

With little more than 330 cells, two thirds within the sensory vesicle, the CNS of the tadpole larva of the ascidian Ciona intestinalis provides us with a chordate nervous system in miniature. Neurulation, neurogenesis and its genetic bases, as well as the gene expression territories of this tiny constituency of cells all follow a chordate plan, giving rise in some cases to frank structural homologies with the vertebrate brain. Recent advances are fueled by the release of the genome and EST expression databases and by the development of methods to transfect embryos by electroporation. Immediate prospects to test the function of neural genes are based on the isolation of mutants by classical genetics and insertional mutagenesis, as well as by the disruption of gene function by morpholino antisense oligo-nucleotides. Coupled with high-speed video analysis of larval swimming, optophysiological methods offer the prospect to analyze at single-cell level the function of a CNS built on a vertebrate plan.

/pdf/the-neurobiology-of-the-ascidian-tadpole-larva-recent-ao3skr4keq.pdf

The neurobiology of the ascidian tadpole larva: recent developments in an ancient chordate.

The circumoral nerve ring and the radial nerve cords (RNCs) of Eupentacta fraudatrix and Pseudocnus lubricus (Holothuroidea) were examined as an example of holothurian nervous tissue. The RNC is composed of outer ectoneural and inner hyponeural layers, which are interconnected with one another via short neural bridges. The circumoral nerve ring is purely ectoneural. Both ectoneural and hyponeural components are epithelial tubes with a thick neuroepithelium at one side. A thin ciliated non-neuronal epithelium complements the neuroepithelium to form a tube, thereby enclosing the epineural and hyponeural canals. The whole of the ectoneural and hyponeural subsystems is separated from the surrounding tissue by a continuous basal lamina. The nerve ring and the ectoneural and hyponeural parts of the radial nerves are all neuroepithelia composed of supporting cells and neurons. Supporting cells are interpreted as being glial cells. Based on ultrastructural characters, three types of neurons can be distinguished: (1) putative primary sensory neurons, whose cilium protrudes into the epineural or hyponeural canal; (2) non-ciliated neurons with swollen rough endoplasmic reticulum cisternae; (3) monociliated neurons that are embedded in the trunk of nerve fibers. Different types of synapses occur in the neuropile area. They meet all morphological criteria of classical chemical synapses. Vacuolated cells occur in the neuroepithelium of E. fraudatrix, but are absent in P. lubricus; their function is unknown. The cells of the non-neuronal epithelia that overlie the ectoneural and hyponeural canals are hypothesized to belong to the same cell type as the supporting cells of the neuroepithelium.

Ultrastructure of the circumoral nerve ring and the radial nerve cords in holothurians (Echinodermata)

Brachyury (T) expression in embryos of a larvacean urochordate, Oikopleura dioica, and the ancestral role of T.

Reissner's Fiber in Ascidian Tadpole Larvae

A peculiar cell type is described from the sensory vesicle of the brain of the appendicularian tunicate Oikopleura dioica. The cells carry globular, modified cilia and resemble in several other respects the coronet cells of Saccus vasculosus in many fishes. This finding, together with earlier reports of similar cells in the sensory vesicle of ascidian tadpoles, makes it probable that a primitive form of coronet cells is a regular constituent of the tunicate brain cavity. The existence of coronet cells in the tunicate line of evolution is of interest from a phylogenetical as well as functional point of view. The author proposes that the tunicate sensory vesicle corresponds to part of the craniate third ventricle and that the tunicate coronet cells are involved in the regulation of the ventricle fluid composition.

Primitive Coronet Cells in the Brain of Oikopleura (Appendicularia, Tunicata)

Light and electron microscopic studies showed that the oral gland cells have two quite different zones. Medially, the basal zone is in contact with body fluids and the endostyle. Its strongly pyroninophile cytoplasm contains the extremely digitated nucleus and numerous small mitochondria. Laterally, the apical zone contacts the epidermis and it may also send a process between epidermal cells and deliver cell fragments into the primordium of the new house. This cell zone contains numerous membranes. It is concluded that the oral gland cells are light producing glands and that the membrane-rich cell fragments which are incorporated into the house wall are the source of the bioluminiscence which has been reported from empty houses. The ontogenetically related subchordal cells have a similar structure and it is possible that also these cells are light producers.

The Oral Gland Cells of Oikopleura dioica (Tunicata Appendicularia)

Oikopleura dioica has two large subchordal cells which were studied in vivo and with light and electron microscopy. They have fixed positions within the haemocoel of the tail but change their morphology continually by protruding and withdrawing processes in an amoeboid manner. Also the fine structure varies considerably from one animal to the next. The cell surface sometimes indicates a strong pinocytotic activity with many coated pits and vesicles. In other cases there are many small vesicles which are interpreted as exocytotic. They are found both in the cytoplasm close to the plasma membrane and in the haemocoelic fluid. Subchordal cells with no pronounced surface activity have a large amount of rough endoplasmic reticulum, which suggests that they synthesize proteins. The presence of subchordal cells in the tail is correlated with the presence of the ontogenetically related bioluminescent oral gland cells in the pharynx; either both types exist simultaneously or both are lacking. It is speculated that the two cell types are also functionally co-ordinated. Oikopleura albicans has a multitude of tiny subchordal cells which have essentially the same fine structure as the two large Oikopleura dioica cells.

The subchordal cells of Oikopleura dioica and O. albicans (Appendicularia, Chordata)

Previous studies of epithelial nervous systems have focused on the neuronal elements, but generally neglected the origin of neuro–glial interactions. In this study, we use a polyclonal antiserum directed against Reissner's substance to label non-neuronal bipolar cells in the ectoneural part of the radial nerve cord in the sea star Asterias rubens. Ultrastructural results show secretory activity in these bipolar cells. Immunolabelled material is released into the extracellular matrix in the hyaline layer as well as in the region of the basal end-feet. As a first step towards characterising the antigen, a specific protein band of 36 kD was demonstrated with immunoprecipitation. Cells of this type: (1) traverse the epithelium to full extent from the outer surface to the basal lamina; (2) carry a single apical cilium; (3) contain conspicuous bundles of intermediate filament; (4) produce a secretion which is, at least in part, homologous to the Reissner's substance which is produced by a primitive radial glia cell type in chordates. It is concluded that the bipolar cells in the ectoneural part of the surface epithelium of the sea star Asterias rubens are secretory radial glia, which evidently have a common origin to the radial glia which secretes Reissner's substance in chordates.

Secretory Radial Glia in the Ectoneural System of the Sea Star Asterias rubens (Echinodermata)

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