A comparative study of the organization of the sarcotubular system in ascidian muscle

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

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.

Multiple actin genes encoding the same alpha-muscle isoform are expressed during ascidian development

Ascidian embryos develop rapidly into tadpole larvae containing striated tail muscle cells. We have isolated and characterized five actin cDNA clones from a Styela clava tailbud-stage library. The nucleotide sequences of these clones and genomic Southern blot analysis indicate that they represent at least four different muscle actin genes, which are designated ScTb1, ScTb24, ScTb30, and ScTb12/34. The derived protein sequences of these genes indicate that they encode the same alpha-muscle actin which has features related to each of the three different classes of vertebrate alpha-muscle actins. Northern and in situ hybridization with probes prepared from the 3' untranslated region (UTR) of several of the ScTb clones showed that these muscle actin genes are expressed in different temporal and spatial patterns during development. ScTb1 was detected in eggs, embryos, and adults, ScTb24 and ScTb12/34 were detected in embryos and adults, and ScTb30 was detected only in embryos. The maternal transcripts disappeared shortly after fertilization and zygotic mRNAs were first detected during gastrulation and continued to accumulate during subsequent tail muscle differentiation. ScTb30 mRNA, which is expressed in the embryo, peaks during the tailbud stage and is present at low levels in the tadpole larva. In contrast, ScTb1, ScTb24, and ScTb12/34 mRNAs, which are expressed in embryos and adults, peak during the late tailbud stage and are present in substantial quantities in the larva. The ScTb24 gene was detected only in tail muscle cells, whereas the ScTb30 gene was detected in embryonic tail muscle, mesenchyme, epidermal, and neural cells. The ScTb24 mRNA also accumulates primarily in vascular tissue in the branchial sac and mantle of adults. The existence of a gene family encoding the same alpha-muscle actin isoform is unique among the chordates and may function to maximize muscle actin production during the rapid differentiation phase of ascidian larval muscle cells.

Morphological and physiological properties of non-striated muscle from the tunicate, Ciona intestinalis: parallels with vertebrate skeletal muscle

Non-striated muscle from the longitudinal body wall muscle in an adult ascidian (tunicate) is characterized morphologically and physiologically. These muscles are unique among chordate non-striated (i.e. 'smooth') muscles in that they are composed of discrete bundles of several small diameter (4-10 microns) muscle cells (fibers) arranged in parallel functional units. Each bundle is wrapped by a basal lamina, and at least some appear to be directly innervated at a neuromuscular junction similar to an end plate. In these regards, a bundle of Ciona smooth muscle cells is analogous to a skeletal muscle fiber of a vertebrate. This analogy also extends to physiological properties. Ciona muscle generates a rapid all-or-none Ca action potential which gives rise to a brisk twitch with brief latency. These anatomical and physiological adaptations are discussed in terms of the evolution of vertebrate skeletal muscle.

Histochemical investigations of Rotifera Bdelloidea. I. Localization of cholinesterase activity

Cholinesterase activity has been investigated in Rotifera Bdelloidea (Philodina roseola, Philodina tubercolata, Rotaria rotatoria and other unidentified species) by histochemical methods and in vivo observations. Parallel histological studies have been carried out. The enzyme specificity was tested by employing different substrates and inhibitors. The effects in vivo of tubocurarin, bungarotoxin and acetylcholine were also observed. Acetylcholinesterase activity is localized in the nervous and muscular tissues, in sensory organs and in all the ciliated cells. Secretory cells (subcerebral, salivary and pedal glands) and gonad cells (nuclei of the syncytial vitellarium and follicular layer, oocytes and eggs) show both acetyl- and butyrylcholinesterase activities. The effects in vivo of cholinesterase inhibitors, as well as those of tubocurarin, bungarotoxin and acetylcholine, are consistent with the histochemical results, indicating a cholinergic system of transmission and acetylcholinesterase, as well as butyrylcholinesterase, activity.