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

Ependym und Circumventriculäre Organe

Publisher Summary The structure of the cerebrospinal spinal fluid (CSF)-contacting endings differs in the two parts of the nervous system In the hypothalamus, the CSF contacting dendritic terminals are characterized by an atypical cilium and those in the central canal by a kinocilium and numerous stereocilia These structures express different functional tasks to be solved in the two main areas of the CSF-contacting neuronal system The CSF-contacting endings are formed by the dendritic processes of the neurons Their dendritic nature is proved by the presence of the endoplasmic reticulum, ribosomes, and Golgi areas, as well as by the lack of synaptic vesicles The number of CSF-contacting neurons is large in the hypothalamus of lower vertebrates, while in higher vertebrates the number of dendritic CSF-contacting terminals decreases Light and electron microscope studies are used to derive morphological results on CSF contacting neurons and neuronal system It is the task of further morphophysiological examinations to clarify the functional relations of the CSF-contacting neuronal system

Comparative ultrastructure of the cerebrospinal fluid-contacting neurons.

An analysis of fine structure and function was conducted on three types of receptors in the cerebral vesicle of two species of ascidian tadpoles (Ciona intestinalis andDistaplia occidentalis). Theocellus is composed of one pigmented, cup-shaped supportive cell, an estimated 15–20 sensory cells, and three lens cells, each with a large body of granules (glycogen ?). The outer segments of the photoreceptoral processes are modified cilia, one per sensory cell, consisting of many lamellae, formed by infoldings of the ciliary membranes, and axonemes of 9 + 0 doublets of microtubules. The lamellae are homologous to retinal disks of vertebrates.Hydrostatic pressure (?)receptors are modified cilia containing tubules which open to the lumen of the cerebral vesicle. These receptors closely resemble the globular, ciliary processes of coronet cells in the saccus vasculosus of fishes. Thestatocyte is a onecell gravity receptor. The part extending into the lumen of the brain contains the nucleus and a large black body which is thought to function as a float. The foot-piece of the cell is firmly anchored in the brain wall; the neck is probably the site of generation of signals.

Ultrastructure of sensory receptors in ascidian tadpoles

Ascidian eggs develop into tadpole larvae. They have a simple central nervous system (CNS) at the dorsal midline. The CNS is formed through neural tube formation at the neurula stage, as in vertebrates. The total number of cells in the CNS is approximately 300. In Halocynthia roretzi, the anterior part of the CNS, which consists of the brain (sensory vesicle) and the visceral ganglion, is exclusively derived from 10 blastomeres at the 110-cell stage. The anterior CNS is relatively complex and shows remarkable left–right asymmetry, with the lumen of the sensory vesicle, the otolith, and the ocellus on the right side, and the presumed hydrostatic pressure organ on the left side. We labeled these 10 precursor blastomeres – six in the animal hemisphere (a-line) and four in the vegetal hemisphere (A-line) – with lineage tracer, and examined the fates in swimming larvae. The clonal organization of the anterior CNS is essentially invariant among individuals, although slight variation in the clonal boundary was observed. There was no extensive mixing between descendants of each precursor. We observed no evidence of cell migration except for two neuronal cells derived from a8.25 blastomeres. The eventual fates of the bilateral blastomeres produced extensive left–right asymmetry. The results suggest that the anterior neural tube rotates in a clockwise direction when viewed from the posterior pole. Staged observation indicated that this rotation takes place during the last 5 h of embryogenesis. We describe detailed positions of descendants of each precursor blastomere. In particular, specific cells of sensory structures were identified by their morphology and staining with specific antibodies and probes. The otolith and ocellus pigment cells were derived from left and right a8.25 blastomeres. Lens cells of the ocellus have a right A8.8 origin, and most of the photoreceptor cells originated from the right A8.7. The presumed pressure organ was formed by descendants of left and right a8.19 and left a8.17 blastomeres. The description of cell lineages of the CNS would facilitate future research to analyze the mechanisms of development of the simple CNS of ascidian tadpole larvae.

Tracing cell fate in brain formation during embryogenesis of the ascidian Halocynthia roretzi.

The saccus vasculosus of the rainbow trout, a sac-like outgrowing of the basis of the diencephalon consists of a strongly undulated epithelial wall, separating the cerebrospinal fluid from the endomeningeal matrix fluid. This epithelium is built up by highly specialized coronet cells, glial cells and peculiar so-called pseudo-coronet cells. The coronet cells extend from the ventricular- to the meningeal border. Morphological evidence is put forward indicating that transport of low molecular material might be the main function of these cells. Several cytochemical data support this hypothesis. A possible repulsion and renewal of globules of the coronet cells may represent steps in the functional cycle of these cells. Concerning the transport function, the observation of open connections between the lumina of the blood sinuses and the endomeningeal spaces might be important.

Light- and electronmicroscopical observations on the saccus vasculosus of the rainbow trout

The following characteristics of the adenosine triphosphatases (ATPase) in the saccus vasculosus were studied in Salmo gairdneri Richardson: 1) distributional pattern, 2) cytochemical properties in relation to different substrates, inhibitors, pH and bivalent metal ions, and 3) ultrastructural localization. Ultracytochemical studies using modifications of the Washstein-Meisel technique showed that within the pH range 7.1-8.0 several Mg++ or Ca++-activated ATPase are localized on the intracellular surface of membranes and in the cytoplasm of ependymal coronet cells and tanycytes ("supporting cells", "Zwischenzellen", glial cells"). The high ATPase activity at the level of the specialized luminal plasma membranes of coronet cell globules and of tanycyte microvilli is discussed in relation to phenomena of active transport and a possible resulting transfer of low-molecular weight substances into and/or from the cerebrospinal fluid (CSF). The localization of ATPase on the specialized membranes of primary vesicles is considered in connection with available structural and enzyme-cytochemical data on a possible function of these cell organelles in storage and release of substances (including Ca++ ions?). The cytoplasmic ATPase activity in coronet cells is ascribed to microtubules and/or possible existing contractile proteins/filaments, presumably concerned with internal transport or motility processes. In tanycytes ATPase activity is believed to be associated with the characteristic microfilamentous system of still unknown function. The ATPase activity in the (9 + 0) ciliary apparatus of globules could not be interpreted in terms of motility. The present study provides further support to the proposed hypothesis of the transport function of the saccus vasculosus, and an extension of the concept in the sense that not only the principal coronet cells, but also the tanycytes of this circumventricular organ are involved in CSF-homeostasis.

W. F. G. Flight

Papers

Light- and electronmicroscopical observations on the saccus vasculosus of the rainbow trout

Fine structural localization of adenosine triphosphatase activities in the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson.