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Hair cells in an ascidian (Tunicata) and their evolution in chordates
Full text linkIn ascidians, mechanoreceptors of the oral area are involved in monitoring the incoming water flow. Sensory cells are represented by scattered, ciliated primary cells (sending their own axons to the cerebral ganglion) or secondary sensory cells (axonless cells forming afferent and efferent synapses with neurons, whose somata are located in the ganglion) of the coronal organ. Coronal cells have varying morphologies: in species of the Enterogona order, they are multiciliate, whereas those of Pleurogona possess an apical apparatus composed of one or two cilia accompanied by stereovilli, in some cases also graded in length. The coronal organ has been proposed as a homologue to the vertebrate octavo-lateralis system, because coronal cells resemble vertebrate hair cells for morphology, embryonic origin and arrangement. In the ascidian Molgula socialis (Pleurogona), we now describe the morphology of the coronal organ, which contains a
few associated rows of sensory cells that run the whole length of the oral velum and the branched tentacles. Three kinds of sensory cells, accompanied by specialised supporting cells, are present. Comparisons between the coronal organ and other chordate mechanosensory structures suggest that hair cells originated in the common ancestor of chordates
Hair cells in non-vertebrate models: lower chordates and molluscs
Full text linkIn: Special issue of Hearing Research featuring comparative hearing. (on invitation by Geoffrey Manley). Hearing Research.
The study of hair cells in invertebrates is important, because it can shed light on the debated question about the evolutionary origin of vertebrate hair cells. Here, we review the morphology and significance of hair cells in two groups of invertebrates, the lower chordates (tunicates and cephalochordates) and the molluscs. These taxa possess complex mechanoreceptor organs based on both primary (sensory neurons) and/or secondary, axonless, sensory cells, bearing various apical specializations. Compared with vertebrates, these taxa show interesting examples of convergent evolution and possible homologies of sensory systems. For example, the “lateral line organ” of Octopoda and Decapoda, composed of primary sensory cells aligned on the arms and the head, is considered a classic example of convergent evolution to mechanoreception. Similarly, in ascidians, the cupular organ, formed of primary sensory cells embedded in a gelatinous cupula, is seen as an analog of neuromasts in vertebrates. However, the coronal organ of the oral siphon of ascidians, represented by a line of secondary sensory cells with a hair bundle also comprising graded stereovilli, is currently the best candidate for tracing the evolutionary origin of the vertebrate octavo-lateralis system. Several features, such as embryological origin, position, gene expression and morphology, support this hypothesis
New insights into morpho-functional features of haemocytes from the blue crab Callinectes sapidus
No full textIn this study, we provided a comprehensive morpho-functional characterization of haemocytes in the blue crab Callinectes sapidus. Three haemocyte types were identified in the haemolymph: hyalinocytes (50 ± 4.7 %), lacking evident cytoplasmic granules; semigranulocytes (22.8 ± 2.02 %), containing a variable number of refractile granules; and granulocytes (27.2 ± 2 %), distinguished by their abundance of refractile granules. Haemocytes were predominantly oval or round. No significant size differences were observed among cell types, with granulocytes and semigranulocytes ranging from 7 to 22 μm, and hyalinocytes from 8 to 20 μm. Additionally, haemocytes were categorized into three cytochemical subpopulations: acidophils (38 %), basophils (36 %), and neutrophils (26 %). Notably, Neutral Red staining failed to reveal lysosomes in vivo, suggesting low membrane permeability under these conditions. Transmission electron microscopy corroborated the presence of the three haemocyte types. Both granulocytes and hyalinocytes exhibited phagocytic activity against yeast cells, although the phagocytic index remained low (∼4 %), implying that phagocytosis may not be the primary immune mechanism in C. sapidus. All haemocyte types generated superoxide anion and tested positive for several hydrolytic enzymes and phenoloxidase activity. Overall, these findings confirm the presence of three distinct haemocyte types in C. sapidus haemolymph and suggest that alternative immune pathways, beyond phagocytosis, may play a central role. Further research is needed to investigate additional immune functions, such as degranulation and inflammatory responses in C. sapidus
External amoebocytes perform immunosurveillance of the pharynx entry in ascidians (Urochordata)
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Hair cells in ascidians and the evolution of lateral line placodes
No full textThe vertebrate hair cells are ciliary highly differentiated mechanoreceptors whose name derives from the peculiar microvilli, called stereovilli, that protrude into the fluid-filled cavities of the inner ear or lateral line organs. They differ from the primary sensory cells found in most invertebrates in that they are axonless, thus being secondary sensory cells that synapse with the dendrites of neurons whose cell bodies are located in the central nervous system (CNS). Although their morphology varies in different vertebrate species, hair cells typically have a single eccentric cilium and a collar of stereovilli graded in length from one side to the other.
Hair cells derive from placodes of the acustico-lateralis system that, together with the other neurogenic placodes, are generally believed to originate ontogenetically from a wide panplacodal field and are usually considered exclusive to craniates. However, recent molecular and morphological data suggest that cell populations with the properties of neurogenic placodes are also present in cephalochordates and tunicates. Extending our study now to eight species belonging to the three orders of ascidians, we find that all possess coronal organs with hair cells. All molecular and morphological data are consistent with the idea that the ancestor of chordates possessed the ability to differentiate hair cells and that the latter derive embryologically from an area having the characteristics of a neurogenic placode. We propose that the ascidian embryonic stomodeum contains a population of cells homologous to the vertebrate acustico-lateralis placodes
Innervation of ascidian siphons and their responses to stimulation
Full text linkThe distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon has about 8000 primary sensory neurons. A coronal organ of the compound type is present on the oral tentacles. Convergence in the afferent pathway is estimated at >10:1. A new category of cells associated with the velar sphincter muscle is described at the tentacle bases. Responses to stimulation were recorded using flow meters. Both siphons are sensitive to touch and near-field vibrations. Removal of the oral tentacles did not diminish vibration sensitivity. Gentle stimulation of the oral siphon evokes crossed responses in which the atrial siphon closes and the velar sphincter contracts. Stronger stimulation produces squirts with closure of both siphons and branchial ciliary arrest. Experiments with polystyrene beads show that the oral tentacles are sensitive to contact with inflowing par..
Testing an unusual in vivo vessel network model: a method to study angiogenesis in the colonial tunicate Botryllus schlosseri
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