Cytokinesis inMicrasterias rotata problems of directed primary wall deposition

Summary The ultrastructure of cytokinesis inM. rotata is described with particular attention to the early stages. At prophase a girdle of primary wall material is deposited at the cell isthmus. The girdle thickens medially, a septum is established and grows inward to separate the daughter semicells by telophase. It is argued that the wall microtubules present at the isthmus are not responsible for delimiting the area over which the girdle is initially deposited, but may participate in girdle deposition and thickening. An important role is ascribed to the secondary wall which, at the isthmus, is devoid of pore bodies and slime. The inner wall surface could, at the isthmus, provide the substratum required for unitial girdle deposition. Though microtubules may be associated with septum initiation, they appear to play no part in subsequent septum growth.     

The dorsal compartment locomotory control system in amphioxus larvae

Amphioxus myotomes consist of separate sets of superficial and deep muscle fibers, each with its own innervation, that are thought to be responsible for slow swimming and escape behavior, respectively. Tracings from serial EM sections of the anterior nerve cord in the larva show that the motoneurons and premotor interneurons controlling the superficial fibers (the dorsal compartment, or DC pathway) are linked by specialized junctions of a previously undescribed type, referred to here as juxta-reticular (JR) junctions for the characteristic presence of a cisterna of endoplasmic reticulum on each side. JR junctions link the DC motoneurons with each other, with the largest of the anterior paired neurons (LPN3s) and with one class of ipsilateral projection neurons (IPNs), but occur nowhere else. Because of the paucity of synaptic input to the DC system, larval behavior can only be explained if the JR junctions act as functional links between cells. An analysis of the pattern of cell contacts also suggests that the LPN3s are probably pacemakers for both slow and fast locomotion, but act through junctions in the former case and conventional synapses in the latter. The only major synaptic input to the DC system identified in somites 1 and 2 was from four neurons located in the cerebral vesicle, referred to here as Type 2 preinfundibular projection neurons (PPN2s). They have unusually large varicosities, arranged in series, that make periodic contacts with the DC motoneurons. More caudally, the DC motoneurons receive additional input via similar large varicosities from the receptor cells of the first dorsal ocellus, located in somite 5. The overall circuitry of the locomotory control system suggests that the PPN2s may be instrumental in sustaining slow swimming, whereas mechanical stimulation, especially of the rostrum, preferentially activates the fast mode.     

Vetulicolians—are they deuterostomes? chordates?

A recent paper by Shu et al.(1) reinterprets the fossil Vetulicola and related forms, all from the Lower Cambrian, as basal deuterostomes, assigning them their own phylum, Vetulicolia. Their conclusion is based on the presence of structures resembling gill slits and a trunk-like region that shows evidence of segmentation. This report summarizes the fossil evidence for their interpretation and evaluates a possible alternative, that vetulicolians may instead be tunicate-like chordates. Implications for our understanding of the nature of the primitive deuterostome (and chordate) body plan are discussed. .     

Anterior neural centres in echinoderm bipinnaria and auricularia larvae: cell types and organization

Serial and interval electron micrograph series were used to examine the anterior part of the ciliary band system in the bipinnaria larva of Pisaster ochraceus and the auricularia larva of Stichopus californicus for evidence of ganglion‐like organization. The bipinnaria has paired concentrations of Multipolar with Apical Processes (MAP) cells in this region that correspond in position with previously identified clusters of serotonergic and peptidergic neurones. MAP cells located in the centre of the band have well‐developed apical processes, but no cilium. Those at the sides of the band have fewer processes, but some have recumbent cilia that extend under the glycocalyx, suggesting a sensory function. Comparable cell types are not found elsewhere in the band, a clear indication that the apical parts of the ciliary band system are organized in a distinctive fashion. Two neuronal cell types were identified in the apical region of the auricularia larva, a conventional bipolar neurone that corresponds with previously described serotonergic apical cells, and more numerous MAP cells for which there is no previous record and hence, no known transmitter. Previous immunocytochemical studies are summarized and re‐examined in the light of these results. Relevant evolutionary issues are also discussed, but the data fail to provide strong evidence either for or against Garstang’s hypothesis that the chordate brain and spinal cord derive from larval ciliary bands resembling those of modern echinoderms.     

Acid-sensing ion channel 3 mediates peripheral anti-hyperalgesia effects of acupuncture in mice inflammatory pain

Background  

Peripheral tissue inflammation initiates hyperalgesia accompanied by tissue acidosis, nociceptor activation, and inflammation mediators. Recent studies have suggested a significantly increased expression of acid-sensing ion channel 3 (ASIC3) in both carrageenan- and complete Freund's adjuvant (CFA)-induced inflammation. This study tested the hypothesis that acupuncture is curative for mechanical hyperalgesia induced by peripheral inflammation.     

Protein structure similarity from principle component correlation analysis

Background  

Owing to rapid expansion of protein structure databases in recent years, methods of structure comparison are becoming increasingly effective and important in revealing novel information on functional properties of proteins and their roles in the grand scheme of evolutionary biology. Currently, the structural similarity between two proteins is measured by the root-mean-square-deviation (RMSD) in their best-superimposed atomic coordinates. RMSD is the golden rule of measuring structural similarity when the structures are nearly identical; it, however, fails to detect the higher order topological similarities in proteins evolved into different shapes. We propose new algorithms for extracting geometrical invariants of proteins that can be effectively used to identify homologous protein structures or topologies in order to quantify both close and remote structural similarities.     

The oral nerve plexus in amphioxus larvae: function,cell types and phylogenetic significance

Serial electron microscope reconstructions were used to examine the organization and cell types of the nerve plexus that surrounds the mouth in amphioxus larvae. The plexus is involved in a rejection response that occurs during feeding: a number of oral spines project across the mouth, and debris impinging on them triggers a contraction of the gill slit and pharyngeal musculature that forces water through the mouth, dislodging the debris. The oral spine cells are secondary sense cells that synapse with neurites belonging to a class of peripheral interneurons intrinsic to the oral nerve plexus. These in turn synapse with a second class of peripheral neurons with large axons that we interpret as sensory cells and which probably transmit signals to the nerve cord. The intrinsic cells also appear to synapse with each other, implying that local integrative activities of some complexity occur in the oral plexus. In comparative terms, the intrinsic neurons most closely resemble the Merkel-like accessory cells of vertebrate taste buds, and we postulate a homology between oral spine cells and taste buds, despite differences in function. There are also similarities between the amphioxus oral plexus and adoral nerves and ganglia of echinoderm larvae, suggesting homology of both the oral nerve plexus and the mouth itself between lower deuterostome phyla and chordates.     

The auricularia-to-doliolaria transformation in two aspidochirote holothurians, Holothuria mexicana and Stichopus californicus

Abstract. The fragmentation and rearrangement of the ciliary bands that occurs during the auricularia-to-doliolaria transformation is described for the non-feeding auricularia larva of Holothuria mexicana and the more typical planktotrophic auricularia of Stichopus californicus. The ciliary band of the auricularia larva runs along a series of ridges that project from the sides of the body. Fragmentation results from a loss of ciliary band cells from the zones between the ridges. The remaining fragments then reorient, elongate, and fuse to form the 5 circumferential bands of the doliolaria. The fate of the band cells lost during this process could not be determined with certainty, but they disappear after being sequestered beneath the epithelium for a time, probably through histolysis. Cell counts indicate that significant numbers of cells are also lost from the ridges. Normal swimming ceases just before transformation begins, probably because the nerve supply to all or parts of the band is disrupted, and this may play a role in initiating morphogenesis.     

Locomotory and feeding effectors of the tornaria larva of Balanoglossus biminiensis

Lacalli, T. C. and Gilmour, T. H. J. 2001. Locomotory and feeding effectors of the tornaria larva of Balanoglossus biminiensis . — Acta Zoologica (Stockholm) 82 : 117–126
The tornaria ciliary bands and oesophagus were examined ultrastructurally to identify the neural components that control larval behaviour. The circumoral ciliary band is known to be innervated in part by fibres from the apical plate and adoral nerve centres. Within the band itself, however, the only neurones we could find were multipolar cells, an unusual cell type with apical processes that traverse the surface of the band. Similar cells occur in the circumoral bands of echinoderm larvae. The tornaria telotroch has a much larger nerve, but no neurones were found either in the band or nearby, so the source of the fibres in the telotroch nerve remains unknown. In addition to having different innervation, the two bands also respond differently to cholinergic agonists, which elicit telotroch arrests but have no visible effect on the circumoral band. The oesophagus has a well-developed musculature and an extensive nerve plexus. During feeding, the oesophagus repeatedly contracts, forcing excess water out along two lateral channels prior to swallowing. These channels are also sites of gill slit formation, so there is evidently a continuity between the water bypass mechanism of the larva and that of the postmetamorphic juvenile.