Partial regeneration of a genal spine by the trilobite Ceraurus plattinensis

A slender extension of a rounded stump on the proximal portion of the left genal spine of Ceraurus plattinensis Foerste, 1920 from the Middle Ordovician of Ontario, Canada records the partial regeneration of a genal spine. The regenerated spine possesses almost normal prosopon of fine granules and resembles the distal portion of the normal right genal spine. It demonstrates the following aspects about this trilobite's physiology: partial regeneration of genal spines could take place even if severed proximally. partial regeneration of genal spines could take place even in late holaspid individuals and the pattern of regeneration took place in a distoproximal order.     

Structure, ontogeny, and moulting of the olenid trilobite Ctenopyge (Eoctenopyge) angusta Westergård, 1922 from the upper Cambrian of Västergötland, Sweden

The genus Ctenopyge is known mainly from disarticulated sclerites and from rare complete specimens flattened in shales. Hitherto, very few specimens have been found preserved intact and in three dimensions. In a recently discovered fauna, however, in the Peltura minor Subzone in Västergötland, central Sweden, there occur several species of Ctenopyge , of which many are complete and superbly preserved; moreover they occur at all stages of growth. Of these the abundant Ctenopyge ( Eoctenopyge ) angusta Westergård, 1922 is described and reconstructed here as an adult, and the entire ontogeny is documented for all post–protaspid growth stages. Many characters typical of the adult, such as the long genal spines and the caudal spine, develop very early in ontogeny, and the relative dimensions of the cranidium do not greatly change during growth. Macropleural spines, however, develop later. The transitory pygidium, relatively large and shield–shaped in the early meraspid, later becomes very small as the ten thoracic segments are liberated; a median spine develops on the last thoracic segment only at the holaspid stage. Instar groupings can be clearly distinguished for the early stages. Recurrent associations of sclerites are interpreted as moulting configurations. As reconstructed, the genal spines are horizontal and parallel with the extended thorax; an adaptation which presumably allowed the trilobite to rest on the sea floor.     

Post-embryonic development of the Furongian (late Cambrian) trilobite Tsinania canens: implications for life mode and phylogeny

SUMMARY The current concept of the order Asaphida was proposed to accommodate some Cambrian and Ordovician trilobite clades that are characterized by the possession of a ventral median suture. The family Tsinaniidae was recently suggested to be a member of the order Asaphida on the basis of its close morphological similarity to Asaphidae. Postembryonic development of the tsinaniid trilobite, Tsinania canens , from the Furongian (late Cambrian) Hwajeol Formation of Korea, reveals that this trilobite had an adult-like protaspis. Notable morphological changes with growth comprise the effacement of dorsal furrows, sudden degeneration of pygidial spines, regression of genal spines, and loss of a triangular rostral plate to form a ventral median suture. Programmed cell death may be responsible for degenerating the pygidial and genal spines during ontogeny. Morphological changes with growth, such as the loss of pygidial spines, modification of pleural tips, and effacement of dorsal furrows, suggest that T. canens changed its life mode during ontogeny from benthic crawling to infaunal. The protaspid morphology and the immature morphology of T. canens retaining genal and pygidial spines suggest that tsinaniids bear a close affinity to leiostegioids of the order Corynexochida. Accordingly, development of a ventral median suture in T. canens demonstrates that the ventral median suture could have evolved polyphyletically, and thus the current concept of the order Asaphida needs to be revised.     

Structural dynamics of dendritic spines: Molecular composition,geometry and functional regulation

The development of dendritic spines with specific geometry and membrane composition is critical for proper synaptic function. Specific spine membrane architecture, sub-spine microdomains and spine head and neck geometry allow for well-coordinated and compartmentalized signaling, disruption of which could lead to various neurological diseases. Research from neuronal cell culture, brain slices and direct in vivo imaging indicates that dendritic spine development is a dynamic process which includes transition from small dendritic filopodia through a series of structural refinements to elaborate spines of various morphologies. Despite intensive research, the precise coordination of this morphological transition, the changes in molecular composition, and the relation of spines of various morphologies to function remain a central enigma in the development of functional neuronal circuits. Here, we review research so far and aim to provide insight into the key events that drive structural change during transition from immature filopodia to fully functional spines and the relevance of spine geometry to function.     

The subspine organization of actin fibers regulates the structure and plasticity of dendritic spines

Synapse function and plasticity depend on the physical structure of dendritic spines as determined by the actin cytoskeleton. We have investigated the organization of filamentous (F-) actin within individual spines on CA1 pyramidal neurons in rat hippocampal slices. Using two-photon photoactivation of green fluorescent protein fused to beta-actin, we found that a dynamic pool of F-actin at the tip of the spine quickly treadmilled to generate an expansive force. The size of a stable F-actin pool at the base of the spine depended on spine volume. Repeated two-photon uncaging of glutamate formed a third pool of F-actin and enlarged the spine. The spine often released this "enlargement pool" into the dendritic shaft, but the pool had to be physically confined by a spine neck for the enlargement to be long-lasting. Ca2+/calmodulin-dependent protein kinase II regulated this confinement. Thus, spines have an elaborate mechanical nature that is regulated by actin fibers.     

Faunal turnovers and trilobite morphologies in the upper Cambrian Leptoplastus Zone at Andrarum, southern Sweden

The Furongian (upper Cambrian) Leptoplastus Zone marks a time of critical changes in the evolution of olenid trilobites. This zone, unexposed at Andrarum in Skåne, southern Sweden, has been re-excavated and the sequence of faunas and sediments logged in detail. The faunal succession accords with that previously described from borehole cores by Westergård, and the subzones of L. paucisegmentatus, L. raphidophorus, L. crassicornis, L. ovatus, L. angustatus, and L. stenotus have been recognized. In the first two subzones the olenid assemblages are monospecific. At the base of the L. crassicornis Subzone more than one species is present and morphotypes with long genal spines appear for the first time. Faunal turnover is rapid, but the incoming of new species is invariably linked to an abrupt change in sedimentation, or follows an unfossiliferous interval; species either arose or migrated in after a time of environmental perturbation. Particular faunal associations are often confined to discrete sedimentary packages though some species may range through a succession of sedimentary changes. Leptoplastus crassicornis has very long genal spines, adapted for resting on the sea floor; it may have competed with the coeval, and very similar, L. angustatus. Subsequently, L. angustatus is accompanied by the stout-bodied, short-spined L. ovatus, which presumably occupied a different niche within the same environment. Leptoplastus stenotus is convergent on the much earlier L. paucisegmentatus, and likewise is found as a monospecific assemblage, presumably being adapted to a similar niche.     

A complete styginid trilobite from the Ordovician of Sweden

Styginidae are a small group of trilobite species, which are usually rare and most of which are incompletely known. Here we describe a complete specimen of Raymondaspis grandigena n.sp. from the Middle Ordovician (Darriwilian, upper Arenig) of Sweden. Among the group it has an unusual combination of exceptionally large genal spines, a thorax with notably short pleural spines in the anterior tergites, and a wide concave pygidial margin. The hypostome is documented for the first time in situ in a styginid, and its attachment can be best described as semi-impendent.     

Modelling dendritic spines with the finite element method,investigating the impact of geometry on electric and calcic responses

Understanding the relationship between shape and function of dendritic spines is an elusive topic. Several modelling approaches have been used to investigate the interplay between spine geometry, calcium diffusion and electric signalling. We here use a second order finite element method to solve the Poisson–Nernst–Planck equations and describe electrodiffusion in dendritic spines. With this, we obtain relationships between dendritic geometry and calcic as well as electric responses to synaptic events. Our findings support the hypothesis that spine geometry plays a role shaping the electrical responses to synaptic events. Our method was also able to reveal the fine scale distribution of calcium in spines with irregular shapes.

     

The Spines of the Channel Catfish, Ictalurus punctatus, as an Anti-Predator Adaptation: an Experimental Study

Although experimental evidence is lacking, the stout pectoral spine of catfishes has been interpreted as a defensive adaptation. The spine can be rigidly locked and abducted to produce stridulation sounds, which have been hypothesized to serve a warning function. We studied spine function in channel catfish (Ictalurus punctatus) as a deterrent to predation by largemouth bass (Micropterus salmoides) by presenting individuals with pairs of catfish, one with its pectoral spines clipped and the other intact. The number of initial attacks on clipped and intact fish was similar, suggesting that bass do not recognize the spine visually. Bass showed evidence of learning across trials, striking clipped fish fewer times and consuming them. Conversely, intact fish were attacked more than clipped ones because intact fish were repeatedly disgorged and attacked again, suggesting that bass become sensitized to the spine. Ingestion times were longer for intact than clipped fish, and fewer intact fish were eaten. Eighty‐eight percent of intact fish survived in the mouth of a bass one or more times. Catfish did not stridulate or use their spines to deter initial attacks, refuting the warning hypothesis. Locking and stridulation motions, only observed when catfish were held inside the mouth of a bass, did not deter subsequent attacks indicating that neither the spine nor stridulation carry a warning function. It is possible, therefore, that stridulation sounds function as a distress call. The spine functions against a gape‐limited predator by increasing the difficulty of ingestion but not capture.     

Structure and development of the ctenial spines on the scales of a teleost fish,the cichlid Cichlasoma nigrofasciatum

Sire, J.‐Y. and Arnulf, I. 2000. Structure and development of the ctenial spines on the scales of a teleost fish, the cichlid Cichlasoma nigrofasciatum. — Acta Zoologica (Stockholm) 81 : 139–158 Numerous teleost species possess ctenoid scales characterized by the presence of ctenial spines arranged in rows (the cteni) along their posterior, free margin. Whilst the morphology and function of the ctenial spines are similar to those of odontodes (extra‐oral teeth), e.g. in armored catfish, their homology is questionable. To address this problem, we have studied ctenial spine development, structure, attachment to a bony support, and replacement with the aim of comparing these features to those described for odontodes. The ctenial spines have been studied in a growth series of the cichlid Cichlasoma nigrofasciatum, using light, scanning and transmission electron microscopy. Ctenial spines are entirely constituted of a collagen matrix. They lack a pulp cavity and, although their distal end can be in contact with the epidermal basal layer cells, they are not covered by an enameloid‐like tissue. They are attached to the scale by means of a narrow strand of unmineralized collagen matrix acting as a ligament and allowing spines to be movable. The ctenial spines develop as prolongations of the external layer of the scale, a woven‐fibroid collagen matrix, and subsequently grow by addition of parallel‐fibred collagen matrix. New ctenial spines are added at the posterior scale border in waves that follow the same rhythm as the deposition of circuli in the anterior region. From the focus region to the scale border, the ctenial spines constitute lines in which only the most posterior ctenial spine is functional. The other spines that are no longer functional are not shed but resorbed from the top, and their attachment region mineralizes and thickens by deposition of new material. The remnants of spines constitute the main part of the superficial layer of the scale in which anchoring bundles attach; this region is covered afterwards by the limiting layer, a tissue devoid of collagen fibrils. Because of their tooth‐like morphology (shape and size), their posterior orientation and their attachment to the scale surface, the ctenial spines resemble odontodes. Moreover, both elements perform a similar hydrodynamic function. Nevertheless, the structure and development of the ctenial spines differ completely from those of odontodes and consequently, they cannot be considered homologous elements. Ctenial spines and odontodes in teleosts provide us with a beautiful example of homoplasy; they share shape and function, but have a different origin as evidenced by their different structure and process of development.     

Periodic shedding and replacement of venomous caudal spines,with special reference to South American freshwater stingrays,Potamotrygon spp.

Synopsis The number of venomous caudal spines and their length and position relative to one another were determined in seven species of South American freshwater rays (Potamotrygonidae) and eight marine or euryhaline species of four families from the Caribbean Coast of South and Central America. Most species have two visible spines at certain stages in the shedding-replacement cycle and only one visible spine at other stages (following shedding). If we include the embryological beginnings of the spines before they erupt and become visible, the spine counts of most rays are actually 2 rather than 1 or 2. Since most species apparently follow this pattern, spine counts are of little use in distinguishing between species except in the relatively few that may have only one, or no spines. Eight captive Potamotrygon specimens maintained in simulated tropical temperature conditions over 12 months showed periodic shedding and replacement of spines. The molts were biannual for a given ray but annual for a given spine. They alternated between two spine loci and their cycles were approximately six months out of phase with each other. Recent studies on Dasyatis sabina by others report only one molt per year, with replacement spines forming always posterior to the primary spine rather than alternating between posterior and anterior. Supernumerary spines (counts of more than two, up to five) are also discussed, as are counts of one and zero.     

Theoretical study on electrical properties of dendritic spines

In most parts of mammalian central nervous system the majority of synapses are located on dendritic spines. Several suggestions have been made about the functional significance of the dendritic spines. We investigate electrical properties of dendritic spines in the neurons with arbitrary dendritic geometry. Following Butz & Cowan (1974), all dendritic branches, including spines, are treated as cylinders of uniform passive membrane. We show that the postsynaptic potential due to the synapse on the spine is represented as a convolution integral of the following two functions. The first is the postsynaptic potential caused by the same synapse on the branching point where the spine stalk is attached to the main dendritic trunk. The second function is determined mainly by the morphological and electrical properties of the spine and it represents the attenuation effect of the spine. On the assumption that the diameter of the spine stalk is sufficiently small compared to that of the parent dendrite to which the spine stem is attached, we obtain an approximation of the second function and conclude that morphological change of the spine does not produce an effective change of the postsynaptic potential, hence does not provide the neural basis for learning or memory simply by changing cable properties of dendrites. Moreover, we show that synapses on the dendritic spine are not effectively isolated from other synapses on the same assumption.     

Electrical properties of dendritic spines with bulbous end terminals

Several suggestions have been made about the functional significance of dendritic spines in connection with synaptic plasticity. We investigated transient electrical behavior of spines with bulbous terminals in neurons with arbitrary dendritic geometries. It is shown that postsynaptic potential transform caused by a synapse on a spine can be resolved into a product of two transfer functions and the synaptic input current transform. The first transfer function was determined to be independent of the spine. The second transfer function represents the straightforward attenuation effect of the spine, which determines the effective synaptic current reaching the parent dendrite. Using what is known of the size and the shape of spines from histology, we conclude that almost all of the synaptic current flow into the parent dendrite, and that therefore the straightforward attenuation effect is negligible. Consequently, when the synaptic current remained unaltered, as was the case for a large synaptic resistance as compared with the spine stem resistance, a morphological change of the spine did not produce an effective change in the postsynaptic potential. On the other hand, when the synaptic resistance is compared with the spine stem impedance, the morphological change of the spine might induce changes of the synaptic current and the postsynaptic potential.     

Structure and molecular organization of dendritic spines

Dendritic spines mediate most excitatory synapses in the CNS and are therefore likely to be of major importance for neural processing. We review the structural aspects of dendritic spines, with particular emphasis on recent advances in the characterization of their molecular components. Spine morphology is very diverse and spine size is correlated with the strength of the synaptic transmission. In addition, the spine neck biochemically isolates individual synapses. Therefore, spine morphology directly reflects its function. A large number of molecules have been described in spines, involving several biochemical families. Considering the small size of a spine, the variety of molecules found is astounding, suggesting that spines are paramount examples of biological nanotechnology. Single-molecular studies appear necessary for future progress. The purpose of this rich molecular diversity is still mysterious but endows synapses with a diverse and flexible biochemical machinery.     

Articulation,interlocking devices and enrolment in Monkaspis daulis (Walcott, 1905) from the Guzhangian,middle Cambrian of North China

Monkaspis daulis (Walcott 1905 ) is the first enrolled trilobite to be documented from the Kushan Formation (Guzhangian, Cambrian Series 3) of Northern China. It has a wide pygidium that allowed a sphaeroidal enrolment type to be achieved, covering the cephalon and anterior trunk segments. Monkaspis daulis is micropygous, but the pygidium is proportionally larger than that of many other Cambrian trilobites, and there is a slightly variable number of segments in the trunk. Articulation structures are very well developed through the trunk, but interlocking or coaptative devices are poorly developed with the exception of the terrace lines and a novel structure, a ‘posterior arch’, in the pygidium, which would have prevented shear. The absence of genal spines that would otherwise have inhibited sinking into the mud substrate may have enabled Monkaspis daulis to use the enrolment procedure to excavate a hole in the sediment for taking shelter.     

Spine motility. Phenomenology,mechanisms, and function

Throughout the history of neuroscience, dendritic spines have been considered stable structures, but in recent years, imaging techniques have revealed that spines are constantly changing shape. Spine motility is difficult to categorize, has different forms, and possibly even represents multiple phenomena. It is influenced by synaptic transmission, intracellular calcium, and a multitude of ions and other molecules. An actin-based cascade mediates this phenomenon, and while the precise signaling pathways are still unclear, the Rho family of GTPases could well be a "common denominator" controlling spine morphology. One role of spine motility might be to enable a searching function during synaptogenesis, allowing for more efficacious neuronal connectivity in the neuronal thicket. This idea revisits concepts originally formulated by Cajal, who proposed over a hundred years ago that spines might help to increase and modify synaptic connections.     

The biophysical properties of spines as a basis for their electrical function: a comment on Kawato & Tsukahara (1983)

In a theoretical study of the passive cable properties of dendritic spines Kawato & Tsukahara (1983) claim to have proved that "the dendritic spine has no significant electrical function" (from their discussion). However, Kawato & Tsukahara restrict their analysis to current inputs to spines. Since the dimensions of spines are very small, their input resistance is expected to be very large and the synaptic input to spines has to be modeled as conductance change. Under this assumption, spines show interesting (non-linear) electrical properties: i) the somatic potential induced by an excitatory synapse on a spine may depend strongly on the shape of the spine and ii) the effect of inhibition might be confined to the spine.     

A new species and a new record of Nycteridopsylla Oudemans, 1906 (Siphonaptera: Ischnopsyllidae) from China

A new species, Nycteridopsylla quadrispina, found on the vespertilionid bat Ia io, is described from China. N. iae Beaucournu & Kock, 1992, a new record for China, was also found on the same host. The new species is distinguished by the presence of four spines on the genal comb and by the shape and chaetotaxy of the head.     

Early development of dorsal and pelvic fins and their supports in hatchery-reared red-spotted grouper, Epinephelus akaara (Perciformes: Serranidae)

Early morphogenesis of dorsal and pelvic fins and their supports in the larval and juvenile red-spotted grouper, Epinephelus akaara, was examined using a hatchery-reared series. The dorsal spine anlage first appeared suspended in the middle part of the finfold at ca. 2.5 mm TL. Dorsal and pelvic supports appeared by the time the fish reached ca. 3 mm and started to ossify at ca. 3.5 mm. Elongated spines and their supports developed synchronously in both dorsal and pelvic fins. The formation of dorsal fin supports proceeded from anterior to posterior. The ossification of supports was completed by ca. 33 mm. Spinelets on the second dorsal spine and pelvic spine appeared by ca. 3 mm. In specimens larger than 36 mm, all spinelets on the second dorsal spine and pelvic spine had disappeared. The maximum size of the second dorsal spine and pelvic spine lengths relative to TL were ca. 45% and 44% at 3.3 mm in fish size, respectively. Thereafter, their proportions decreased gradually. Considering the order of development of the elongated spines and mucous cells in the early life stages, the elongated spines might function as antipredator devices. Received: June 20, 2000 / Revised: April 28, 2001 / Accepted: June 11, 2001