Stellate Structure Research Articles

Analysis of anti-clogging performance and particle transport characteristics of a stellate irrigation emitter

The service life and irrigation efficiency of drip irrigation systems are largely determined by the anti-clogging performance of the emitters. The degree and speed of clogging of the emitters are governed by the motion state of the particles in the flow channel. Computational fluid dynamics coupled discrete element method (CFD-DEM) and experiments were used to analyse anti-clogging performance and particles transport characteristics of a stellate irrigation emitter. With the angle of repose of sediment particles as the optimization objective, the key parameters of physical properties of sediment particles and container were obtained in the discrete phase model using a Plackett-Burman test, steepest climb test and Box-Behnken test. A novel evaluation method for assessing anti-clogging performance based on particles mass passing rate is proposed and its accuracy experimentally verified. Using three particles groups with different particle size distributions, the anti-clogging performance of stellate emitter was found to better than the widely used toothed emitter. In particular, the anti-clogging performance of the stellate emitter was found to be superior with the sensitive particle size of 0.03 mm. It was shown that sediment particles demonstrated retention, bonding and deposition in the inlet far away from the flow channel, the backwater area at the vertical vertex of the stellate structure and the backwater area at the joint of the stellate emitter flow channel structure units.

Machine Learning Approach to Predict Flow Regime Index of a Stellate Water-Retaining Labyrinth Channel Emitter

Accurate calculation of the flow regime index in the design and development stage of a drip irrigation emitter plays an essential role. In this study, machine learning technologies were used to establish the relationship between flow channel structural parameters of the novel stellate water-retaining labyrinth channel (SWRLC) irrigation emitter and its flow regime index. The training dataset and test dataset were built by computational fluid dynamics (CFD) simulation and experimental study. The extreme learning machine (ELM), backpropagation neural network (BPNN), and traditional multiple linear regression (MLR) models were developed for the prediction of the flow regime index of the SWRLC emitter. The input parameters matrix consisted of the length of the trapezoid baseline, angle between the hypotenuses of adjacent trapezoids, trapezoid height, radius of stellate water-retaining structure, spacing of two symmetric trapezoids, path depth, and SWRLC unit number, while flow regime index x was the output of the models. The comprehensive indicator (CI) was proposed, and root mean square error (RMSE), mean absolute error (MAE), mean bias error value (MBE), and coefficient of determination (R2) were used to introduce the reliable assessment of the three models. The comparison results showed that the ELM model had the lowest errors, with the CI, RMSE, MAE, and R2 were 1.96 × 10−11, 0.00163, 0.00126, and 91.49%, respectively. The BPNN model had the lowest MBE error with the value of 1.03 × 10−4. The ELM and BPNN models were available and had acceptable accuracy for predicting the flow regime index of the emitter, saving both time and cost and increasing efficiency in the design and development stage. According to the CI, the ELM model performed best, followed by the BPNN model with a minor discrepancy.

Molecular characterization and functional analysis of interferon regulatory factor-4 in the Red Claw Crayfish (Cherax quadricarinatus)

Vibrio parahaemolyticus (V. parahaemolyticus) is a primary pathogen in crustaceans and causes acute hepatopancreatic necrosis disease. Interferon regulatory factor-4 (IRF4) belongs to the regulatory factor family and plays a key role in the innate immunity of crustaceans. To better understand the IRF4 regulatory mechanism and its immunomodulatory role in response to challenge with Vibrio in crustaceans, the cDNA of Cherax quadricarinatus (C. quadricarinatus) IRF4 (CqIRF4) was isolated and the immune effect of this protein in the C. quadricarinatus response to V. parahaemolyticus was studied. The length of CqIRF4 was 2354 bp, with a 1251 bp open reading frame encoding 417 amino acids. After infection with V. parahaemolyticus, the expression levels of CqIRF4 in the hepatopancreas and hemolymph of C. quadricarinatus were significantly upregulated (P < 0.05), while no significant fluctuations were detected in muscle or gill tissues. Additionally, histopathological damage was detected in the hepatopancreas, such as loss of the stellate structure, significant vacuolization of cells, intercellular hemorrhaging, hypertrophy, and necrosis in some tissues. After injecting recombinant CqIRF4 protein in vivo, colonization of the hepatopancreas by V. parahaemolyticus decreased dramatically. The transcription levels of the interleukin-like genes IL-4, -6, -23, -12, and -15, as well as the lysozyme, AlP, ALF, SOD1, laccase, and C-type lectin genes, were significantly upregulated (P < 0.05). This study shows that the CqIRF4 as an immunomodulatory factor, was highly activated by a Vibrio challenge, and inhibited V. parahaemolyticus colonization in the hepatopancreas of C. quadricarinatus by regulating the corresponding immune genes.

Six‐arm Stellat Dendritic‐PbS Flexible Infrared Photodetector for Intelligent Healthcare Monitoring

AbstractThe flexible photodetector plays an important role in improving human medical health status. However, the narrow spectral detection range, poor stress stability, and non‐degradation of traditional flexible photodetectors greatly hinder the further development of wearable medical devices. In this paper, a novel flexible infrared photodetector is proposed for intelligent healthcare monitoring using high purity lead sulfide (PbS) nanoparticles on paper‐based flexible substrate synthesized by hydrothermal method and physical friction. The excellent performance of the detector is attributed to the 1.01 eV band gap and six‐arm stellate dendritic structure of PdS, a good combination between PbS and paper substrate via physical friction. As a result, our photodetector demonstrates wide‐spectrum detection capabilities from 365 to 1550 nm. The photodetector at 980 nm (50.4 μW cm−2) shows responsivity of 6.45 mA W−1, detectivity of 6.4 × 1010 Jones, response recovery time of 0.36 s/0.41 s, with good mechanical stability. By comparison, our detector has a wider detection range, better weak signal detection performance, and shorter response time than the performance of the former paper‐based detector. Furthermore, the paper‐based PbS photodetector as a dual‐wavelength photoplethysmography sensor is applied to analyze the oxygen saturation and develop an intelligent bandage to monitor wound healing. This paper‐based PbS photodetector has tremendous potential in the field of wearable medical devices and intelligent medical applications are expected.

Synthesis and characterization of a penetrating and pre-wetting agent for coal seam water injection

Coal seam water injection, as an indispensable process in underground mining, is of great significance to the treatment of dust hazard in working face. However, the permeability of water in the coal seam is unsatisfactory due to hydrophobic properties of coal and many other factors. Here we synthesized a water injection additive with stellate structure by modified β-cyclodextrin. The molecular structure characteristics of the product were analyzed by FTIR, XRD and TG-DTG-DSC. The permeability and wettability of materials, which are closely related to water injection were tested. The contact angle of 0.1% product solution was only 25°. Moreover, combined with the characteristics of molecular structure and the effects, the principle of action was explained. And the optimum reaction conditions were determined. After performance testing, the synthesized material has stronger penetrating and pre-wetting ability than common surfactants, and the mechanism of its action was explained based on the molecular characteristics of the material.

Maintaining of internal consistency of algebraic Bayesian networks with linear and stellate structure

Subject of Research. When working with algebraic Bayesian networks, it is necessary to ensure their correctness in terms of the consistency of the probability estimates of their constituent elements.There are several approaches to automating the maintenance of consistency, characterized by their computational complexity (execution time). This complexity depends on the network structure and the chosen type of consistency. The time for internal consistency maintenance in algebraic Bayesian networks with linear and stellate structure is compared with the time for consistency maintenance of a knowledge pattern covering such networks. The comparison is based on statistical estimates. Method.The essence of the method lies in reducing the number of variables and conditions in linear programming problems which solution ensures the maintenance of internal consistency. An experiment was carried out demonstrating the differences between the time of consistency maintenance for different algebraic Bayesian networks with a global structure. Main Results. An improved version of the algorithm for internal consistency maintenance is presented.Solvable linear programming problems are simplified in comparison with the previous version of the algorithm. Two theorems are formulated and proved, refining the estimates of the number of variables and conditions in the linear programming problems to be solved, as well as the number of the problems themselves. An experiment is performed, which showed that the proposed software implementation of internal consistency maintenanceis superior in working time to software implementation of the consistency maintenanceof a complete knowledge pattern. Practical Relevance. The results obtained can be applied in machine learning of algebraic Bayesian networks (including the synthesis of their global structures). The proposed method provides optimal synthesis of global network structures for which it is enough to use the maintenance of internal consistency during learning and further network processing. Owing to the method application these processes will have acceptable computational complexity.

New data on the stratigraphic distribution of the nannofossil genus &amp;lt;i&amp;gt;Catinaster&amp;lt;/i&amp;gt; and on evolutionary relationships among its species

Abstract. Examination of Upper Miocene–Lower Pliocene sediments at IODP Site U1338, in the Eastern Equatorial Pacific, provided new data on the distribution range of the calcareous nannofossil genus Catinaster. In addition to the the well-known occurrence of Catinaster coalitus and Catinaster calyculus in the early Late Miocene, we document Catinaster mexicanus in both the mid-late Miocene and the Early Pliocene. We confirm its taxonomic validity, rejecting previous interpretations of Pliocene C. mexicanus specimens as the result of dissolution of Discoaster altus. Instead, the Pliocene appearance of C. mexicanus seems to originate from the D. altus lineage. The short interval of occurrence (c. 50 ka) in the Late Miocene may document a preliminary evolutionary emergence of C. mexicanus that lacks any relationship with the other Catinaster species. Clear ancestor species to validate its independent origin from Discoaster are, however, missing. In the stratigraphic intervals where Catinaster species are found, their co-occurrence with Discoaster species bearing a prominent star-shaped boss on one side is noteworthy. This suggests that Catinaster and Discoaster at times developed a common morphological feature (a stellate structure, with or without hexaradiate symmetry), possibly under recurrent changes in climatic/environmental conditions. The data presented on C. mexicanus suggest a wider geographical distribution than previously thought, extending from the tropical Pacific to the Gulf of Mexico, equatorial Atlantic and tropical Indian oceans.

βA3/A1-crystallin is required for proper astrocyte template formation and vascular remodeling in the retina

Nuc1 is a spontaneous rat mutant resulting from a mutation in the Cryba1 gene, coding for βA3/A1-crystallin. Our earlier studies with Nuc1 provided novel evidence that astrocytes, which express βA3/A1-crystallin, have a pivotal role in retinal remodeling. The role of astrocytes in the retina is only beginning to be explored. One of the limitations in the field is the lack of appropriate animal models to better investigate the function of astrocytes in retinal health and disease. We have now established transgenic mice that overexpress the Nuc1 mutant form of Cryba1, specifically in astrocytes. Astrocytes in wild type mice show normal compact stellate structure, producing a honeycomb-like network. In contrast, in transgenics over-expressing the mutant (Nuc1) Cryba1 in astrocytes, bundle-like structures with abnormal patterns and morphology were observed. In the nerve fiber layer of the transgenic mice, an additional layer of astrocytes adjacent to the vitreous is evident. This abnormal organization of astrocytes affects both the superficial and deep retinal vascular density and remodeling. Fluorescein angiography showed increased venous dilation and tortuosity of branches in the transgenic retina, as compared to wild type. Moreover, there appear to be fewer interactions between astrocytes and endothelial cells in the transgenic retina than in normal mouse retina. Further, astrocytes overexpressing the mutant βA3/A1-crystallin migrate into the vitreous, and ensheath the hyaloid artery, in a manner similar to that seen in the Nuc1 rat. Together, these data demonstrate that developmental abnormalities of astrocytes can affect the normal remodeling process of both fetal and retinal vessels of the eye and that βA3/A1-crystallin is essential for normal astrocyte function in the retina.

Further contribution to the knowledge of Chitaleypushpam mohgaoense Paradkar

This paper deals with the investigation of the petrified flower Chitaleypushpam mohgaoense Paradkar and redescribes the flower in as much detail as possible. The present petrified flower was collected from well-known fossiliferous locality Mohgaonkalan, M.P., India, where Deccan Intertrappean beds are exposed. The previous diagnosis of the genus and the species is emended. The additional characters of Chitaleypushpam are especially about structure of carpels. In the present fossil flowers gynoecia are seven carpeled and nine carpeled in Specimen-A and Specimen-B, respectively. Besides this, in cross section the ovary reveals stellate structure. From the above discussion it is clear that present fossil flowers closely resemble Chitaleypushpam mohgaoense Paradkar in many characters and hence are placed under the same genus and species with emended diagnosis.

Review of the Septibranchia (Mollusca: Pelecypoda) from the deep sea of Campos Basin, Brazil: Family Lyonsiellidae, with description of a new species

A review is made of the members of Lyonsiellidae present on the continental slope (750-2000 m) of Campos Basin (22°S), off south-eastern Brazil. A total of six taxa are recognized. Two taxa were previously unknown to science ( Lyonsiella pipoca n. sp., described herein, and Policordia sp.). Lyonsiella frielei and Policordia gemma are recorded from Brazilian waters for the first time. Lyonsiella abyssicola is distributed throughout the Atlantic Ocean, but the present record fills a gap in its distribution in the southwest Atlantic. Lyonsiella cf. formosa has a confused taxonomic status, and we believe that more than one species has been called L. formosa. Lyonsiella abyssicola, L. frielei and L. cf. formosa have the shell surface covered by small spines that typically have a hexagonal column and a stellate structure at the distal end.

Review of the Septibranchia (Pelecypoda: Mollusca) from deep sea of Campos Basin, Brazil: family Verticordiidae, with description of a new species

Despite the increase in the number of deep-sea studies, little is known about the hadal molluscs. For Brazil, only a few studies have treated the Septibranchia. This contribution is part of an effort to increase the knowledge of this group off the Brazilian coast, although it is focused only on the Campos Basin (22°S), State of Rio de Janeiro. Four species of the family Verticordiidae are recognized here. ForSpinosipella tingaSimone &amp; Cunha, 2008, the known depth distribution is extended to 1050 m. ForVerticordia woodiiSmith, 1885, previously reported from Brazil, the known distribution is extended southwards to the Campos Basin.Verticordia quadrataSmith, 1885, is reported from Brazil for the first time. A previously unknown species,Verticordia ouricurisp. nov., is described. The presence of small spines, typically with a hexagonal column and a stellate structure at their distal end emerges as an additional taxonomic character for the verticordiids.

A case of serous cystadenoma of the pancreas with a central stellate scar detected on contrast-enhanced ultrasound with perflubutane.

An asymptomatic 66-year-old woman was admitted to our hospital for detailed evaluation of a 63-mm mass in the tail of the pancreas detected on abdominal computed tomography (CT). Abdominal ultrasound (US) revealed a hypoechoic solid mass, but on contrast-enhanced ultrasound (CE-US) with perflubutane, a stellate structure within the tumor, characteristic of a serous cystadenoma, was observed. A distal pancreatectomy was performed, and histologic examination confirmed a serous cystadenoma of the pancreas. This case highlights the usefulness of CE-US with perflubutane for diagnosis of pancreatic serous cystadenomas.

Phylogeny and ultrastructure of Nephroselmis and Pseudoscourfieldia (Chlorophyta), including the description of Nephroselmis anterostigmatica sp. nov. and a proposal for the Nephroselmidales ord. nov.

T. Nakayama, S. Suda, M. Kawachi, and I. Inouye. 2007. Phylogeny and ultrastructure of Nephroselmis and Pseudoscourfieldia (Chlorophyta), including the description of Nephroselmis anterostigmatica sp. nov. and a proposal for the Nephroselmidales ord. nov. Phycologia 46: 680–697. DOI: 10.2216/04-25.1A new species of Nephroselmis, Nephroselmis anterostigmatica, is described from coastal waters of the Palau Islands and the southern part of Japan. The long flagellum was stretched out at angle to the cell and showed a characteristic sigmoid line. A cup-shaped yellowish green chloroplast with two narrow sinuses included a pyrenoid with tubular invaginations of thylakoids. In contrast to other species of Nephroselmis, the eyespot was located at the anterior side of the cell. The transitional region of the flagellum possessed a single transitional plate and a stellate structure. The cell body was covered by two types of square scales (windmill/Maltese cross scales absent), small stellate scales and two types of large spiny scales. The flagellar surface was covered by inner square and outer rod-shaped scales, and ornamented with T- and Pl-hairs. Tip hairs are absent. Nephroselmis anterostigmatica is superficially similar to Nephroselmis pyriformis (N. Carter) Ettl, but a detailed comparison of morphological characters, such as chloroplast shape, position of eyespot, composition of hair scales, presence or absence of the two types of small square scales and the large spiny scales, reveals clear differences between the two species. The molecular phylogenetic analysis on the basis of small subunit ribosomal DNA (SSU rDNA) sequences showed that N. anterostigmatica was not closely related to N. pyriformis, but was allied to Nephroselmis astigmatica Inouye & Pienaar. Comparisons of the hair scales and SSU rDNA sequences confirm that the identity of strain CCMP717 is N. pyriformis, and Pseudoscourfieldia marina (Throndsen) Manton has flagella ornamented with a different type of hair scales. The morphological evolution within the genus Nephroselmis is discussed and the order Nephroselmidales is proposed.

Cellular flow patterns and their evolutionary scenarios in three-dimensional Rayleigh-Bénard convection.

The evolution of three-dimensional, cellular convective flows in a plane horizontal layer of a Boussinesq fluid heated from below is studied numerically. Slow motion in the form of a spatially periodic pattern of hexagonal cells is introduced initially. In a further development, the flow can undergo a sequence of transitions between various cell types. The features of the flow evolution agree with the idea of the flow seeking an optimal scale. In particular, two-vortex polygonal cells may form at some evolution stages, with an annular planform of the upflow region and downflows localized in both central and peripheral regions of the cells. If short-wave hexagons are stable, they exhibit a specific, stellate fine structure.

Ethologic and ecologic interpretation of complex stellate structures in Pleistocene deep-sea sediments (Otadai Formation), Boso Peninsula, central Japan

A large stellate structure, which is characterized by long, horizontal radial structures with a single vertical shaft, occurs in mud-dominated lower submarine fan sediments (Lower Pleistocene Otadai Formation, Boso Peninsula, Japan) deposited on an active plate margin during a highstand interval of a eustatic sea-level cycle. A single radial structure is divided into more than 20 meniscate packages filled with pellets of fecal origin. The Otadai specimens closely resemble the trace fossil Phoebichnus trochoides in its configuration, which has been reported only from Mesozoic rocks of shallow marine origin. However, the radials of the Otadai material have no mantle or lining. Hence, the Otadai material can be discriminated from P. trochoides at the ichnospecies level. Thus the Otadai material is introduced as a new ichnospecies, namely Phoebichnus bosoensis. Considering the anatomy and mode of occurrence of P. bosoensis, the trace-making animal adopted a sedentary life-style and apparently fed on detritus at the seafloor and conveyed the fecal material downward into the radials. Internal structures in the radials of P. bosoensis appear to have been produced by the repetition of systematic excretory behavior of the trace-making animal. The vertical shaft is thought to be a dwelling structure. During a highstand interval of a eustatic sea-level cycle, large amounts of terrigenous sediments were deposited by turbidity currents at a lower submarine fan setting of the Otadai Formation, as suggested by sedimentologic data. Large amounts of terrigenous sediment supply to a lower submarine fan setting of the Otadai Formation was due to a narrower shelf and a steeper slope in comparison to a passive plate margin. Therefore sufficient influx to the seafloor of organic matter as the food for the benthos is assumed for the highstand interval of a eustatic sea-level cycle. This inference means that the nutrient level at the seafloor was suitable for surface deposit-feeders. Sufficient food supply must have been available to support the long-term sedentary life-style for the Otadai Phoebichnus producers, which produced the stellate structure as a dump for fecal matter.

A population of cells resident within embryonic and newborn rat skeletal muscle is capable of differentiating into multiple mesodermal phenotypes

We have previously shown a population of putative mesenchymal stem cells in the connective tissue surrounding embryonic avian skeletal muscle. These cells differentiate into at least five recognizable phenotypes in culture: fibroblasts, chondrocytes, myotubes, osteoblasts, and adipocytes. We have now isolated a similar population of cells from fetal and newborn rat skeletal muscle. Cells from rat leg muscle were dissected, minced, and then enzymatically digested with a collagenase-dispase solution. The dissociated cells were plated and allowed to differentiate into two recognizable populations: myotubes and stellate mononucleated cells. The cells were then trypsinized, filtered through a 20 microm filter to remove the myotubes, frozen at -80 degrees C, then thawed and replated. In culture the cells maintained their stellate structure. However, under treatment with dexamethasone, a nonspecific differentiating agent, seven morphologic conditions emerged: cells with refractile vesicles that stained with Sudan black B (adipocytes), multinucleated cells that spontaneously contracted in culture and stained with an antibody to myosin (myotubes), round cells whose extracellular matrix stained with Alcian blue, pH 1.0 (chondrocytes), polygonal cells whose extracellular matrix stained with Von Kossa's stain (osteoblasts), cells with filaments that stained with an antibody to smooth muscle a-actin (smooth muscle cells), cells that incorporated acetylated low density lipoprotein (endothelial cells), and spindle-shaped cells that grew in a swirl pattern (fibroblasts). The initial population is tentatively classified as putative mesenchymal stem cells. The presence of these cells point to the existence of stem cells in the postembryonic mammal that could provide a basis for tissue regeneration as opposed to scar tissue formation during wound healing.

Fine structural studies on an ultraplanktonic species ofPyramimonas, P. virginica(Prasinophyceae), with a discussion of subgenera within the genusPyramimonas

The ultrastructure of Pyramimonas virginica, the only known ultraplanktonic species of Pyramimonas, has been examined in detail. This species differs from other species of Pyramimonas in the structure of the flagellar apparatus, the scaly periplast, the pyrenoid, the eyespot, location of the eyespot in the cell, and mitosis. The flagellar apparatus is simple and shows signs of reduction. A 2-2-2-2 microtubular root system is present but a rhizoplast was not found. A very short transitional helix (perhaps two rings) is present in the flagellar transition region, together with a short one-piece stellate structure. Numbering of individual flagella in different species of Pyramimonas has shown a surprising variation in the orientation of the flagellar apparatus relative to the rest of the cell. Thus the eyespot in the various species is located differently relative to the flagellar apparatus and to other organelles such as the nucleus (in Pyramimonas there is no connection between the flagellar apparatus and ...

The flagellar apparatus of Prymnesium nemamethecum (Prymnesiophyceae)

Abstract The flagellar apparatus of Prymnesium nemamethecum Pienaar et Birkhead is more similar to that of P. patelliferum Green, Hibberd et Pienaar than to any other prymnesiophyte yet described. This includes the arrangement of the appendages, the origins and pathways of the microtubular roots, and the system of fibrous and striated connectives. There is, however, a major difference in the presence of a compound broad root in interphase cells of P. nemamethecum. Other differences include the number of constituent microtubules in some roots, the extent of development of a root (herein designated R7) associated with the right basal body, and an additional accessory band (AB3) in P. nemamethecum. The transitional region of the flagella is also described. It resembles that of P. parvum Carter, as both species have two transitional plates separated by a form of stellate structure. P. nemamethecum also has a distal axosome and a series of tubular rings between the outer doublets and the flagellar membrane. Th...

Centrin plays an essential role in microtubule severing during flagellar excision in Chlamydomonas reinhardtii

Previously, we reported that flagellar excision in Chlamydomonas reinhardtii is mediated by an active process whereby microtubules are severed at select sites within the flagellar-basal body transition zone (Sanders, M. A., and J. L. Salisbury. 1989. J. Cell Biol. 108:1751-1760). At the time of flagellar excision, stellate fibers of the transition zone contract and displace the microtubule doublets of the axoneme inward. The resulting shear force and torsional load generated during inward displacement leads to microtubule severing immediately distal to the central cylinder of the transition zone. In this study, we have used a detergent-extracted cell model of Chlamydomonas that allows direct experimental access to the molecular machinery responsible for microtubule severing without the impediment of the plasma membrane. We present four independent lines of experimental evidence for the essential involvement of centrin-based stellate fibers of the transition zone in the process of flagellar excision: (a) Detergent-extracted cell models excise their flagella in response to elevated, yet physiological, levels of free calcium. (b) Extraction of cell models with buffers containing the divalent cation chelator EDTA leads to the disassembly of centrin-based fibers and to the disruption of transition zone stellate fiber structure. This treatment results in a complete loss of flagellar excision competence. (c) Three separate anti-centrin monoclonal antibody preparations, which localize to the stellate fibers of the transition zone, specifically inhibit contraction of the stellate fibers and block calcium-induced flagellar excision, while control antibodies have no inhibitory effect. Finally, (d) cells of the centrin mutant vfl-2 (Taillon, B., S. Adler, J. Suhan, and J. Jarvik. 1992. J. Cell Biol. 119:1613-1624) fail to actively excise their flagella following pH shock in living cells or calcium treatment of detergent-extracted cell models. Taken together, these observations demonstrate that centrin-based fiber contraction plays a fundamental role in microtubule severing at the time of flagellar excision in Chlamydomonas.

Centrin-mediated microtubule severing during flagellar excision in Chlamydomonas reinhardtii.

Chlamydomonas cells excise their flagella in response to a variety of experimental conditions (e.g., extremes of temperature or pH, alcohol or detergent treatment, and mechanical shear). Here, we show that flagellar excision is an active process whereby microtubules are severed at select sites within the transition zone. The transition zone is located between the flagellar axoneme and the basal body; it is characterized by a pair of central cylinders that have an H shape when viewed in longitudinal section. Both central cylinders are connected to the A tubule of each microtubule doublet of the transition zone by fibers (approximately 5 nm diam). When viewed in cross section, these fibers are seen to form a distinctive stellate pattern characteristic of the transition zone (Manton, I. 1964. J. R. Microsc. Soc. 82:279-285; Ringo. D. L. 1967. J. Cell Biol. 33:543-571). We demonstrate that at the time of flagellar excision these fibers contract and displace the microtubule doublets of the axoneme inward. We believe that the resulting shear force and torsional load act to sever the axonemal microtubules immediately distal to the central cylinder. Structural alterations of the transition zone during flagellar excision occur both in living cells and detergent-extracted cell models, and are dependent on the presence of calcium (greater than or equal to 10(-6) M). Immunolocalization using monoclonal antibodies against the calcium-binding protein centrin demonstrate the presence of centrin in the fiber-based stellate structure of the transition zone of wild-type cells. Examination of the flagellar autotomy mutant, fa-1, which fails to excise its flagella (Lewin, R., and C. Burrascano. 1983. Experientia. 39:1397-1398), demonstrates that the fa-1 lacks the ability to completely contract the fibers of the stellate structure. We conclude that flagellar excision in Chlamydomonas involves microtubule severing that is mediated by the action of calcium-sensitive contractile fibers of the transition zone. These observations have led us to question whether microtubule severing may be a more general phenomenon than previously suspected and to suggest that microtubule severing may contribute to the dynamic behavior of cytoplasmic microtubules in other cells.