Efficient production of CRISPR/Cas9 gene knockouts in the male/female nematode Caenorhabditis nigoni

Although nematode genetics was founded on the use of hermaphrodite genetics for studying animal development and behavior, there is a growing need to extend this work to male/female species. One of the most promising species is C. nigoni, because it is so closely related to the model hermaphroditic C. briggsae. We present methods for using CRISPR/Cas9 gene editing to create mutations, and techniques for balancing, maintaining and studying these mutations.

Functional divergence of orthologous temperature-sensitive mutations in C. elegans and C. briggsae

To learn if orthologous mutations are temperature-sensitive in related species, we studied four C. briggsae mutations orthologous to alleles of important C. elegans genes. Both Cel-glp-4(bn2) and Cbr-glp-4(v473) are temperature-sensitive, causing sterility at 25°C. By contrast, Cel-fog-1 ( q253) is strongly ts , but its ortholog Cbr-fog-1(v442) causes a loss-of-function at all temperatures. Finally, the C. elegans glp-1 alleles bn18 and e2141 are ts sterile. However, their C. briggsae orthologs, Cbr-glp-1(v429) and Cbr-glp-1(v438) respectively, are wild-type at all temperatures. Thus, a ts mutation in one species provides clues about how to design ts alleles in another, but all theoretical outcomes are possible.

Sex Determination in Nematode Germ Cells

BACKGROUND

Animal germ cells differentiate as sperm or as oocytes. These sexual fates are controlled by complex regulatory pathways to ensure that the proper gametes are made at the appropriate times.

SUMMARY

Nematodes like Caenorhabditis elegans and its close relatives are ideal models for studying how this regulation works, because the XX animals are self-fertile hermaphrodites that produce both sperm and oocytes. In these worms, germ cells use the same signal transduction pathway that functions in somatic cells. This pathway determines the activity of the transcription factor TRA-1, a Gli protein that can repress male genes. However, the pathway is extensively modified in germ cells, largely by the action of translational regulators like the PUF proteins. Many of these modifications play critical roles in allowing the XX hermaphrodites to make sperm in an otherwise female body. Finally, TRA-1 cooperates with chromatin regulators in the germ line to control the activity of fog-1 and fog-3, which are essential for spermatogenesis. FOG-1 and FOG-3 work together to determine germ cell fates by blocking the translation of oogenic transcripts. Key Messages: Although there is great diversity in how germ cell fates are controlled in other animals, many of the key nematode genes are conserved, and the critical role of translational regulators may be universal.

Sex Determination: TRA-1 Is a Non-binary Regulator of Sexual Identity

In nematodes, TRA-1 represses the transcription of genes involved in male differentiation, allowing XX animals to undergo normal hermaphrodite development. New reports show that this transcription factor also acts in XO males, to control the differentiation of many neurons.

Mechanically-induced osteophyte in the rat knee

OBJECTIVES

Osteophytes are common anatomical signs of advanced osteoarthritis. It remains unclear whether they develop from physio-molecular, and/or mechanical stimuli. This study examined the effects of mechanical impact on the knee joint periosteum leading to osteophyte formation.

DESIGN

Eighteen mature rats received one single impact load of 53 N (30 MPa) to the periosteum of the experimental medial femoral condyles. Contralateral knees were used as controls. Animals were sacrificed at 24 h, 3, 6 and 9 weeks post-impact. Distal femurs were harvested and prepared for histology. Hematoxylin and Eosin, and Masson's trichrome stained slides were examined by light microscopy. Nuclear density was quantified to assess the tissue reaction.

RESULTS

24 h: The synovium membrane, fibrous and cambium periosteum were damaged. Blood infiltration pooled in the impacted medial collateral ligament (MCL) region. Week 3: A cartilaginous tissue spur, chondrophyte, was found in every rat at the impacted site of the MCL. Chondrophytes were composed of fibrocartilage and cartilage matrix, with signs of cartilage mineralization and remodelling activity. Week 6: Chondrophytes presented signs of more advanced mineralisation, recognized as osteophytes. Week 9: Osteophytes appeared to be more mineralized with almost no cartilage tissue.

CONCLUSIONS

Osteophytes can be induced with a single mechanical impact applied to the periosteum in rat knees. These data indicate that a moderate trauma to the periosteal layer of the joint may play a role in osteophyte development.

Evolution of Yin and Yang isoforms of a chromatin remodeling subunit precedes the creation of two genes

Genes can encode multiple isoforms, broadening their functions and providing a molecular substrate to evolve phenotypic diversity. Evolution of isoform function is a potential route to adapt to new environments. Here we show that de novo, beneficial alleles in the nurf-1 gene became fixed in two laboratory lineages of C. elegans after isolation from the wild in 1951, before methods of cryopreservation were developed. nurf-1 encodes an ortholog of BPTF, a large (>300 kD) multidomain subunit of the NURF chromatin remodeling complex. Using CRISPR-Cas9 genome editing and transgenic rescue, we demonstrate that in C. elegans, nurf-1 has split into two, largely non-overlapping isoforms (NURF-1.D and NURF-1.B, which we call Yin and Yang, respectively) that share only two of 26 exons. Both isoforms are essential for normal gametogenesis but have opposite effects on male/female gamete differentiation. Reproduction in hermaphrodites, which involves production of both sperm and oocytes, requires a balance of these opposing Yin and Yang isoforms. Transgenic rescue and genetic position of the fixed mutations suggest that different isoforms are modified in each laboratory strain. In a related clade of Caenorhabditis nematodes, the shared exons have duplicated, resulting in the split of the Yin and Yang isoforms into separate genes, each containing approximately 200 amino acids of duplicated sequence that has undergone accelerated protein evolution following the duplication. Associated with this duplication event is the loss of two additional nurf-1 transcripts, including the long-form transcript and a newly identified, highly expressed transcript encoded by the duplicated exons. We propose these lost transcripts are non-functional side products necessary to transcribe the Yin and Yang transcripts in the same cells. Our work demonstrates how gene sharing, through the production of multiple isoforms, can precede the creation of new, independent genes.

Reproduction: Sperm with Two X Chromosomes and Eggs with None

A new study shows that the nematode Auanema rhodensis manipulates X chromosome segregation in surprising ways that depend on both the sex of the parent and the type of gamete. The result is a complex mating system that produces unusual sex ratios and inheritance patterns.

The zinc transporter ZIPT-7.1 regulates sperm activation in nematodes

Sperm activation is a fascinating example of cell differentiation, in which immotile spermatids undergo a rapid and dramatic transition to become mature, motile sperm. Because the sperm nucleus is transcriptionally silent, this transition does not involve transcriptional changes. Although Caenorhabditis elegans is a leading model for studies of sperm activation, the mechanisms by which signaling pathways induce this transformation remain poorly characterized. Here we show that a conserved transmembrane zinc transporter, ZIPT-7.1, regulates the induction of sperm activation in Caenorhabditis nematodes. The zipt-7.1 mutant hermaphrodites cannot self-fertilize, and males reproduce poorly, because mutant spermatids are defective in responding to activating signals. The zipt-7.1 gene is expressed in the germ line and functions in germ cells to promote sperm activation. When expressed in mammalian cells, ZIPT-7.1 mediates zinc transport with high specificity and is predominantly located on internal membranes. Finally, genetic epistasis places zipt-7.1 at the end of the spe-8 sperm activation pathway, and ZIPT-7.1 binds SPE-4, a presenilin that regulates sperm activation. Based on these results, we propose a new model for sperm activation. In spermatids, inactive ZIPT-7.1 is localized to the membranous organelles, which contain higher levels of zinc than the cytoplasm. When sperm activation is triggered, ZIPT-7.1 activity increases, releasing zinc from internal stores. The resulting increase in cytoplasmic zinc promotes the phenotypic changes characteristic of activation. Thus, zinc signaling is a key step in the signal transduction process that mediates sperm activation, and we have identified a zinc transporter that is central to this activation process.

Sperm are specialized cells with transcriptionally silent DNA that has been packaged for delivery into the egg. In their final step of development, immature sperm undergo a rapid transition from nonmotile cells to mature, motile sperm capable of fertilization. The signals that trigger this change are not clearly understood. By identifying mutants in the roundworm Caenorhabditis elegans that are defective in sperm activation, we discovered a conserved transmembrane protein, ZIPT-7.1, that transports zinc and promotes sperm activation in both sexes. ZIPT-7.1 is expressed in the germ line and functions there to control sperm activation. When expressed ectopically in mammalian cells, the protein specifically transports zinc across membranes and localizes primarily to membranes within the cell. Previous genetic studies had identified two pathways that mediate sperm activation in C. elegans, and our results suggest that zipt-7.1 acts at the end of one of these two, the spe-8 pathway. We propose that when this pathway triggers sperm activation, it acts through ZIPT-7.1, which mediates the release of zinc from internal stores in the immature sperm. This released zinc functions as a second messenger to promote the differentiation of mature, motile sperm.

"The persistence of memory"-Hermaphroditism in nematodes

Self-fertility has evolved many times in nematodes. This transition often produces an androdioecious species, with XX hermaphrodites and XO males. Although these hermaphrodites resemble females in most respects, early germ cells differentiate as sperm, and late ones as oocytes. The sperm then receive an activation signal, populate the spermathecae, and are stored for later use in self-fertilization. These traits are controlled by complex modifications to the sex-determination and sperm activation pathways, which have arisen independently during the evolution of each hermaphroditic species. This transformation in reproductive strategy then promotes other major changes in the development, evolution, and population structure of these animals. Mol. Reprod. Dev. 84: 144-157, 2017. © 2016 Wiley Periodicals, Inc.

Size isn't everything

Male nematode worms may make larger sperm than hermaphrodite worms, but this is not the only reason that sperm from males have a competitive edge.

The influence of osteophyte depiction in CT for patient-specific guided hip resurfacing procedures

PURPOSE

An accurate fit of a patient-specific instrument guide during an intervention is one of the critical factors affecting accuracy of the surgical procedure. In this study, we investigated how well osteophytes, which are abnormal bone growths that form along joints, are depicted in clinical preoperative CT scans and estimated the influence of such depiction errors on the intraoperative accuracy of the guide.

METHODS

In 34 hip resurfacing patients, 227 osteophyte surface points on the anterior aspect of the femoral neck were collected intraoperatively, using an optoelectronic navigation system. These points were registered to a preoperative CT scan of the patient, and distances between collected points and segmented virtual bone surface, as well as Hounsfield units for these points, were determined. We simulated the registration error of a patient-specific guide, using a modified registration algorithm, to test placement on the anterior aspect of the femoral neck without removing any osteophytes. This error was then applied to the surgical plan of the femoral central-pin position and orientation for evaluation.

RESULTS

The average distance between the collected points and the segmented surface was 2.6 mm. We estimated the average error for the entrance point of the central-pin to be 0.7 mm in the distal direction and 3.2 mm in the anterior direction. The average orientation error was 2.8° in anteversion.

CONCLUSIONS

The depiction of osteophytes in clinical preoperative CT scans for proximal femurs can be unreliable and can possibly result in significant intraoperative instrument alignment errors during image-guided surgeries.

Wide-angle ITER-prototype tangential infrared and visible viewing system for DIII-D

An imaging system with a wide-angle tangential view of the full poloidal cross-section of the tokamak in simultaneous infrared and visible light has been installed on DIII-D. The optical train includes three polished stainless steel mirrors in vacuum, which view the tokamak through an aperture in the first mirror, similar to the design concept proposed for ITER. A dichroic beam splitter outside the vacuum separates visible and infrared (IR) light. Spatial calibration is accomplished by warping a CAD-rendered image to align with landmarks in a data image. The IR camera provides scrape-off layer heat flux profile deposition features in diverted and inner-wall-limited plasmas, such as heat flux reduction in pumped radiative divertor shots. Demonstration of the system to date includes observation of fast-ion losses to the outer wall during neutral beam injection, and shows reduced peak wall heat loading with disruption mitigation by injection of a massive gas puff.

The evolutionary origins and consequences of self-fertility in nematodes

Self-fertile hermaphrodites have evolved from male/female ancestors in many nematode species, and this transition occurred on three independent occasions in the genus Caenorhabditis. Genetic analyses in Caenorhabditis show that the origin of hermaphrodites required two types of changes: alterations to the sex-determination pathway that allowed otherwise female animals to make sperm during larval development, and the production of signals from the gonad that caused these sperm to activate and fertilize oocytes. Comparisons of C. elegans and C. briggsae hermaphrodites show that the ancestral sex-determination pathway has been altered in multiple unique ways. Some of these changes must have precipitated the production of sperm in XX animals, and others were modifying mutations that increased the efficiency of hermaphroditic reproduction. Reverse genetic experiments show that XX animals acquired the ability to activate sperm by co-opting one of the two redundant pathways that normally work in males. Finally, the adoption of a hermaphroditic lifestyle had profound effects on ecological and sexual interactions and genomic organization. Thus, nematode mating systems are ideal for elucidating the origin of novel traits, and studying the influence of developmental processes on evolutionary change.

Dependence of the sperm/oocyte decision on the nucleosome remodeling factor complex was acquired during recent Caenorhabditis briggsae evolution

The major families of chromatin remodelers have been conserved throughout eukaryotic evolution. Because they play broad, pleiotropic roles in gene regulation, it was not known if their functions could change rapidly. Here, we show that major alterations in the use of chromatin remodelers are possible, because the nucleosome remodeling factor (NURF) complex has acquired a unique role in the sperm/oocyte decision of the nematode Caenorhabditis briggsae. First, lowering the activity of C. briggsae NURF-1 or ISW-1, the core components of the NURF complex, causes germ cells to become oocytes rather than sperm. This observation is based on the analysis of weak alleles and null mutations that were induced with TALENs and on RNA interference. Second, qRT-polymerase chain reaction data show that the C. briggsae NURF complex promotes the expression of Cbr-fog-1 and Cbr-fog-3, two genes that control the sperm/oocyte decision. This regulation occurs in the third larval stage and affects the expression of later spermatogenesis genes. Third, double mutants reveal that the NURF complex and the transcription factor TRA-1 act independently on Cbr-fog-1 and Cbr-fog-3. TRA-1 binds both promoters, and computer analyses predict that these binding sites are buried in nucleosomes, so we suggest that the NURF complex alters chromatin structure to allow TRA-1 access to Cbr-fog-1 and Cbr-fog-3. Finally, lowering NURF activity by mutation or RNA interference does not affect this trait in other nematodes, including the sister species C. nigoni, so it must have evolved recently. We conclude that altered chromatin remodeling could play an important role in evolutionary change.

Rogue sperm indicate sexually antagonistic coevolution in nematodes

Intense reproductive competition often continues long after animals finish mating. In many species, sperm from one male compete with those from others to find and fertilize oocytes. Since this competition occurs inside the female reproductive tract, she often influences the outcome through physical or chemical factors, leading to cryptic female choice. Finally, traits that help males compete with each other are sometimes harmful to females, and female countermeasures may thwart the interests of males, which can lead to an arms race between the sexes known as sexually antagonistic coevolution. New studies from Caenorhabditis nematodes suggest that males compete with each other by producing sperm that migrate aggressively and that these sperm may be more likely to win access to oocytes. However, one byproduct of this competition appears to be an increased probability that these sperm will go astray, invading the ovary, prematurely activating oocytes, and sometimes crossing basement membranes and leaving the gonad altogether. These harmful effects are sometimes observed in crosses between animals of the same species but are most easily detected in interspecies crosses, leading to dramatically lowered fitness, presumably because the competitiveness of the sperm and the associated female countermeasures are not precisely matched. This mismatch is most obvious in crosses involving individuals from androdioecious species (which have both hermaphrodites and males), as predicted by the lower levels of sperm competition these species experience. These results suggest a striking example of sexually antagonistic coevolution and dramatically expand the value of nematodes as a laboratory system for studying postcopulatory interactions.

The regulation of spermatogenesis and sperm function in nematodes

In the nematode C. elegans, both males and self-fertile hermaphrodites produce sperm. As a result, researchers have been able to use a broad range of genetic and genomic techniques to dissect all aspects of sperm development and function. Their results show that the early stages of spermatogenesis are controlled by transcriptional and translational processes, but later stages are dominated by protein kinases and phosphatases. Once spermatids are produced, they participate in many interactions with other cells - signals from the somatic gonad determine when sperm activate and begin to crawl, signals from the female reproductive tissues guide the sperm, and signals from sperm stimulate oocytes to mature and be ovulated. The sperm also show strong competitive interactions with other sperm and oocytes. Some of the molecules that mediate these processes have conserved functions in animal sperm, others are conserved proteins that have been adapted for new roles in nematode sperm, and some are novel proteins that provide insights into evolutionary change. The advent of new techniques should keep this system on the cutting edge of research in cellular and reproductive biology.

Germline quality control: eEF2K stands guard to eliminate defective oocytes

The control of germline quality is critical to reproductive success and survival of a species; however, the mechanisms underlying this process remain unknown. Here, we demonstrate that elongation factor 2 kinase (eEF2K), an evolutionarily conserved regulator of protein synthesis, functions to maintain germline quality and eliminate defective oocytes. We show that disruption of eEF2K in mice reduces ovarian apoptosis and results in the accumulation of aberrant follicles and defective oocytes at advanced reproductive age. Furthermore, the loss of eEF2K in Caenorhabditis elegans results in a reduction of germ cell death and significant decline in oocyte quality and embryonic viability. Examination of the mechanisms by which eEF2K regulates apoptosis shows that eEF2K senses oxidative stress and quickly downregulates short-lived antiapoptotic proteins, XIAP and c-FLIPL by inhibiting global protein synthesis. These results suggest that eEF2K-mediated inhibition of protein synthesis renders cells susceptible to apoptosis and functions to eliminate suboptimal germ cells.

Rapid creation of forward-genetics tools for C. briggsae using TALENs: lessons for nonmodel organisms

Although evolutionary studies of gene function often rely on RNA interference, the ideal approach would use reverse genetics to create null mutations for cross-species comparisons and forward genetics to identify novel genes in each species. We have used transcription activator-like effector nucleases (TALENs) to facilitate both approaches in Caenorhabditis nematodes. First, by combining golden gate cloning and TALEN technology, we can induce frameshifting mutations in any gene. Second, by combining this approach with bioinformatics we can predict and create the resources needed for forward genetic analysis in species like Caenorhabditis briggsae. Although developing genetic model organisms used to require years to isolate marker mutations, balancers, and tools, with TALENs, these reagents can now be produced in months. Furthermore, the analysis of nonsense mutants in related model organisms allows a directed approach for making these markers and tools. When used together, these methods could simplify the adaptation of other organisms for forward and reverse genetics.

Evolutionary change within a bipotential switch shaped the sperm/oocyte decision in hermaphroditic nematodes

A subset of transcription factors like Gli2 and Oct1 are bipotential--they can activate or repress the same target, in response to changing signals from upstream genes. Some previous studies implied that the sex-determination protein TRA-1 might also be bipotential; here we confirm this hypothesis by identifying a co-factor, and use it to explore how the structure of a bipotential switch changes during evolution. First, null mutants reveal that C. briggsae TRR-1 is required for spermatogenesis, RNA interference implies that it works as part of the Tip60 Histone Acetyl Transferase complex, and RT-PCR data show that it promotes the expression of Cbr-fog-3, a gene needed for spermatogenesis. Second, epistasis tests reveal that TRR-1 works through TRA-1, both to activate Cbr-fog-3 and to control the sperm/oocyte decision. Since previous studies showed that TRA-1 can repress fog-3 as well, these observations demonstrate that it is bipotential. Third, TRR-1 also regulates the development of the male tail. Since Cbr-tra-2 Cbr-trr-1 double mutants resemble Cbr-tra-1 null mutants, these two regulatory branches control all tra-1 activity. Fourth, striking differences in the relationship between these two branches of the switch have arisen during recent evolution. C. briggsae trr-1 null mutants prevent hermaphrodite spermatogenesis, but not Cbr-fem null mutants, which disrupt the other half of the switch. On the other hand, C. elegans fem null mutants prevent spermatogenesis, but not Cel-trr-1 mutants. However, synthetic interactions confirm that both halves of the switch exist in each species. Thus, the relationship between the two halves of a bipotential switch can shift rapidly during evolution, so that the same phenotype is produce by alternative, complementary mechanisms.

Analyzing shoulder translation with navigation technology

PURPOSE

Asymmetric stress imposed on the shoulder can lead to anterior shoulder instability in young athletes who perform repetitive overhead motions. A common treatment, surgical anterior capsule tightening, assumes that the instability is caused by abnormal anterior laxity. This study investigated the possibility that one element of overall imbalance, posterior capsular tightness, could be an underlying reason for shoulder instability. Surgical navigation technology, which is more accurate than whole-body motion-capture systems, was used to study anterior translational motions.

METHOD

The study was used four cadaver shoulders, with the scapula and rotator cuff muscles intact. Opto-electronic surgical navigation localization devices were mounted on the scapula and humerus to accurately capture positions and orientations. The shoulders were passively moved through 7 motions, 5 of simple angulation and 2 combinations of clinical interest. Each motion was repeated in 4 different soft-tissue states: rotator cuff intact, capsule intact, and surgically induced capsular tightnesses of 5 and 10mm.

RESULTS

The shoulders had significantly greater anterior translation when the posterior capsule was artificially tightened (p < 0.05); this was particularly in movements that combined abduction with internal or external rotation, which are typical overhead sports motions. Overall translation was indifferent to whether the shoulders were intact or dissected down to the capsule, as was translation during flexion was indifferent to dissection state (p > 0.95).

CONCLUSION

Surgical navigation technology can easily be used to analyze cadaveric shoulder motion, with opportunities for adaptation to anesthetized patients. Results suggest that the inverse of artificial tightening, such as surgical release of the posterior capsule, may be an effective minimally invasive treatment of chronic shoulder dislocation subsequent to sports motions.

A bias caused by ectopic development produces sexually dimorphic sperm in nematodes

Self-fertile hermaphrodites have evolved independently several times in the genus Caenorhabditis [1, 2]. These XX hermaphrodites make smaller sperm than males [3, 4], which they use to fertilize their own oocytes. Because larger sperm outcompete smaller sperm in nematodes [3-5], it had been assumed that this dimorphism evolved in response to sperm competition. However, we show that it was instead caused by a developmental bias. When we transformed females of the species Caenorhabditis remanei into hermaphrodites [6], their sperm were significantly smaller than those of males. Because this species never makes hermaphrodites in the wild, this dimorphism cannot be due to selection. Instead, analyses of the related nematode Caenorhabditis elegans suggest that this dimorphism might reflect the development of sperm within the distinct physiological environment of the hermaphrodite gonad. These results reveal a new mechanism for some types of developmental bias-the effects of a novel physical location alter the development of ectopic cells in predictable ways.

Otx-dependent expression of proneural bHLH genes establishes a neuronal bilateral asymmetry in C. elegans

Bilateral asymmetry in Caenorhabditis elegans arises in part from cell lineages that differ on the left and right sides of the animal. The unpaired MI neuron descends from the right side of an otherwise left-right symmetric cell lineage that generates the MI neuron on the right and the e3D epithelial cell on the left. We isolated mutations in three genes that caused left-right symmetry in this normally asymmetric cell lineage by transforming MI into an e3D-like cell. These genes encode the proneural bHLH proteins NGN-1 and HLH-2 and the Otx homeodomain protein CEH-36. We identified the precise precursor cells in which ceh-36 and ngn-1 act, and showed that CEH-36 protein is asymmetrically expressed and is present in an MI progenitor cell on the right but not in its bilateral counterpart. This asymmetric CEH-36 expression promotes asymmetric ngn-1 and hlh-2 expression, which in turn induces asymmetric MI neurogenesis. Our results indicate that this left-right asymmetry is specified within the two sister cells that first separate the left and right branches of the cell lineage. We conclude that the components of an evolutionarily conserved Otx/bHLH pathway act sequentially through multiple rounds of cell division on the right to relay an initial apparently cryptic asymmetry to the presumptive post-mitotic MI neuron, thereby creating an anatomical bilateral asymmetry in the C. elegans nervous system.

Independent recruitment of F box genes to regulate hermaphrodite development during nematode evolution

Elucidating the molecular mechanisms that created ancient complex traits like insect wings is difficult. Fortunately, some complex traits have arisen recently. For example, hermaphroditic reproduction evolved independently many times during recent nematode evolution. Although C. elegans hermaphrodites require fog-2, which encodes an F box protein that regulates the translation of tra-2 mRNAs, the related species C. briggsae lacks fog-2. We identified a critical regulator of hermaphrodite development in C. briggsae, named she-1. Analysis of double mutants indicates that she-1 acts upstream of tra-2 in C. briggsae, just as fog-2 does in C. elegans. Molecular cloning shows that she-1 encodes a novel F box protein that was created by a recent gene duplication. Whereas FOG-2 acts through GLD-1 in C. elegans, SHE-1 does not bind GLD-1 in C. briggsae. Thus, both species recruited F box genes produced by recent duplication events into the sex-determination pathway to control hermaphrodite development, but these genes have distinct activities. This result implies that some gene families are more likely to give rise to novel regulatory genes than other families. Finally, we note that null mutations of she-1 are temperature sensitive, so C. briggsae might once have been a facultative hermaphrodite.

Caenorhabditis elegans FOS-1 and JUN-1 regulate plc-1 expression in the spermatheca to control ovulation

Fos and Jun are components of activator protein-1 (AP-1) and play crucial roles in the regulation of many cellular, developmental, and physiological processes. Caenorhabditis elegans fos-1 has been shown to act in uterine and vulval development. Here, we provide evidence that C. elegans fos-1 and jun-1 control ovulation, a tightly regulated rhythmic program in animals. Knockdown of fos-1 or jun-1 blocks dilation of the distal spermathecal valve, a critical step for the entry of mature oocytes into the spermatheca for fertilization. Furthermore, fos-1 and jun-1 regulate the spermathecal-specific expression of plc-1, a gene that encodes a phospholipase C (PLC) isozyme that is rate-limiting for inositol triphosphate production and ovulation, and overexpression of PLC-1 rescues the ovulation defect in fos-1(RNAi) worms. Unlike fos-1, regulation of ovulation by jun-1 requires genetic interactions with eri-1 and lin-15B, which are involved in the RNA interference pathway and chromatin remodeling, respectively. At least two isoforms of jun-1 are coexpressed with fos-1b in the spermatheca, and different AP-1 dimers formed between these isoforms have distinct effects on the activation of a reporter gene. These findings uncover a novel role for FOS-1 and JUN-1 in the reproductive system and establish C. elegans as a model for studying AP-1 dimerization.

Cartilage collagen matrix reorientation and displacement in response to surface loading

An investigation of collagen fiber reorientation, as well as fluid and matrix movement of equine articular cartilage and subchondral bone under compressive mechanical loads, was undertaken using small angle X-ray scattering measurements and optical microscopy. Small angle X-ray scattering measurements were made on healthy and diseased samples of equine articular cartilage and subchondral bone mounted in a mechanical testing apparatus on station ID18F of ESRF, Grenoble, together with fiber orientation analysis using polarized light and displacement measurements of the cartilage matrix and fluid using tracers. At surface pressures of up to approximately 1.5 MPa, there was reversible compression of the tangential surface fibers and immediately subjacent zone. As load increased, deformation in these zones reached a maximum and then reorientation propagated to the radial deep zone. Between surface pressures of 4.8 MPa and 6.0 MPa, fiber orientation above the tide mark rotated 10 deg from the radial direction, with an overall loss of alignment. With further increase in load, the fibers "crimped" as shown by the appearance of subsidiary peaks approximately +/-10 deg either side of the principal fiber orientation direction. Failure at higher loads was characterized by a radial split in the deep cartilage, which propagated along the tide mark while the surface zone remained intact. In lesions, the fiber organization was disrupted and the initial response to load was consistent with early rupture of fibers, but the matrix relaxed to an organization very similar to that of the unloaded tissue. Tracer measurements revealed anisotropic solid and fluid displacement, which depended strongly on depth within the tissue.

Apoptosis maintains oocyte quality in aging Caenorhabditis elegans females

In women, oocytes arrest development at the end of prophase of meiosis I and remain quiescent for years. Over time, the quality and quantity of these oocytes decreases, resulting in fewer pregnancies and an increased occurrence of birth defects. We used the nematode Caenorhabditis elegans to study how oocyte quality is regulated during aging. To assay quality, we determine the fraction of oocytes that produce viable eggs after fertilization. Our results show that oocyte quality declines in aging nematodes, as in humans. This decline affects oocytes arrested in late prophase, waiting for a signal to mature, and also oocytes that develop later in life. Furthermore, mutations that block all cell deaths result in a severe, early decline in oocyte quality, and this effect increases with age. However, mutations that block only somatic cell deaths or DNA-damage-induced deaths do not lower oocyte quality. Two lines of evidence imply that most developmentally programmed germ cell deaths promote the proper allocation of resources among oocytes, rather than eliminate oocytes with damaged chromosomes. First, oocyte quality is lowered by mutations that do not prevent germ cell deaths but do block the engulfment and recycling of cell corpses. Second, the decrease in quality caused by apoptosis mutants is mirrored by a decrease in the size of many mature oocytes. We conclude that competition for resources is a serious problem in aging germ lines, and that apoptosis helps alleviate this problem.

Visualization of protein interactions in living Caenorhabditis elegans using bimolecular fluorescence complementation analysis

The bimolecular fluorescence complementation (BiFC) assay is a powerful tool for visualizing and identifying protein interactions in living cells. This assay is based on the principle of protein-fragment complementation, using two nonfluorescent fragments derived from fluorescent proteins. When two fragments are brought together in living cells by tethering each to one of a pair of interacting proteins, fluorescence is restored. Here, we provide a protocol for a Venus-based BiFC assay to visualize protein interactions in the living nematode, Caenorhabditis elegans. We discuss how to design appropriate C. elegans BiFC cloning vectors to enable visualization of protein interactions using either inducible heat shock promoters or native promoters; transform the constructs into worms by microinjection; and analyze and interpret the resulting data. When expression of BiFC fusion proteins is induced by heat shock, the fluorescent signals can be visualized as early as 30 min after induction and last for 24 h in transgenic animals. The entire procedure takes 2-3 weeks to complete.

Dynamic simulation of a displacement-controlled total knee replacement wear tester

This paper presents a dynamic finite element method (FEM) model of a commercial displacement-controlled total knee replacement (TKR) wear tester. The first goal of the study was to validate the model, which included both the wear tester and the TKR components. Convergence simulations and experimental testing were performed. These included a novel experimental determination of the coefficient of friction and an evaluation of predicted joint contact areas by comparing simulation results with experimental data collected using pressure-sensitive film. The second goal of this study was to develop a procedure for implementing force-based testing protocols on a displacement-controlled TKR wear tester. A standard force-based cyclic wear-testing protocol was simulated using the FEM model and resulting displacement waveforms were extracted. These were used as control inputs to the physical wear tester and an experimental wear test was performed. Reaction loads on the tibial components were measured and compared with the simulated results. The model was capable of accurately predicting the tibial loads throughout the test cycle, verifying the model's contact mechanics. The study demonstrated the use of computational modelling to convert a force-based testing protocol into displacement-based control parameters for use in a displacement-controlled mechanical testing system.

Sex determination in the Caenorhabditis elegans germ line

Sexual identity is one of the most important factors that determine how an animal will develop. Although it controls many dimorphic tissues in the body, its most ancient role is in the germ line, where it species that some cells become sperm, and others become eggs. In most animals, these two fates occur in distinct sexes. However, certain nematodes like C. elegans produce XX hermaphrodites, which make both types of gametes. In these animals, a core sex-determination pathway regulates the development of both the body and the germ line. However, modifier genes alter the activity of this pathway in germ cells, and these changes are critical for allowing XX animals to produce oocytes and sperm in an otherwise female body. In this review, I focus on (1) the core sex-determination pathway, (2) the activity of the transcription factor TRA-1 and its immediate targets fog-1 and fog-3 in germ cells, (3) how the regulation of tra-2 activity allows XX spermatogenesis, and (4) how the regulation of fem-3 activity maintains the appropriate balance between TRA-2 and FEM-3 in the germ line. Finally, I consider the major questions in this field that are driving new research.

A navigation system for shoulder arthroscopic surgery

The general framework and experimental validation of a novel navigation system designed for shoulder arthroscopy are presented. The system was designed to improve the surgeon's perception of the three-dimensional space within the human shoulder. Prior to surgery, a surface model of the shoulder was created from computed tomography images. Intraoperatively optically tracked arthroscopic instruments were calibrated. The surface model was then registered to the patient using tracked freehand ultrasound images taken from predefined landmark regions on the scapula. Three-dimensional models of the surgical instruments were displayed, in real time, relative to the surface model in a user interface. Laboratory experiments revealed only small registration and calibration errors, with minimal time needed to complete the intraoperative tasks.

Validation of a new surgical procedure for percutaneous scaphoid fixation using intra-operative ultrasound

A new technique for percutaneous fixation of non-displaced scaphoid fractures is described. The technique used pre-operative planning from computed tomography images, registration to intra-operatively acquired three-dimensional ultrasound images, and intra-operative guidance using an optical tracking system. Two stand-alone software applications were developed. The first one was used to determine the surgical plan pre-operatively and the second one was used to guide the surgeon during screw insertion. Laboratory validation of the technique included measurements of the inter-operator and intra-operator variability in the outcome of scaphoid fixation using the proposed procedure, and also included comparison of the performance of this procedure with the conventional percutaneous fixation technique using fluoroscopy. The results showed that the tight accuracy requirements of percutaneous scaphoid fixation were met and that the consistency was superior to the conventional technique.

Two-dimensional order in mammalian pre-ocular tear film

We report a grazing incidence x-ray diffraction (GIXD) investigation of the surface lipid layer of the pre-ocular tear film. For the first time we demonstrate the existence of 2D order over a wide range of surface pressures in this system, with typical spicing of 3.75A and 4.16A independent of the monolayer surface pressure. Analogous lipid ordering is also found in an artificial lipid mixture of the major lipid components of the tear film, suggesting that the 2D ordering is set by generic lipid-lipid interactions. Fluorescence microscopy of the natural and artificial tear film mixture reveals the co-existence of a dilute and a much more condensed phase in the amphiphilic lipid matrix over the pressure range of 15-45mN/m investigated by GIXD, plus an additional structure due to the much more hydrophobic part of the mixture. This evidence supports the previous hypothesis that tear film has a layered structure.

Regional variations of collagen orientation in normal and diseased articular cartilage and subchondral bone determined using small angle X-ray scattering (SAXS)

OBJECTIVE

To determine regional differences in the orientation of collagen in the articular cartilage of the equine metacarpophalangeal joint as well as describing cartilage orientation in lesions using small angle X-ray scattering (SAXS).

DESIGN

SAXS diffraction patterns were taken at the European Synchrotron Radiation Facility (ESRF), with increasing depth into cartilage and bone cross sections. Results for healthy samples were taken at different regions along the joint which receive different loads and differences in collagen orientation were determined. Results were also taken from diseased samples and the collagen orientation changes from that of healthy samples observed.

RESULTS

Regions subject to low loads show a lower degree of orientation and regions exposed to the highest loads possess oriented collagen fibres especially in the radial layer. In early lesions the orientations of the collagen fibres are disrupted. Subchondral bone fibres are twisted in regions where the joint receives shear forces. Changes in fibre orientation are also observed in the calcified cartilage even in regions where the cartilage is intact. In more advanced lesions where there is loss of cartilage the fibres in the calcified layer are realigned tangential to the surface.

CONCLUSIONS

Regional variations in collagen arrangement show that the highly ordered layers of the articular cartilage are the most important elements in supporting high variable loads. In lesions changes occur in the deep tissue whilst the overlying cartilage appeared normal. We therefore suggest that the interface region is a key element in the early stages of the disease.

Enigma variations: control of sexual fate in nematode germ cells

A new study showing that neither FEM-2 nor FEM-3 is required for spermatogenesis in Caenorhabditis briggsae, unlike in Caenorhabditis elegans, implies that the sex-determination pathway in these species is evolving rapidly, and supports the proposal that they evolved hermaphroditism independently.

Patellofemoral joint kinematics in individuals with and without patellofemoral pain syndrome

BACKGROUND

Patellofemoral pain syndrome is a prevalent condition in young people. While it is widely believed that abnormal patellar tracking plays a role in the development of patellofemoral pain syndrome, this link has not been established. The purpose of this cross-sectional case-control study was to test the hypothesis that patterns of patellar spin, tilt, and lateral translation make it possible to distinguish individuals with patellofemoral pain syndrome and clinical evidence of patellar malalignment from those with patellofemoral pain syndrome and no clinical evidence of malalignment and from individuals with no knee problems.

METHODS

Three-dimensional patellofemoral joint kinematics in one knee of each of sixty volunteers (twenty in each group described above) were assessed with use of a new, validated magnetic resonance imaging-based method. Static low-resolution scans of the loaded knee were acquired at five different angles of knee flexion (ranging between -4 degrees and 60 degrees). High-resolution geometric models of the patella, femur, and tibia and associated coordinate axes were registered to the bone positions on the low-resolution scans to determine the patellar motion as a function of knee flexion angle. Hierarchical modeling was used to identify group differences in patterns of patellar spin, tilt, and lateral translation.

RESULTS

No differences in the overall pattern of patellar motion were observed among groups (p>0.08 for all global maximum likelihood ratio tests). Features of patellar spin and tilt patterns varied greatly between subjects across all three groups, and no significant group differences were detected. At 19 degrees of knee flexion, the patellae in the group with patellofemoral pain and clinical evidence of malalignment were positioned an average of 2.25 mm more laterally than the patellae in the control group, and this difference was marginally significant (p=0.049). Other features of the pattern of lateral translation did not differ, and large overlaps in values were observed across all groups.

CONCLUSIONS

It cannot be determined from our cross-sectional study whether the more lateral position of the patella in the group with clinical evidence of malalignment preceded or followed the onset of symptoms. It is clear from the data that an individual with patellofemoral pain syndrome cannot be distinguished from a control subject by examining patterns of spin, tilt, or lateral translation of the patella, even when clinical evidence of mechanical abnormality was observed.

Genetic interactions due to constitutive and inducible gene regulation mediated by the unfolded protein response in C. elegans

The unfolded protein response (UPR) is an adaptive signaling pathway utilized to sense and alleviate the stress of protein folding in the endoplasmic reticulum (ER). In mammals, the UPR is mediated through three proximal sensors PERK/PEK, IRE1, and ATF6. PERK/PEK is a protein kinase that phosphorylates the alpha subunit of eukaryotic translation initiation factor 2 to inhibit protein synthesis. Activation of IRE1 induces splicing of XBP1 mRNA to produce a potent transcription factor. ATF6 is a transmembrane transcription factor that is activated by cleavage upon ER stress. We show that in Caenorhabditis elegans, deletion of either ire-1 or xbp-1 is synthetically lethal with deletion of either atf-6 or pek-1, both producing a developmental arrest at larval stage 2. Therefore, in C. elegans, atf-6 acts synergistically with pek-1 to complement the developmental requirement for ire-1 and xbp-1. Microarray analysis identified inducible UPR (i-UPR) genes, as well as numerous constitutive UPR (c-UPR) genes that require the ER stress transducers during normal development. Although ire-1 and xbp-1 together regulate transcription of most i-UPR genes, they are each required for expression of nonoverlapping sets of c-UPR genes, suggesting that they have distinct functions. Intriguingly, C. elegans atf-6 regulates few i-UPR genes following ER stress, but is required for the expression of many c-UPR genes, indicating its importance during development and homeostasis. In contrast, pek-1 is required for induction of approximately 23% of i-UPR genes but is dispensable for the c-UPR. As pek-1 and atf-6 mainly act through sets of nonoverlapping targets that are different from ire-1 and xbp-1 targets, at least two coordinated responses are required to alleviate ER stress by distinct mechanisms. Finally, our array study identified the liver-specific transcription factor CREBh as a novel UPR gene conserved during metazoan evolution.

The endoplasmic reticulum (ER) is an intracellular organelle where proteins fold and assemble prior to transport to the cell surface. The ER contains a finely tuned quality control apparatus to ensure that improperly folded proteins are retained in the ER lumen. A variety of physiological demands, environmental perturbations, and pathological conditions compromise protein folding in the ER and lead to the accumulation of unfolded proteins. The unfolded protein response (UPR) is an evolutionarily conserved intracellular adaptive signaling pathway that alleviates protein-folding defects in the ER. The unfolded protein signal is transmitted from the ER to the nucleus by three pathways involving the proteins ATF-6, PEK-1, and IRE-1/XBP-1. However, it is not known how these three pathways coordinate downstream transcriptional activation to mediate either cell adaptation or cell death. The authors have studied the nematode Caenorhabditis elegans to present a comprehensive genetic and gene expression analysis of the three UPR pathways. The findings demonstrate that the UPR regulates the expression of hundreds of genes in the presence, as well as the absence, of ER stress in a manner that is more complex and diverse than previously known.

Needle artifact localization in 3T MR images

This work explores an image-based approach for localizing needles during MRI-guided interventions, for the purpose of tracking and navigation. Susceptibility artifacts for several needles of varying thickness were imaged, in phantoms, using a 3 tesla MRI system, under a variety of conditions. The relationship between the true needle positions and the locations of artifacts within the images, determined both by manual and automatic segmentation methods, have been quantified and are presented here.

Computer-assisted guidewire insertion for hip fracture fixation

OBJECTIVE

: This study was designed to test in a laboratory setting a novel computer-assisted fluoroscopic technique and a conventional fluoroscopic technique for open reduction and internal fixation (ORIF) of hip fractures. Our hypothesis is that a novel computer-assisted fluoroscopic technique will achieve acceptable guidewire placement in one pass, with decreased fluoroscopic time and with accuracy and precision better than conventional technique.

DESIGN

Prospective, randomized trials.

SETTING

Laboratory.

PARTICIPANTS

Thirty, Sawbone, femur phantoms.

INTERVENTION

Dynamic hip screw guidewires were inserted into 15 femur phantoms under fluoroscopic guidance by using computer-assisted fluoroscopic ORIF technique, and 15 femurs were inserted by using a conventional fluoroscopic-assisted ORIF technique.

MAIN OUTCOME MEASUREMENTS

Ideal guidewire placement was defined as the center of the femoral head, 5 mm from the apical bone edge on anteroposterior and lateral views. Accuracy was measured as distance to ideal placement, and the number of passes and fluoroscopic time were noted for each trial.

RESULTS

The computer-assisted technique achieved an average guidewire placement that was as accurate as the conventional technique in fewer passes, 1.1 +/- 0.2 (mean +/- standard deviation) compared with 2.4 +/- 1.1 (P < 0.0001), respectively, and with fewer fluoroscopic images, 2 +/- 0 compared with 13.5 +/- 3 (P < 0.0002), respectively. Guidewire placement in both groups was within the tip-apex distance defined by Baumgaertner et al.

CONCLUSIONS

The computer-assisted technique was significantly more accurate and precise than conventional technique. It also required fewer drill tracks through the femur and exposed the patient and the surgical team to significantly less ionizing radiation.

An estimation/correction algorithm for detecting bone edges in CT images

The normal direction of the bone contour in computed tomography (CT) images provides important anatomical information and can guide segmentation algorithms. Since various bones in CT images have different sizes, and the intensity values of bone pixels are generally nonuniform and noisy, estimation of the normal direction using a single scale is not reliable. We propose a multiscale approach to estimate the normal direction of bone edges. The reliability of the estimation is calculated from the estimated results and, after re-scaling, the reliability is used to further correct the normal direction. The optimal scale at each point is obtained while estimating the normal direction; this scale is then used in a simple edge detector. Our experimental results have shown that use of this estimated/corrected normal direction improves the segmentation quality by decreasing the number of unexpected edges and discontinuities (gaps) of real contours. The corrected normal direction could also be used in postprocessing to delete false edges. Our segmentation algorithm is automatic, and its performance is evaluated on CT images of the human pelvis, leg, and wrist.

Magnetic resonance imaging for in vivo assessment of three-dimensional patellar tracking

We have developed a non-invasive measurement technique which can ultimately be used to quantify three-dimensional patellar kinematics of human subjects for a range of static positions of loaded flexion and assessed its accuracy. Knee models obtained by segmenting and reconstructing one high-resolution scan of the knee were registered to bone outlines obtained by segmenting fast, low-resolution scans of the knee in static loaded flexion. We compared patellar tracking measurements made using the new method to measurements made using Roentgen stereophotogrammetric analysis in three cadaver knee specimens loaded through a range of flexion in a test rig. The error in patellar spin and tilt measurements was less than 1.02 degrees and the error in lateral patellar shift was 0.88 mm. Sagittal plane scans provided more accurate final measurements of patellar spin and tilt, whereas axial plane scans provided more accurate measurements of lateral translation and patellar flexion. Halving the number of slices did not increase measurement error significantly, which suggests that scan times can be reduced without reducing accuracy significantly. The method is particularly useful for multiple measurements on the same subject because the high-resolution bone-models need only be created once; thus, the potential variability in coordinate axes assignment and model segmentation during subsequent measurements is removed.

A point-selection algorithm based on spatial-stiffness analysis of rigid registration

OBJECTIVE

We propose a model of shape-based registration that leads to a task-specific algorithm for preoperatively selecting a set of model registration points.

MATERIALS AND METHODS

We performed five sets of computer simulations using registration points generated by our algorithm and two noise amplification index (NAI) algorithms on the basis of the research of Simon 20. We used several different bone surface models (distal radius, proximal femur and tibia) computed from CT images of patient volunteers. The number of registration points used varied between 6 and 30.

RESULTS

Our algorithm was faster than the NAI-based algorithms by factors of approximately 4 and 200. It had equal or better performance in terms of target registration error (TRE) when compared with the other algorithms. Our simulations also showed that point selection can have a large effect on TRE behavior; in particular, poor point selection does not necessarily decrease TRE as more registration points are added.

CONCLUSIONS

Our point-selection algorithm produces model registration points with similar or better TRE behavior than the NAI-based algorithms we tested, and it does so with significantly less computation time.

Computer-assisted deformity correction using the ilizarov method

The Taylor spatial frame is a fixation device used to implement the Ilizarov method of bone deformity correction to gradually distract an osteotomized bone at regular intervals, according to a prescribed schedule. We improve the accuracy of Ilizarov's method of osteogenesis by preoperatively planning the correction, intraoperatively measuring the location of the frame relative to the patient, and computing the final shape of the frame. In four of five tibial phantom experiments, we were able to achieve correction errors of less than 2 degrees of total rotation. We also demonstrate how registration uncertainty can be propagated through the planned transformation to visualize the range of possible correction outcomes. Our method is an improvement over an existing computer-assisted technique (Iyun et al.) in that the surgeon has the same flexibility as in the conventional technique when fixating the frame to the patient.

A phylogeny of caenorhabditis reveals frequent loss of introns during nematode evolution

Since introns were discovered 26 years ago, people have wondered how changes in intron/exon structure occur, and what role these changes play in evolution. To answer these questions, we have begun studying gene structure in nematodes related to Caenorhabditis elegans. As a first step, we cloned a set of five genes from six different Caenorhabditis species, and used their amino acid sequences to construct the first detailed phylogeny of this genus. Our data indicate that nematode introns are lost at a very high rate during evolution, almost 400-fold higher than in mammals. These losses do not occur randomly, but instead, favor some introns and do not affect others. In contrast, intron gains are far less common than losses in these genes. On the basis of the sequences at each intron site, we suggest that several distinct mechanisms can cause introns to be lost. The small size of C. elegans introns should increase the rate at which each of these types of loss can occur, and might account for the dramatic difference in loss rate between nematodes and mammals.

Accurate assessment of patellar tracking using fiducial and intensity-based fluoroscopic techniques

Accuracies of a point-based and an intensity-based fluoroscopic methods of assessing patella tracking were determined by comparing the pattern of patellar motion with respect to orientation (flexion, internal rotation, and lateral tilt) and translation (lateral, proximal, and anterior) with the pattern of patellar motion measured using Roentgen stereophotogrammetric analysis in three cadaver knee specimens. Each pose in the patellar motion could be obtained from single as well as multiple calibrated fluoroscopic images. The errors using the intensity-based method were slightly higher than those of the point-based method, but they appear to be sufficiently low to detect clinically significant differences in patellar kinematics.

Oxygen sensation and social feeding mediated by a C. elegans guanylate cyclase homologue

Specialized oxygen-sensing cells in the nervous system generate rapid behavioural responses to oxygen. We show here that the nematode Caenorhabditis elegans exhibits a strong behavioural preference for 5-12% oxygen, avoiding higher and lower oxygen levels. 3',5'-cyclic guanosine monophosphate (cGMP) is a common second messenger in sensory transduction and is implicated in oxygen sensation. Avoidance of high oxygen levels by C. elegans requires the sensory cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase homologue, gcy-35. The GCY-35 haem domain binds molecular oxygen, unlike the haem domains of classical nitric-oxide-regulated guanylate cyclases. GCY-35 and TAX-4 mediate oxygen sensation in four sensory neurons that control a naturally polymorphic social feeding behaviour in C. elegans. Social feeding and related behaviours occur only when oxygen exceeds C. elegans' preferred level, and require gcy-35 activity. Our results suggest that GCY-35 is regulated by molecular oxygen, and that social feeding can be a behavioural strategy for responding to hyperoxic environments.

Robust registration for computer-integrated orthopedic surgery: laboratory validation and clinical experience

In order to provide navigational guidance during computer-integrated orthopedic surgery, the anatomy of the patient must first be registered to a medical image or model. A common registration approach is to digitize points from the surface of a bone and then find the rigid transformation that best matches the points to the model by constrained optimization. Many optimization criteria, including a least-squares objective function, perform poorly if the data include spurious data points (outliers). This paper describes a statistically robust, surface-based registration algorithm that we have developed for orthopedic surgery. To find an initial estimate, the user digitizes points from predefined regions of bone that are large enough to reliably locate even in the absence of anatomic landmarks. Outliers are automatically detected and managed by integrating a statistically robust M-estimator with the iterative-closest-point algorithm. Our in vitro validation method simulated the registration process by drawing registration data points from several sets of densely digitized surface points. The method has been used clinically in computer-integrated surgery for high tibial osteotomy, distal radius osteotomy, and excision of osteoid osteoma.

Turning clustering loops: sex determination in Caenorhabditis elegans

The nematode Caenorhabditis elegans has two sexes: males and hermaphrodites. Hermaphrodites are essentially female animals that produce sperm and oocytes. In the past few years tremendous progress has been made towards understanding how sexual identity is controlled in the worm. These analyses have revealed that the regulatory pathway controlling sexual development is far from linear and that it contains a number of loops and branches that play crucial roles in regulating sexual development. This review summarizes our current understanding of the mechanisms that regulate sexual cell fate in C. elegans.