Polycomb-mediated repression of paternal chromosomes maintains haploid dosage in diploid embryos of Marchantia.

Complex mechanisms regulate gene dosage throughout eukaryotic life cycles. Mechanisms controlling gene dosage have been extensively studied in animals, however it is unknown how generalizable these mechanisms are to diverse eukaryotes. Here, we use the haploid plant Marchantia polymorpha to assess gene dosage control in its short-lived diploid embryo. We show that throughout embryogenesis, paternal chromosomes are repressed resulting in functional haploidy. The paternal genome is targeted for genomic imprinting by the Polycomb mark H3K27me3 starting at fertilization, rendering the maternal genome in control of embryogenesis. Maintaining haploid gene dosage by this new form of imprinting is essential for embryonic development. Our findings illustrate how haploid-dominant species can regulate gene dosage through paternal chromosome inactivation and initiates the exploration of the link between life cycle history and gene dosage in a broader range of organisms.

The reproductive cells of organisms that reproduce sexually ­ the egg and the sperm ­ each contain one copy of the organism's genome. An embryo forms upon fertilization of an egg by a sperm cell. This embryo contains two copies of the genome, one from each parent. Under most circumstances, it does not matter which parent a gene copy came from: both gene copies are expressed. However, in some species genes coming from only one of the parents are switched on. This unusual mode of gene expression is called genomic imprinting. The best-known example of this occurs in female mammals, which repress the genes on the paternal X chromosome. Genomic imprinting also exists in flowering plants. Both mammals and flowering plants evolved tissues that channel nutrients from the mother to the embryo during development; the placenta and the endosperm, respectively. Genomic imprinting had, until now, only been described in these two types of organisms. It was unknown whether imprinting also happens in other organisms, and specifically those in which embryos develop inside the mother but without the help of a placenta or endosperm. Here Montgomery et al. addressed this question by studying the liverwort, Marchantia polymorpha, a moss-like plant. Initial experiments showed that cells in the liverwort embryo mostly expressed the genes coming from the egg, and not the sperm. All the genetic material coming from the sperm had a molecular marker or tag called H3K27me3. This mark, which also appears on the paternal X chromosome in female mammals, switches off the genes it tags. M. polymorpha embryos thus suppress gene expression from all of the genetic material from the father, relying only on maternal genetic material for development. When Montgomery et al. deleted the maternal genes necessary for making the H3K27me3 mark, the paternal genes switched on, and this led to the death of the embryos. The survival of M. polymorpha embryos therefore depended on keeping only one set of genes active. Taken together these experiments indicate that genomic imprinting evolved about 480 million years ago, about 320 million years earlier than previously thought, in organisms for which embryo development depended only on one parent. This means there are likely many more organisms that control gene expression in this way, opening up opportunities for further research. Understanding imprinting in more detail will also shed light on how sexual reproduction evolved.

New hybrid reformations of peripheral CT angiography: do we still need axial images?

PURPOSE: To quantify the detectability of peripheral artery stenosis on hybrid CT angiography (CTA) reformations. METHODS: Hybrid reformations were developed by combining multipath curved planar reformations (mpCPR) and maximum intensity projections (MIP). Fifty peripheral CTAs were evaluated twice: either with MIP, mpCPR and axial images or with hybrid reformations only. Digital subtraction angiography served as gold standard. RESULTS: Using hybrid reformations, two independent readers detected 88.0% and 81.3% of significant stenosis, respectively. However, CTA including axial images detected statistically significant more lesions (98%). CONCLUSION: Peripheral CTA reading including axial images is still recommended. Further improvement of these hybrid reformations is necessary.

Enhanced voxelization and representation of objects with sharp details in truncated distance fields.

This paper presents a new method for voxelization of solid objects containing sharp details. Voxelization is a sampling process that transforms a continuously defined object into a discrete one represented as a voxel field. The voxel field can be used for rendering or other purposes, which often involve a reconstruction of a continuous approximation of the original object. Objects to be voxelized need to fulfill certain representability conditions; otherwise, disturbing artifacts appear during reconstruction. The method proposed here extends the traditional distance-based voxelization by an a-priori detection of sharp object details and their subsequent modification in such a way that the resulting object to be voxelized fulfills the representability conditions. The resulting discrete objects are represented by means of truncated (i.e., narrow-band) distance fields, which provide reduction of memory requirements and further processing by level set techniques. This approach is exemplified by two classes of solid objects that normally contain such sharp details: implicit solids and solids resulting from CSG operations. In both cases, the sharp details are rounded to a specific curvature dictated by the sampling distance.

A cell architecture modeling system based on quantitative ultrastructural characteristics.

The architecture of living cells is difficult to describe and communicate; therefore, realistic computer models may help their understanding. 3D models should correspond both to qualitative and quantitative experimental data and therefore should include specific authoring tools such as appropriate visualization and stereological measures. For this purpose we have developed a problem solving environment for stereology-based modeling (PSE-SBM), which is an automated system for quantitative modeling of cell architecture. The PSE-SBM meets the requirement to produce models that correspond in stereological and morphologic terms to real cells and their organelles. Instead of using standard interactive graphing tools, our approach relies on functional modeling. We have built a system of implicit functions and set operations, organized in a hierarchical tree structure, which describes individual cell organelles and their 3D relations. Natural variability of size, shape, and position of organelles is achieved by random variation of the specific parameters within given limits. The resulting model is materialized by evaluation of these functions and is adjusted for a given set of specific parameters defined by the user. These principles are explained in detail, and modeling of segments of a muscle cell is used as an example to demonstrate the potential of the PSE-SBM for communication of architectural concepts and testing of structural hypotheses.

Multipath curved planar reformation of the peripheral arterial tree in CT angiography.

The study was approved by the institutional review board, and informed consent was obtained. The purpose of the study was to prospectively quantify the angular visibility range, determine the existence of orthogonal viewing pairs, and characterize the conditions that cause artifacts in multipath curved planar reformations (MPCPRs) of the peripheral arterial tree in 10 patients (eight men and two women; mean age, 69 years; range, 54-80 years) with peripheral arterial occlusive disease. Percentage of segments with the maximal possible visibility score of 1 was significantly greater (odds ratio, 1.42; P<.001) for MPCPRs than for maximum intensity projections. One or more orthogonal viewing pairs were identified for all above-knee arterial segments, and artifactual vessel distortion was observed when the vessel axis approached a horizontal course in MPCPRs.

3D distance fields: a survey of techniques and applications.

A distance field is a representation where, at each point within the field, we know the distance from that point to the closest point on any object within the domain. In addition to distance, other properties may be derived from the distance field, such as the direction to the surface, and when the distance field is signed, we may also determine if the point is internal or external to objects within the domain. The distance field has been found to be a useful construction within the areas of computer vision, physics, and computer graphics. This paper serves as an exposition of methods for the production of distance fields, and a review of alternative representations and applications of distance fields. In the course of this paper, we present various methods from all three of the above areas, and we answer pertinent questions such as How accurate are these methods compared to each other? How simple are they to implement?, and What is the complexity and runtime of such methods?

Magnetic resonance imaging of trabecular and cortical bone in mice: comparison of high resolution in vivo and ex vivo MR images with corresponding histology.

Measurements of bone morphometry and remodeling have been shown to reflect bone strength and can be used to diagnose degenerative bone disease. In this study, in vivo and ex vivo magnetic resonance imaging (MRI) techniques to assess trabecular and cortical bone properties have been compared to each other and to histology as a novel means for the quantification of bone. Femurs of C57Bl/6 mice were examined both in vivo and ex vivo on an 11.7 T MRI scanner, followed by histologic processing and morphometry. A thresholding analysis technique was applied to the MRI images to generate contour lines and to delineate the boundaries between bone and marrow. Using MRI, an optimal correlation with histology was obtained with an in vivo longitudinal sectioned short echo time gradient-echo versus an in vivo long echo time spin-echo sequence or an ex vivo pulse sequence. Gradient-echo images were acquired with a maximum in-plane resolution of 35 microm. Our results demonstrated that in both the in vivo and ex vivo data sets, the percent area of marrow increases and percent area of trabecular bone and cortical bone thickness decreases moving from the epiphyseal growth plate to the diaphysis. These changes, observed with MRI, correlate with the histological data. Investigations using in vivo MRI gradient-echo sequences consistently gave the best correlation with histology. Our quantitative evaluation using both ex vivo and in vivo MRI was found to be an effective means to visualize non-invasively the normal variation in trabecular and cortical bone as compared to a histological "gold standard" The experiments validated in vivo MRI as a potential high resolution technique for investigating both soft tissue, such as marrow, and bone without radiation exposure.

Single point imaging with suppressed sound pressure levels through gradient-shape adjustment.

Acoustic noise produced during single point imaging (SPI) experiments was modulated by changes in the spatial encoding gradients. Parameters of both linear and sine-shaped gradient ramps were modified to minimize the acoustic noise levels. Acoustic noise measurements during SPI were measured on three different gradient systems and revealed that for small gradient-bore systems a considerable acoustic noise reduction of more than 20 dB can easily be achieved. SPI in conjunction with an optimized gradient waveform can be a superb alternative to the previously introduced single point ramped imaging with T(1) enhancement (SPRITE) method when sound levels and overheating of gradients are a concern.

Magnetic resonance imaging of seeds by use of single point acquisition.

In general, magnetic resonance imaging (MRI) is used to obtain a spatial representation of the water distribution in an object. Water in soft materials (living matter) often shows a high degree of translational mobility, giving rise to relatively long magnetic relaxation times. This allows the use of conventional MRI techniques such as the spin-echo, to acquire an image. However, when hydration levels become low, water becomes less mobile, resulting in much shorter magnetic relaxation times and a corresponding signal loss. To avoid problems arising from rapid decaying signals, we investigated the use of single point imaging (SPI) in the study of seeds. We were able to obtain SPI images of nonimbibed and imbibed seeds. Using SPI with shaped gradients significantly reduced the acoustic noise level.