A perivascular origin for mesenchymal stem cells in multiple human organs

Mesenchymal stem cells (MSCs), the archetypal multipotent progenitor cells derived in cultures of developed organs, are of unknown identity and native distribution. We have prospectively identified perivascular cells, principally pericytes, in multiple human organs including skeletal muscle, pancreas, adipose tissue, and placenta, on CD146, NG2, and PDGF-Rbeta expression and absence of hematopoietic, endothelial, and myogenic cell markers. Perivascular cells purified from skeletal muscle or nonmuscle tissues were myogenic in culture and in vivo. Irrespective of their tissue origin, long-term cultured perivascular cells retained myogenicity; exhibited at the clonal level osteogenic, chondrogenic, and adipogenic potentials; expressed MSC markers; and migrated in a culture model of chemotaxis. Expression of MSC markers was also detected at the surface of native, noncultured perivascular cells. Thus, blood vessel walls harbor a reserve of progenitor cells that may be integral to the origin of the elusive MSCs and other related adult stem cells.

Complete genome sequence of the model actinomycete Streptomyces coelicolor A3(2)

Streptomyces coelicolor is a representative of the group of soil-dwelling, filamentous bacteria responsible for producing most natural antibiotics used in human and veterinary medicine. Here we report the 8,667,507 base pair linear chromosome of this organism, containing the largest number of genes so far discovered in a bacterium. The 7,825 predicted genes include more than 20 clusters coding for known or predicted secondary metabolites. The genome contains an unprecedented proportion of regulatory genes, predominantly those likely to be involved in responses to external stimuli and stresses, and many duplicated gene sets that may represent 'tissue-specific' isoforms operating in different phases of colonial development, a unique situation for a bacterium. An ancient synteny was revealed between the central 'core' of the chromosome and the whole chromosome of pathogens Mycobacterium tuberculosis and Corynebacterium diphtheriae. The genome sequence will greatly increase our understanding of microbial life in the soil as well as aiding the generation of new drug candidates by genetic engineering.

α-Synuclein impairs macroautophagy: implications for Parkinson's disease

Parkinson's disease (PD) is characterized pathologically by intraneuronal inclusions called Lewy bodies, largely comprised of α-synuclein. Multiplication of the α-synuclein gene locus increases α-synuclein expression and causes PD. Thus, overexpression of wild-type α-synuclein is toxic. In this study, we demonstrate that α-synuclein overexpression impairs macroautophagy in mammalian cells and in transgenic mice. Our data show that α-synuclein compromises autophagy via Rab1a inhibition and Rab1a overexpression rescues the autophagy defect caused by α-synuclein. Inhibition of autophagy by α-synuclein overexpression or Rab1a knockdown causes mislocalization of the autophagy protein, Atg9, and decreases omegasome formation. Rab1a, α-synuclein, and Atg9 all regulate formation of the omegasome, which marks autophagosome precursors.

Cancer potential in liver, lung, bladder and kidney due to ingested inorganic arsenic in drinking water

In order to compare risk of various internal organ cancers induced by ingested inorganic arsenic and to assess the differences in risk between males and females, cancer potency indices were calculated using mortality rates among residents in an endemic area of chronic arsenicism on the southwest coast of Taiwan, and the Armitage-Doll multistage model. Based on a total of 898,806 person-years as well as 202 liver cancer, 304 lung cancer, 202 bladder cancer and 64 kidney cancer deaths, a significant dose-response relationship was observed between arsenic level in drinking water and mortality of the cancers. The potency index of developing cancer of the liver, lung, bladder and kidney due to an intake of 10 micrograms kg day of arsenic was estimated as 4.3 x 10(-3), 1.2 x 10(-2), 1.2 x 10(-2), and 4.2 x 10(-3), respectively, for males; as well as 3.6 x 10(-3), 1.3 x 10(-2), 1.7 x 10(-2), and 4.8 x 10(-3), respectively, for females in the study area. The multiplicity of inorganic arsenic-induced carcinogenicity without showing any organotropism deserves further investigation.

Trabeculectomy with simultaneous topical application of mitomycin-C in refractory glaucoma

From May 1981 to Feb. 1989 trabeculectomy with per-operative topical application of mitomycin C was performed on eyes with refractory glaucoma. All eyes had undergone prior one, two or more than two antiglaucoma surgeries without success for IOP control. Other risk factors in the present series of eyes included young age, aphakia, rubeosis and glaucoma secondary to trauma or associated with anterior uveitis. 59 eyes underwent trabeculectomy with mitomycin application. 14 eyes were lost. 45 eyes of 42 cases, 33 eyes of 31 males and 12 eyes of 11 females were followed for a period of 1 to 8 years, an average 3.0 +/- 1.9 years. The success for IOP control less than 21 mmHg was 77.8%, 35/45 eyes. The success rates were 92%, 11/12 eyes in females and 73%, 24/33 eyes in males. Complications reported in 5-fluorouracil studies such as corneal epithelial toxicity and delayed healing of conjunctival wound were not encountered. The histo-pathological changes of wound healing after trabeculectomy with topical mitomycin application was discussed. In the healing process success to achieve filtration is more likely by pharmacolocally interferring with earlier steps in the process. We recommend the use of per-operative topical application of mitomycin C during trabeculectomy in eyes with uncontrolled glaucoma and poor prognosis, specifically after previous cataract extraction or unsuccessful antiglaucoma surgery.

The tunica adventitia of human arteries and veins as a source of mesenchymal stem cells

We previously demonstrated that human pericytes, which encircle capillaries and microvessels, give rise in culture to genuine mesenchymal stem cells (MSCs). This raised the question as to whether all MSC are derived from pericytes. Pericytes and other cells defined on differential expression of CD34, CD31, and CD146 were sorted from the stromal vascular fraction of human white adipose tissue. Besides pericytes, CD34+ CD31- CD146- CD45- cells, which reside in the outmost layer of blood vessels, the tunica adventitia, natively expressed MSC markers and gave rise in culture to clonogenic multipotent progenitors identical to standard bone marrow-derived MSC. Despite common MSC features and developmental properties, adventitial cells and pericytes retain distinct phenotypes and genotypes through culture. However, in the presence of growth factors involved in vascular remodeling, adventitial cells acquire a pericytes-like phenotype. In conclusion, we demonstrate the co-existence of 2 separate perivascular MSC progenitors: pericytes in capillaries and microvessels and adventitial cells around larger vessels.

Perivascular ancestors of adult multipotent stem cells

Independent studies by numerous investigators have shown that it is possible to harvest multipotent progenitor cells from diverse dissociated and cultured fetal, perinatal, and principally adult developed tissues. Despite the increasingly recognized medical value of these progenitor cells, the archetype of which remains the mesenchymal stem cell, this indirect extraction method has precluded the understanding of their native identity, tissue distribution, and frequency. Consistent with other researchers, we have hypothesized that blood vessels in virtually all organs harbor ubiquitous stem cells. We have identified, marked, and sorted to homogeneity by flow cytometry endothelial and perivascular cells in a large selection of human fetal, perinatal, and adult organs. Perivascular cells, including pericytes in the smallest blood vessels and adventitial cells around larger ones, natively express mesenchymal stem cell markers and produce in culture a long-lasting progeny of multilineage mesodermal progenitor cells. Herein, we review results from our and other laboratories that suggest a perivascular origin for mesenchymal stem cells and other adult progenitor cells. Recent experiments illustrate the therapeutic potential of human pericytes to regenerate skeletal muscle and promote functional recovery in the diseased heart and kidney.

The chromosomal DNA of Streptomyces lividans 66 is linear

Two copies of a DNA sequence similar or identical to one end of the linear plasmid SLP2 were found on the Streptomyces lividans chromosome. Restriction mapping showed that these sequences represented free ends. Electrophoretic retardation and glass-binding studies indicated that the telomeres carry covalently bound proteins. Moreover, the chromosome migrated as an 8 Mb linear DNA in pulsed-field gel electrophoresis. A similar finding with the chromosomes of six other Streptomyces species suggested that a linear chromosome may be characteristic of the genus. The S. lividans chromosome can be circularized by joining the two ends by artificial targeted recombination or by spontaneous deletions spanning both telomeres. Thus the chromosome appears to be able to exist, in viable bacteria, as a linear or a circular molecule.

Human pericytes for ischemic heart repair

Human microvascular pericytes (CD146(+)/34(-)/45(-)/56(-)) contain multipotent precursors and repair/regenerate defective tissues, notably skeletal muscle. However, their ability to repair the ischemic heart remains unknown. We investigated the therapeutic potential of human pericytes, purified from skeletal muscle, for treating ischemic heart disease and mediating associated repair mechanisms in mice. Echocardiography revealed that pericyte transplantation attenuated left ventricular dilatation and significantly improved cardiac contractility, superior to CD56+ myogenic progenitor transplantation, in acutely infarcted mouse hearts. Pericyte treatment substantially reduced myocardial fibrosis and significantly diminished infiltration of host inflammatory cells at the infarct site. Hypoxic pericyte-conditioned medium suppressed murine fibroblast proliferation and inhibited macrophage proliferation in vitro. High expression by pericytes of immunoregulatory molecules, including interleukin-6, leukemia inhibitory factor, cyclooxygenase-2, and heme oxygenase-1, was sustained under hypoxia, except for monocyte chemotactic protein-1. Host angiogenesis was significantly increased. Pericytes supported microvascular structures in vivo and formed capillary-like networks with/without endothelial cells in three-dimensional cocultures. Under hypoxia, pericytes dramatically increased expression of vascular endothelial growth factor-A, platelet-derived growth factor-β, transforming growth factor-β1 and corresponding receptors while expression of basic fibroblast growth factor, hepatocyte growth factor, epidermal growth factor, and angiopoietin-1 was repressed. The capacity of pericytes to differentiate into and/or fuse with cardiac cells was revealed by green fluorescence protein labeling, although to a minor extent. In conclusion, intramyocardial transplantation of purified human pericytes promotes functional and structural recovery, attributable to multiple mechanisms involving paracrine effects and cellular interactions.

Biochemical and molecular changes at the cellular level in response to exposure to environmental estrogen-like chemicals

Estrogen-like chemicals are unique compared to nonestrogenic xenobiotics, because in addition to their chemical properties, the estrogenic property of these compounds allows them to act like sex hormones. Whether weak or strong, the estrogenic response of a chemical, if not overcome, will add extra estrogenic burden to the system. At elevated doses, natural estrogens and environmental estrogen-like chemicals are known to produce adverse effects. The source of extra or elevated concentration of estrogen could be either endogenous or exogenous. The potential of exposure for humans and animals to environmental estrogen-like chemicals is high. Only a limited number of estrogen-like compounds, such as diethylstilbestrol (DES), bisphenol A, nonylphenol, polychlorinated biphenyls (PCBs), and dichlorodiphenyltrichloroethane (DDT), have been used to assess the biochemical and molecular changes at the cellular level. Among them, DES is the most extensively studied estrogen-like chemical, and therefore this article is focused mainly on DES-related observations. In addition to estrogenic effects, environmental estrogen-like chemicals produce multiple and multitype genetic and/or nongenetic hits. Exposure of Syrian hamsters to stilbene estrogen (DES) produces several changes in the nuclei of target organ for carcinogenesis (kidney): (1) Products of nuclear redox reactions of DES modify transcription regulating proteins and DNA; (2) transcription is inhibited; (3) tyrosine phosphorylation of nuclear proteins, including RNA polymerase II, p53, and nuclear insulin-like growth factor-1 receptor, is altered; and (4) DNA repair gene DNA polymerase beta transcripts are decreased and mutated. Exposure of Noble rats to DES also produces several changes in the mammary gland: proliferative activity is drastically altered; the cell cycle of mammary epithelial cells is perturbed; telomeric length is attenuated; etc. It appears that some other estrogenic compounds, such as bisphenol A and nonylphenol, may also follow a similar pattern of effects to DES, because we have recently shown that these compounds alter cell cycle kinetics, produce telomeric associations, and produce chromosomal aberrations. Like DES, bisphenol A after metabolic activation is capable of binding to DNA. However, it should be noted that a particular or multitype hit(s) will depend upon the nature of the environmental estrogen-like chemical. The role of individual attack leading to a particular change is not clear at this stage. Consequences of these multitypes of attack on the nuclei of cells could be (1) nuclear toxicity/cell death; (2) repair of all the hits and then acting as normal cells; or (3) sustaining most of the hits and acting as unstable cells. Proliferation of the last type of cell is expected to result in transformed cells.

Nuclear magnetic resonance and neutron diffraction studies of the complex of ribonuclease A with uridine vanadate, a transition-state analogue

The complex of ribonuclease A (RNase A) with uridine vanadate (U-V), a transition-state analogue, has been studied with 51V and proton NMR spectroscopy in solution and by neutron diffraction in the crystalline state. Upon the addition of aliquots of U-V at pH 6.6, the C epsilon-H resonances of the two active-site histidine residues 119 and 12 decrease in intensity while four new resonances appear. Above pH 8 and below pH 5, these four resonances decrease in intensity as the complex dissociates. These four resonances are assigned to His-119 and His-12 in protonated and unprotonated forms in the RNase-U-V complex. These resonances do not titrate or change in relative area in the pH range 5-8, indicating a slow protonation process, and the extent of protonation remains constant with ca. 58% of His-12 and ca. 26% of His-119 being protonated. The results of diffraction studies show that both His-12 and His-119 occupy well-defined positions in the RNase-U-V complex and that both are protonated. However, while the classic interpretation of the mechanism of action of RNase based on the proposal of Findlay et al. [Findlay, D., Herries, D. G., Mathias, A. P., Rabin, B. R., & Ross, C. A. (1962) Biochem. J. 85, 152-153] requires both His-12 and His-119 to be in axial positions relative to the pentacoordinate transition state, in the diffraction structure His-12 is found to be in an equatorial position, while Lys-41 is close to an axial position. Hydrogen exchange data show that the mobility and accessibility of amides in the RNase-U-V complex do not significantly differ from what was observed in the native enzyme. The results of both proton NMR in solution and neutron diffraction in the crystal are compared and interpreted in terms of the mechanism of action of RNase.

Rapidly fatal outcome of bacteremic Klebsiella pneumoniae pneumonia in alcoholics

Twenty-eight alcoholic patients with community-acquired pneumonia who were admitted to a referral medical center during a 3-year period were reviewed. All were men and with few exceptions were heavy smokers. The overall mortality was 64.3%. The most dramatic group was that of 11 patients (39.3%) with bacteremic Klebsiella pneumoniae pneumonia (BKPP), which had high mortality rate (100%), short onset of illness before hospital admission (42.6 +/- 8.2 h, mean +/- SD), and short survival time after the admission (24.6 +/- 7.9 h). All these 11 patients needed intensive care unit (ICU) management and ventilatory support. ARterial blood gas values showed marked hypoxemia and metabolic acidosis in most of these 11 patients, and presence of shock at arrival in the hospital was noted in 8 patients. Acute renal failure and disseminated intravascular coagulation developed in six patients. Chest radiographs showed pleural effusion and radiographic spread in nearly 50% of patients. Combination chemotherapy consisting of one aminoglycoside and one second- or third-generation cephalosporin was initiated in all patients. The rapidly fatal outcome of these 11 patients with BKPP despite management with adequate antibiotics and intensive care indicated the fulminant nature of this disease. High virulence of the microorganism, altered immune response, and increased susceptibility to infection may all have contributed to the fulminancy in this group of patients. The disease continues to present many difficulties in management.

Paramagnetic metalloporphyrins as potential contrast agents in NMR imaging

Several water-soluble paramagnetic metalloporphyrins were found to increase significantly the relaxation rate (1/T1) of water. Such compounds are known to be selectively retained by tumors in animals, and consequently we identify them as potential contrast agents for tumors in NMR imaging.

Two-dimensional phase unwrapping with use of statistical models for cost functions in nonlinear optimization

Interferometric radar techniques often necessitate two-dimensional (2-D) phase unwrapping, defined here as the estimation of unambiguous phase data from a 2-D array known only modulo 2pi rad. We develop a maximum a posteriori probability (MAP) estimation approach for this problem, and we derive an algorithm that approximately maximizes the conditional probability of its phase-unwrapped solution given observable quantities such as wrapped phase, image intensity, and interferogram coherence. Examining topographic and differential interferometry separately, we derive simple, working models for the joint statistics of the estimated and the observed signals. We use generalized, nonlinear cost functions to reflect these probability relationships, and we employ nonlinear network-flow techniques to approximate MAP solutions. We apply our algorithm both to a topographic interferogram exhibiting rough terrain and layover and to a differential interferogram measuring the deformation from a large earthquake. The MAP solutions are complete and are more accurate than those of other tested algorithms.

The telomeres of Streptomyces chromosomes contain conserved palindromic sequences with potential to form complex secondary structures

The chromosomes of the gram-positive soil bacteria Streptomyces are linear DNA molecules, usually of about 8Mb, containing a centrally located origin of replication and covalently bound terminal proteins (which are presumably involved in the completion of replication of the telomeres). The ends of the chromosomes contain inverted repeats of variable lengths. The terminal segments of five Streptomyces chromosomes and plasmids were cloned and sequenced. The sequences showed a high degree of conservation in the first 166-168bp. Beyond the terminal homology, the sequences diverged and did not generally cross-hybridize. The homologous regions contained seven palindromes with a few nucleotide differences. Many of these differences occur in complementary pairs, such that the palindromicity is preserved. Energy-optimized modelling predicted that the 3' strand of the terminal palindromes can form extensive hairpin structures that are similar to the 3' ends of autonomous parvovirus genomes. Most of the putative hairpins have a GCGCAGC sequence at the loop, with the potential to form a stable single C-residue loop closed by a sheared G:A pairing. The similarity between the terminal structures of the Streptomyces replicons and the autonomous parvoviral genomes suggests that they may share some structural and/or replication features.

Oligomerization of ETO is obligatory for corepressor interaction

Nearly 40% of cases of acute myelogenous leukemia (AML) of the M2 subtype are due to a chromosomal translocation that combines a sequence-specific DNA binding protein, AML1, with a potent transcriptional repressor, ETO. ETO interacts with nuclear receptor corepressors SMRT and N-CoR, which recruit histone deacetylase to the AML1-ETO oncoprotein. SMRT-N-CoR interaction requires each of two zinc fingers contained in C-terminal Nervy homology region 4 (NHR4) of ETO. However, here we show that polypeptides containing NHR4 are insufficient for interaction with SMRT. NHR2 is also required for SMRT interaction and repression by ETO, as well as for inhibition of hematopoietic differentiation by AML1-ETO. NHR2 mediates oligomerization of ETO as well as AML1-ETO. Fusion of NHR4 polypeptide to a heterologous dimerization domain allows strong interaction with SMRT in vitro. These data support a model in which NHR2 and NHR4 have complementary functions in repression by ETO. NHR2 functions as an oligomerization domain bringing together NHR4 polypeptides that together form the surface required for high-affinity interaction with corepressors. As nuclear receptors also interact with corepressors as dimers, oligomerization may be a common mechanism regulating corepressor interactions.

SCP1, a 356 023 bp linear plasmid adapted to the ecology and developmental biology of its host, Streptomyces coelicolor A3(2)

The sequencing of the entire genetic complement of Streptomyces coelicolor A3(2) has been completed with the determination of the 365,023 bp sequence of the linear plasmid SCP1. Remarkably, the functional distribution of SCP1 genes somewhat resembles that of the chromosome: predicted gene products/functions include ECF sigma factors, antibiotic biosynthesis, a gamma-butyrolactone signalling system, members of the actinomycete-specific Wbl class of regulatory proteins and 14 secreted proteins. Some of these genes are among the 18 that contain a TTA codon, making them targets for the developmentally important tRNA encoded by the bldA gene. RNA analysis and gene fusions showed that one of the TTA-containing genes is part of a large bldA-dependent operon, the gene products of which include three proteins isolated from the spore surface by detergent washing (SapC, D and E), and several probable metabolic enzymes. SCP1 shows much evidence of recombinational interactions with other replicons and transposable elements during its history. For example, it has two sets of partitioning genes (which may explain why an integrated copy of SCP1 partially suppressed the defective partitioning of a parAB-deleted chromosome during sporulation). SCP1 carries a cluster of probable transfer determinants and genes encoding likely DNA polymerase III subunits, but it lacks an obvious candidate gene for the terminal protein associated with its ends. This may be related to atypical features of its end sequences.

Placental perivascular cells for human muscle regeneration

Perivascular multipotent mesenchymal progenitors exist in a variety of tissues, including the placenta. Here, we suggest that the abundant vasculature present in the human placenta can serve as a source of myogenic cells to regenerate skeletal muscle. Chorionic villi dissected from the mid-gestation human placenta were first transplanted intact into the gastrocnemius muscles of SCID/mdx mice, where they participated in muscle regeneration by producing myofibers expressing human dystrophin and spectrin. In vitro-cultured placental villi released rapidly adhering and migratory CD146+CD34⁻CD45⁻CD56⁻ cells of putative perivascular origin that expressed mesenchymal stem cell markers. CD146+CD34⁻CD45⁻CD56⁻ perivascular cells isolated and purified from the placental villi by flow cytometry were indeed highly myogenic in culture, and generated dystrophin-positive myofibers, and they promoted angiogenesis after transplantation into SCID/mdx mouse muscles. These observations confirm the existence of mesenchymal progenitor cells within the walls of human blood vessels, and suggest that the richly vascularized human placenta is an abundant source of perivascular myogenic cells able to migrate within dystrophic muscle and regenerate myofibers.

Type I and II collagen regulation of chondrogenic differentiation by mesenchymal progenitor cells

Chondrogenic differentiation by mesenchymal progenitor cells (MPCs) is associated with cytokines such as transforming growth factor-beta 1 (TGF-beta1) and dexamethasone. Extracellular matrix (ECM) also regulates the differentiation by MPCs. To define whether ECM plays a functional role in regulation of the chondrogenic differentiation by MPCs, an in vitro model was used. That model exposed to dexamethasone, recombinant human TGF-beta1(rhTGF-beta1) and collagens. The results showed that MPCs incorporated with dexamethasone and rhTGF-beta1 increased proliferation and expression of glycosaminoglycan (GAG) after 14 days. Type II collagen enhanced the GAG synthesis, but did not increase alkaline phosphatase (ALP) activity. When adding dexamethasone and rhTGF-beta1 MPCs increased mRNA expression of Sox9. Incorporation with type II collagen, dexamethasone and rhTGF-beta1, MPCs induced mRNA expression of aggrecan and enhanced levels of type II collagen, and Sox9 mRNA. In contrast, incorporation with type I collagen, dexamethasone and rhTGF-beta1 MPCs reduced levels of aggrecan, and Sox9 mRNA, and showed no type II collagen mRNA. Altogether, these results indicate that type I and II collagen, in addition to the cytokine effect, may play a functional role in regulating of chondrogenic differentiation by MPCs.

Enhancement of cell growth in tissue-engineering constructs under direct perfusion: Modeling and simulation

Perfusion bioreactors improve mass transfer in cell-scaffold constructs. We developed a mathematical model to simulate nutrient flow through cellular constructs. Interactions among cell proliferation, nutrient consumption, and culture medium circulation were investigated. The model incorporated modified Contois cell-growth kinetics that includes effects of nutrient saturation and limited cell growth. Nutrient uptake was depicted through the Michaelis-Menton kinetics. To describe the culture medium convection, the fluid flow outside the cell-scaffold construct was described by the Navier-Stokes equations, while the fluid dynamics within the construct was modeled by Brinkman's equation for porous media flow. Effects of the media perfusion were examined by including time-dependant porosity and permeability changes due to cell growth. The overall cell volume was considered to consist of cells and extracellular matrices (ECM) as a whole without treating ECM separately. Numerical simulations show when cells were cultured subjected to direct perfusion, they penetrated to a greater extent into the scaffold and resulted in a more uniform spatial distribution. The cell amount was increased by perfusion and ultimately approached an asymptotic value as the perfusion rates increased in terms of the dimensionless Peclet number that accounts for the ratio of nutrient perfusion to diffusion. In addition to enhancing the nutrient delivery, perfusion simultaneously imposes flow-mediated shear stress to the engineered cells. Shear stresses were found to increase with cell growth as the scaffold void space was occupied by the cell and ECM volumes. The macro average stresses increased from 0.2 mPa to 1 mPa at a perfusion rate of 20 microm/s with the overall cell volume fraction growing from 0.4 to 0.7, which made the overall permeability value decrease from 1.35 x 10(-2)cm(2) to 5.51 x 10(-4)cm(2). Relating the simulation results with perfusion experiments in literature, the average shear stresses were below the critical value that would induce the chondrocyte necrosis.

Network approaches to two-dimensional phase unwrapping: intractability and two new algorithms

Two-dimensional (2-D) phase unwrapping, that is, deducing unambiguous phase values from a 2-D array of values known only modulo 2pi, is a key step in interpreting data acquired with synthetic aperture radar interferometry. Noting the recent network formulation of the phase unwrapping problem, we apply here some well-established ideas of network theory to formalize the problem, analyze its complexity, and derive algorithms for its solution. It has been suggested that the objective of phase unwrapping should be to minimize the total number of places where unwrapped and wrapped phase gradients differ. Here we use network constructions to show that this so-called minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly. Therefore we must instead find approximate solutions; we present two new algorithms for doing so. The first uses the network ideas of shortest paths and spanning trees to improve on the Goldstein et al. residue-cut algorithm [Radio Sci. 23, 713 (1988)]. Our improved algorithm is very fast, provides complete coverage, and allows user-defined weights. With our second algorithm, we extend the ideas of linear network flow problems to the nonlinear L0 case. This algorithm yields excellent approximations to the minimum L0 norm. Using interferometric data, we demonstrate that our algorithms are highly competitive with other existing algorithms in speed and accuracy, outperforming them in the cases presented here.

Complications and implications of linear bacterial chromosomes

The emergence of linear bacterial chromosomes has overthrown the dogma of universal circularity of the bacterial chromosomes, and posed mechanistic and evolutionary implications not previously anticipated.

Analysis of cell growth and diffusion in a scaffold for cartilage tissue engineering

Developments in tissue engineering over the past decade have offered promising future for the repair and reconstruction of damaged tissues. To regenerate three dimensional and weight-bearing implants, advances in biomaterials and manufacturing technologies prompted cell cultivations with natural or artificial scaffolds, in which cells are allowed to proliferate, migrate, and differentiate in vitro. In this article, we develop a mathematical model for cell growth in a porous scaffold. By treating the cell-scaffold construct as a porous medium, a continuum model is set up based on basic principles of mass conservation. In addition to cell growth kinetics, we incorporate cell diffusion in the model to describe the effects of cell random walks. Computational results are compared to experimental data found in the literature. With this model, we are able to investigate cell motility, heterogeneous cell distributions, and non-uniform seeding for tissue engineering applications. Results show that random walks tend to enhance uniform cell spreads in space, which in turn increases the probabilities for cells to acquire nutrients; therefore random walks are likely to be a positive contribution to the overall cell growth on scaffolds.