Delayed Accumulation of H3K27me3 on Nascent DNA Is Essential for Recruitment of Transcription Factors at Early Stages of Stem Cell Differentiation.

Recruitment of transcription factors (TFs) to repressed genes in euchromatin is essential to activate new transcriptional programs during cell differentiation. However, recruitment of all TFs, including pioneer factors, is impeded by condensed H3K27me3-containing chromatin. Single-cell and gene-specific analyses revealed that, during the first hours of induction of differentiation of mammalian embryonic stem cells (ESCs), accumulation of the repressive histone mark H3K27me3 is delayed after DNA replication, indicative of a decondensed chromatin structure in all regions of the replicating genome. This delay provides a critical "window of opportunity" for recruitment of lineage-specific TFs to DNA. Increasing the levels of post-replicative H3K27me3 or preventing S phase entry inhibited recruitment of new TFs to DNA and significantly blocked cell differentiation. These findings suggest that recruitment of lineage-specifying TFs occurs soon after replication and is facilitated by a decondensed chromatin structure. This insight may explain the developmental plasticity of stem cells and facilitate their exploitation for therapeutic purposes.

Notch ligand Dll4 impairs cell recruitment to aortic clusters and limits blood stem cell generation.

Hematopoietic stem cells (HSCs) develop from the hemogenic endothelium in cluster structures that protrude into the embryonic aortic lumen. Although much is known about the molecular characteristics of the developing hematopoietic cells, we lack a complete understanding of their origin and the three-dimensional organization of the niche. Here, we use advanced live imaging techniques of organotypic slice cultures, clonal analysis, and mathematical modeling to show the two-step process of intra-aortic hematopoietic cluster (IACH) formation. First, a hemogenic progenitor buds up from the endothelium and undergoes division forming the monoclonal core of the IAHC. Next, surrounding hemogenic cells are recruited into the IAHC, increasing their size and heterogeneity. We identified the Notch ligand Dll4 as a negative regulator of the recruitment phase of IAHC. Blocking of Dll4 promotes the entrance of new hemogenic Gfi1+ cells into the IAHC and increases the number of cells that acquire HSC activity. Mathematical modeling based on our data provides estimation of the cluster lifetime and the average recruitment time of hemogenic cells to the cluster under physiologic and Dll4-inhibited conditions.

The relevance of vascular adjustments to hemodynamic control in the face of temperature change in Crotalus durissus.

The presence of cardiac shunts in ectothermic tetrapods is thought to be consistent with active vascular modulations for proper hemodynamic support. Local control of blood flow modulates tissue perfusion and thus systemic conductance (Gsys) is assumed to increase with body temperature (Tb) to accommodate higher aerobic demand. However, the general increase of Gsys presses for a higher right-to-left (R-L) shunt, which reduces arterial oxygen concentration. In contrast, Tb reduction leads to a Gsys decrease and a left-to-right shunt, which purportedly increases pulmonary perfusion and plasma filtration in the respiratory area. This investigation addressed the role of compensatory vascular adjustments in the face of the metabolic alterations caused by Tb change in the South American rattlesnake (Crotalus durissus). Cardiovascular recordings were performed in decerebrated rattlesnake preparations at 10, 20 and 30°C. The rise in Tb increased metabolic demand, and correlated with an augmentation in heart rate. Although cardiac output increased, systemic stroke volume reduced while pulmonary stroke volume remained stable. Although that resulted in a proportionally higher increase in pulmonary blood flow, the R-L shunt was maintained. While the systemic compliance of large arteries was the most relevant factor in regulating arterial systemic blood pressure, peripheral conductance of pulmonary circulation was the major factor influencing the final cardiac shunt. Such dynamic adjustment of systemic compliance and pulmonary resistance for shunt modulation has not been demonstrated before and contrasts with previous knowledge on shunt control.

CNS macrophages differentially rely on an intronic Csf1r enhancer for their development.

The central nervous system hosts parenchymal macrophages, known as microglia, and non-parenchymal macrophages, collectively termed border-associated macrophages (BAMs). Microglia, but not BAMs, were reported to be absent in mice lacking a conserved Csf1r enhancer: the fms-intronic regulatory element (FIRE). However, it is unknown whether FIRE deficiency also impacts BAM arrival and/or maintenance. Here, we show that macrophages in the ventricular system of the brain, including Kolmer's epiplexus macrophages, are absent in Csf1rΔFIRE/ΔFIRE mice. Stromal choroid plexus BAMs are also considerably reduced. During normal development, we demonstrate that intracerebroventricular macrophages arrive from embryonic day 10.5, and can traverse ventricular walls in embryonic slice cultures. In Csf1rΔFIRE/ΔFIRE embryos, the arrival of both primitive microglia and intracerebroventricular macrophages was eliminated, whereas the arrival of cephalic mesenchyme and stromal choroid plexus BAMs was only partially restricted. Our results provide new insights into the development and regulation of different CNS macrophage populations.

Baroreflex responses of decerebrate rattlesnakes (Crotalus durissus) are comparable to awake animals.

A decerebrate rattlesnake, Crotalus durissus, has previously been used as a model Squamate for cardiovascular studies. It enabled instrumentation for concomitant recordings of diverse variables that showed autonomic responses. However, to validate the preparation and its scope for use, it is necessary to assess how close its cardiovascular variables are to non-decerebrate snakes and the effectiveness of its autonomic responses. Similarly, it is important to analyze its recovery profile after instrumentation and observe if it maintains stability throughout the duration of experimental protocol. Here we have objectively assessed these points by comparing decerebrate preparations and non-decerebrate snakes, after the occlusive cannulation of the vertebral artery. We have assessed cardiovascular variables and the baroreflex to analyze the presence, magnitude and stability of complex autonomic-controlled parameters as indicators of autonomic nervous system (ANS) functionality. After instrumentation, mean heart rates were high but recovered to stable values within 24 h. Mean arterial pressure stabilized within 24 h in control snakes and 48 h in decerebrate preparations. After that, both parameters remained stable. The operational gain and effectiveness index of the baroreflex recovered within the first 6 h after instrumentation in both experimental groups. In addition, the baroreflex capacities and its limits were also equivalent between the groups. These experiments demonstrated that decerebrate preparations and inactive, non-decerebrate snakes showed comparable recovery profiles following anesthesia and cannulation, maintained similar values of cardiovascular variables during experimental manipulation and exhibited functional, ANS modulated reflexes. Accordingly, the present results attest the relevance of this decerebrate preparation for studies on cardiovascular modulation.

A role for macrophages in hematopoiesis in the embryonic head.

Along with the aorta-gonad-mesonephros region, the head is a site of hematopoietic stem and progenitor cell (HS/PC) development in the mouse embryo. Macrophages are present in both these embryonic hemogenic sites, and recent studies indicate a functional interaction of macrophages with hematopoietic cells as they are generated in the aorta. Whereas brain macrophages or "microglia" are known to affect neuronal patterning and vascular circuitry in the embryonic brain, it is unknown whether macrophages play a role in head hematopoiesis. Here, we characterize head macrophages and examine whether they affect the HS/PC output of the hindbrain-branchial arch (HBA) region of the mouse embryo. We show that HBA macrophages are CD45+F4/80+CD11b+Gr1- and express the macrophage-specific Csf1r-GFP reporter. In the HBA of chemokine receptor-deficient (Cx3cr1-/-) embryos, a reduction in erythropoiesis is concomitant with a decrease in HBA macrophage percentages. In cocultures, we show that head macrophages boost hematopoietic progenitor cell numbers from HBA endothelial cells > twofold, and that the proinflammatory factor tumor necrosis factor-α is produced by head macrophages and influences HBA hematopoiesis in vitro. Taken together, head macrophages play a positive role in HBA erythropoiesis and HS/PC expansion and/or maturation, acting as microenvironmental cellular regulators in hematopoietic development.

Limits on the Existence of sub-MeV Sterile Neutrinos from the Decay of

Sterile neutrinos are natural extensions to the standard model of particle physics and provide a possible portal to the dark sector. We report a new search for the existence of sub-MeV sterile neutrinos using the decay-momentum reconstruction technique in the decay of ^{7}Be. The experiment measures the total energy of the ^{7}Li daughter atom from the electron capture decay of ^{7}Be implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. This first experiment presents data from a single STJ operated at a low count rate for a net total of 28 days, and provides exclusion limits on sterile neutrinos in the mass range from 100 to 850 keV that improve upon previous work by up to an order of magnitude.

Nutritional characterisation of Zambian Moringa oleifera: acceptability and safety of short-term daily supplementation in a group of malnourished girls.

In Zambia, chronic malnutrition still is one of the most common problem among children. To fight against malnutrition, the easiest short-term solution could be to combine specific types of food with affordable local plants. A large variety of natural food resources grow in Zambia, such as Moringa oleifera (MO), whose leaves are known for their health benefits, but are not consumed much by local populations. We analysed Zambian MO powder obtained from dried leaves and found that it contains large amounts of protein, minerals and vitamins, such as iron, calcium and carotenoids. These characteristics make MO a good and sustainable complementary solution to malnutrition. We also evaluated the acceptability and the safety of dietary supplementation with MO powder in malnourished children for 30 days. A daily dose of 14 g daily was safe and well accepted. Its regular use in the menu of local populations may be viable proposition.

Histone/protein deacetylase inhibitor therapy for enhancement of Foxp3+ T-regulatory cell function posttransplantation.

T-regulatory (Treg) cells are like other cells present throughout the body in being subject to biochemical modifications in response to extracellular signals. An important component of these responses involves changes in posttranslational modifications (PTMs) of histones and many nonhistone proteins, including phosphorylation/dephosphorylation, ubiquitination/deubiquitination, and acetylation/deacetylation. Foxp3, the key transcription factor of Tregs, is constantly being rapidly turned over, and a number of these PTMs determine its level of expression and activity. Of interest in the transplant setting, modulation of the acetylation or deacetylation of key lysine residues in Foxp3 can promote the stability and function, leading to increased Treg production and increased Treg suppressive activity. This mini-review focuses on recent data concerning the roles that histone/protein deacetylases (HDACs) play in control of Treg function, and how small molecule HDAC inhibitors can be used to promote Treg-dependent allograft survival in experimental models. These data are discussed in the light of increasing interest in the identification and clinical evaluation of isoform-selective HDAC inhibitors, and their potential application as tools to modulate Foxp3+ Treg cell numbers and function in transplant recipients.

Subregional localization and characterization of Ly6aGFP-expressing hematopoietic cells in the mouse embryonic head.

Hematopoietic cell generation in the midgestation mouse embryo occurs through the natural transdifferentiation of temporally and spatially restricted set of hemogenic endothelial cells. These cells take on hematopoietic fate in the aorta, vitelline and umbilical arteries and appear as hematopoietic cell clusters that emerge from the vascular wall. Genetic and live imaging data have supported this. Recently, the embryonic head has been shown to contain fully functional hematopoietic stem cells (HSC). By lineage tracing, cerebrovascular specific endothelial cells were shown to contribute to the postnatal mouse hematopoietic system. Since Ly6aGFP is a marker of all HSCs, some hematopoietic cluster cells and hemogenic endothelial cells in the midgestation mouse aorta, we examine here whether embryonic head HSCs and vascular endothelial cells are positive for this marker. Whereas some head vasculature, single hematopoietic cells and all HSCs are Ly6aGFP expressing, we do not find clusters of hematopoietic cells emerging from the cerebrovasculature that are characteristic of endothelial-to-hematopoietic transition.

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products.

The 205-230 nm photodissociation of vibrationally excited CO2 at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO2 molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO2. Photodissociation was observed from vibrationally excited CO2 with internal energies up to about 20 000 cm-1, and CO(X1Σ+), O(3P), and O(1D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO2 vibrational excitation made this investigation feasible. The internal energies of heated CO2 molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X1Σ+) products in v″ = 0. At 230 nm, CO2 needs to have at least 4000 cm-1 of rovibrational energy to absorb the UV radiation and produce CO(X1Σ+) + O(3P). CO2 internal energies in excess of 16 000 cm-1 were confirmed by observing O(1D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A1B2 and B1A2 states, explaining the nearly isotropic angular distributions of the products. CO(X1Σ+) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X1Σ+), O(3P), and O(1D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO2 temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO2 (and the corresponding available energy) increases. The KER distributions of O(1D) and O(3P) show broad internal energy distributions in the CO(X1Σ+) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO2 VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a3B2 and b3A2, and the ground electronic state are involved in the dynamical pathways that lead to product formation.

Extraction of effective solid-liquid interfacial free energies for full 3D solid crystallites from equilibrium MD simulations.

Molecular dynamics simulations of an embedded atom copper system in the isobaric-isenthalpic ensemble are used to study the effective solid-liquid interfacial free energy of quasi-spherical solid crystals within a liquid. This is within the larger context of molecular dynamics simulations of this system undergoing solidification, where single individually prepared crystallites of different sizes grow until they reach a thermodynamically stable final state. The resulting equilibrium shapes possess the full structural details expected for solids with weakly anisotropic surface free energies (in these cases, ∼5% radial flattening and rounded [111] octahedral faces). The simplifying assumption of sphericity and perfect isotropy leads to an effective interfacial free energy as appearing in the Gibbs-Thomson equation, which we determine to be ∼177 erg/cm2, roughly independent of crystal size for radii in the 50-250 Šrange. This quantity may be used in atomistically informed models of solidification kinetics for this system.

HIV-1 envelope-dependent restriction of CXCR4-using viruses in child but not adult untransformed CD4+ T-lymphocyte lines.

Phytohemagglutin-stimulated child and adult leukocytes equally supported CCR5-dependent (R5) and CXCR4-dependent (X4) HIV-1 replication. In contrast, when phytohemagglutin-stimulated leukocytes from either healthy or congenitally immunodeficient children were cultured on feeder cells, they well supported R5, but not X4 HIV-1 replication, whereas both viruses equally spread in adult cells maintained in similar conditions. Both child and adult cells showed similar levels of proliferation and surface expression of CD4, CCR5, CXCR4, CD25, CD69, and HLA-DR. Lack of X4 HIV-1 replication in child versus adult cells was not caused by a differential expression of several known HIV-1 restriction factors. Similar levels of HIV DNA synthesis occurred in child cells infected with R5 and X4 viruses up to 48 hours after infection when R5 HIV-1 showed a significantly superior capacity to spread in culture than X4 virus. Cultured child cells well supported single round vescicular stomatitis virus-G pseudotyped virus replication, whereas superinfection of R5-infected cells with X4 HIV-1 (or vice versa) rescued the replication of this latter virus. Thus, child cells exposed to feeder cell culture represent a novel model system in which the superior capacity of R5 versus X4 viruses to spread can be investigated in primary, untransformed CD4(+) cells.

From drosophila to humans: a journey through macrophage development.

Macrophages are fascinating immune cells involved in a variety of processes in both health and disease. Although they were first discovered and characterized by their functions as professional phagocytes and antigen-presenting cells, it is now clear that macrophages have multiple roles within embryonic development, tissue homeostasis, regulation of inflammation, and host response to pathogens and tissue insults. Interestingly, macrophages, or macrophage-like cells, exist in a variety of organisms, from echinoderms to humans, and are present also in species that lack an adaptive immune system or hematopoietic stem cells (HSCs). In mammals, macrophages can be generated from bone marrow precursors through a monocyte intermediate, but it is now known that they are also generated during earlier hematopoietic waves in the embryo. Seeding a variety of tissues at different times, macrophages contribute to embryonic organogenesis and tissue homeostasis. Interestingly, in species where embryonic macrophages are generated before HSC specification, they seem to be an important component of the HSC generative microenvironment. There are many excellent reviews reporting the current knowledge on the ontogeny and functions of macrophages in adult tissues. Here, we aim to summarize the current knowledge on the development and functions of embryonic macrophages across the most used animal models, with a special focus on developmental hematopoiesis.

Reducing Salivary Toxicity with Adaptive Radiotherapy (ReSTART): A Randomized Controlled Trial Comparing Conventional IMRT to Adaptive IMRT in Head and Neck Squamous Cell Carcinomas.

BACKGROUND: The utility of Adaptive Radiotherapy (ART) in Head and Neck Squamous Cell Carcinoma (HNSCC) remains to be ascertained. While multiple retrospective and single-arm prospective studies have demonstrated its efficacy in decreasing parotid doses and reducing xerostomia, adequate randomized evidence is lacking. METHODS AND ANALYSIS: ReSTART (Reducing Salivary Toxicity with Adaptive Radiotherapy) is an ongoing phase III randomized trial of patients with previously untreated, locally advanced HNSCC of the oropharynx, larynx, and hypopharynx. Patients are randomized in a 1:1 ratio to the standard Intensity Modulated Radiotherapy (IMRT) arm {Planning Target Volume (PTV) margin 5 mm} vs. Adaptive Radiotherapy arm (standard IMRT with a PTV margin 3 mm, two planned adaptive planning at 10th and 20th fractions). The stratification factors include the primary site and nodal stage. The RT dose prescribed is 66Gy in 30 fractions for high-risk PTV and 54Gy in 30 fractions for low-risk PTV over six weeks, along with concurrent chemotherapy. The primary endpoint is to compare salivary toxicity between arms using salivary scintigraphy 12 months' post-radiation. To detect a 25% improvement in the primary endpoint at 12 months in the ART arm with a two-sided 5% alpha value and a power of 80% (and 10% attrition ratio), a sample size of 130 patients is required (65 patients in each arm). The secondary endpoints include acute and late toxicities, locoregional control, disease-free survival, overall survival, quality of life, and xerostomia scores between the two arms. DISCUSSION: The ReSTART trial aims to answer an important question in Radiation Therapy for HNSCC, particularly in a resource-limited setting. The uniqueness of this trial, compared to other ongoing randomized trials, includes the PTV margins and the xerostomia assessment by scintigraphy at 12 months as the primary endpoint.

Evaluation of anti-infective potential of a tribal folklore Odina wodier Roxb against some selected microbes and herpes simplex virus associated with skin infection.

AIM: To evaluate the in vitro antimicrobial activity of aqueous and methanol extracts of Odina wodier bark (OWB), a folk medicine, against representative bacteria, fungi and herpes simplex virus (HSV) associated with skin infections. METHODS AND RESULTS: The OWB extract(s) was found to inhibit the isolates of Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Klebsiella pneumonia, Escherichia coli at an MIC of 256-5000 µg ml(-1) and Candida albicans at and above 4000 µg ml(-1) by agar and broth dilution assays. The growth curve of Staph. aureus revealed the highest activity within 2-6 h of methanol extract (ME) exposure. Interestingly, the MTT and plaque reduction assay showed that the extracts can inhibit HSV-1 and HSV-2 at EC50 of 22·4 and 28·8 µg ml(-1) , with Selectivity index of 11·7-15. While the time kinetic and binding assays demonstrated that the ME at 50 µg ml(-1) prevents viral attachment into Vero cells. Phytochemical and HPLC analysis of ME revealed the presence of flavonoids, phytosterols, saponins and tannins including the pseudotannin chlorogenic acid. CONCLUSION: The traditional use of OWB for the management of skin infections has scientific basis. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrated the antimicrobial potential of OWB on selected isolates of bacteria, fungi and HSV, associated with skin infections.

Prebiotic inulin: Useful dietary adjuncts to manipulate the livestock gut microflora.

In recent years, there has been a growing appreciation on the relevance of gastrointestinal microflora in both ruminants and non-ruminants owing to revelation of their role in several physiological functions including digestion, nutrient utilization, pathogen exclusion, gastrointestinal development, immunity system, gut gene expression and quality of animal products. The ban imposed on the use of antibiotics and hormones in feed has compelled animal researchers in finding an alternative which could overcome the issues of conventional feed additives. Though the concept of prebiotic was evolved keeping in mind the gastrointestinal flora of human beings, presently animal researchers are exploring the efficiency of prebiotic (inulin) for modulating the gut ecosystem of both ruminants and non-ruminants. It was revealed that prebiotic inulin is found to exhibit desirable changes in the gut of non-ruminants like poultry, swine, rabbit etc for augmenting gut health and improvement of product quality. Similarly, in ruminants the prebiotic reduces rumen ammonia nitrogen, methane production, increase microbial protein synthesis and live weight gains in calves. Unlike other feed additives, prebiotic exhibits its effect in multipronged ways for overall increase in the performances of the animals. In coming days, it is expected that prebiotics could be the part of diets in both ruminants and non-ruminants for enabling modulation of gut microflora vis a vis animals productivity in ecological ways.

Variation of modulation and expression of biomarkers associated with inflammation in bariatric surgery patients: A systematic review and meta-analysis.

BACKGROUND: Bariatric surgery is an effective intervention that causes a series of metabolic changes related to inflammatory processes; however, the variation of biomarkers related to these processes is not entirely understood. Our objective was to investigate the variation of modulation and expression of biomarkers associated with inflammation in patients who underwent bariatric surgery. METHODS: We searched the MEDLINE (via PubMed), EMBASE (via Elsevier), Cochrane Central Register of Controlled Trials, Latin American and Caribbean Literature on Health Sciences (via virtual health library), Cumulative Index to Nursing and Allied Health Literature (via EBSCO), Web of Science core collection, and Scopus (via Elsevier) databases, and the gray literature was examined from inception to January 2022. Three pairs of reviewers performed data screening, extraction, and quality assessment independently. Meta-analysis with random effects models was used for general, subgroup, and sensitivity analyses. The I2 statistic was used to assess heterogeneity between studies. RESULTS: In total, 96 articles were included in this systematic review; of these, 87 studies met the criteria for the meta-analysis, involving 3,533 participants. Five biomarkers were included in the meta-analysis (tumor necrosis factor alpha; interleukin 6; leptin; interleukin 1 beta, and lipopolysaccharides). Only leptin showed a significant decrease in the first month after surgery (mean difference -20.71; [95% confidence interval: -28.10 to -13.32, P < .0001; I2 = 66.7%), with moderate heterogeneity. The 12 months after surgery showed a significant decrease in tumor necrosis factor alpha (mean difference -0.89; [95% confidence interval: -1.37 to -0.42], P = .0002; I2 = 94.7%), interleukin 6 (mean difference -1.62; [95% confidence interval: -1.95 to -1.29], P < .0001; I2 = 94.9%), leptin (mean difference -28.63; [95% confidence interval: -34.02 to -23.25], P < .0001; I2 = 92.7%), and interleukin 1 beta (mean difference -2.46; [95% confidence interval: -4.23 to -0.68], P = .006; I2 = 98.3%), all with high heterogeneity. The type of surgery did not show significant differences for the biomarkers at the first month and 12 months, and the results have not changed with high-quality studies. In the 12-month measurement, variations in tumor necrosis factor alpha and leptin were associated with body mass index. CONCLUSION: The findings of this meta-analysis suggest that Roux-en-Y gastric bypass and sleeve gastrectomy bariatric surgeries are associated with a significant reduction in leptin at 1 month after bariatric surgical intervention and tumor necrosis factor alpha, leptin, and interleukin 1 beta after 12 months.

Embryonic Origins of the Hematopoietic System: Hierarchies and Heterogeneity.

The hierarchical framework of the adult blood system as we know it from current medical and hematology textbooks, displays a linear branching network of dividing and differentiated cells essential for the growth and maintenance of the healthy organism. This view of the hierarchy has evolved over the last 75 years. An amazing increase in cellular complexity has been realized; however, innovative single-cell technologies continue to uncover essential cell types and functions in animal models and the human blood system. The most potent cell of the hematopoietic hierarchy is the hematopoietic stem cell. Stem cells for adult tissues are the long-lived self-renewing cellular component, which ensure that differentiated tissue-specific cells are maintained and replaced through the entire adult lifespan. Although much blood research is focused on hematopoietic tissue homeostasis, replacement and regeneration during adult life, embryological studies have widened and enriched our understanding of additional developmental hierarchies and interacting cells of this life-sustaining tissue. Here, we review the current state of knowledge of the hierarchical organization and the vast heterogeneity of the hematopoietic system from embryonic to adult stages.

Asymmetric HIV-1 co-receptor use and replication in CD4(+) T lymphocytes.

Susceptibility to infection by the human immunodeficiency virus type-1 (HIV-1), both in vitro and in vivo, requires the interaction between its envelope (Env) glycoprotein gp120 Env and the primary receptor (R), CD4, and Co-R, either CCR5 or CXCR4, members of the chemokine receptor family. CCR5-dependent (R5) viruses are responsible for both inter-individual transmission and for sustaining the viral pandemics, while CXCR4-using viruses, usually dualtropic R5X4, emerge in ca. 50% of individuals only in the late, immunologically suppressed stage of disease. The hypothesis that such a major biological asymmetry is explained exclusively by the availability of cells expressing CCR5 or CXCR4 is challenged by several evidences. In this regard, binding of the HIV-1 gp120 Env to the entry R complex, i.e. CD4 and a chemokine R, leads to two major events: virion-cell membrane fusion and a cascade of cell signaling. While the fusion/entry process has been well defined, the role of R/Co-R signaling in the HIV-1 life cycle has been less characterized. Indeed, depending on the cellular model studied, the capacity of HIV-1 to trigger a flow of events favoring either its own latency or replication remains a debated issue. In this article, we will review the major findings related to the role of HIV R/Co-R signaling in the steps following viral entry and leading to viral spreading in CD4(+) T lymphocytes.