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Biomedical subjects

M Rex

Publications and source records attributed to M Rex.

At least 19 recordsLinked to original sources
PubMed2005

Soft colloids driven and sheared by traveling wave fields.

We study the dynamics of soft colloids interacting via a Gaussian pair potential in an external moving potential which is periodic in the spatial coordinate of the direction of motion. Both dynamical density functional theory and Brownian dynamics computer simulations are used to predict the steady-state density profiles. Two different situations are investigated: the first corresponds to a light wave that travels with a constant velocity v through the quiescent solvent containing the colloidal suspension. The second setup consists of two parallel repulsive walls with a periodic topographical substructure. One of the walls is at rest relative to the solvent while the other is in motion, inducing a shearing of the suspension. In the first case, we find that the amplitude of the steady-state density behaves nonmonotonically with the traveling speed v of the wave if the shape of the wave contains an edge: for increasing v , it first grows and then decreases. In the second setup we show that a strongly confined suspension induces a shear resistance which is a nonmonotonic function of the wall velocity. These effects are verifiable in real-space experiments on colloidal suspensions exposed to external laser-optical fields. In both situations, the dynamical density functional theory is in good agreement with the Brownian dynamics simulation data.

Journal Article↗
PubMed2003

Enhanced upper tropical tropospheric COS: impact on the stratospheric aerosol layer.

Carbonyl sulfide (COS) is considered to be a major source of the stratospheric sulfate aerosol during periods of volcanic quiescence. We measured COS at the tropical tropopause and find mixing ratios to be 20 to 50% larger than are assumed in models. The enhanced COS levels are correlated with high concentrations of biomass-burning pollutants like carbon monoxide (CO) and hydrogen cyanide (HCN). The analysis of backward trajectories and global maps of fire statistics suggest that biomass-burning emissions transported upward by deep convection are the source of the enhanced COS in the upper tropical troposphere.

Journal Article↗
PubMed2002

The transcription factor cSox2 and Neuropeptide Y define a novel subgroup of amacrine cells in the retina.

The retina has been extensively used as a model to study the mechanisms responsible for the production of different neural cell phenotypes. The importance of both extrinsic and intrinsic cues in these processes is now appreciated and numerous transcription factors have been identified which are required for both neuronal determination and cell differentiation. In this study we have analysed the expression of the transcription factor Sox2 during development of the chick retina. Expression was found in the proliferating cells of the retina during development and was down regulated by nearly all cell types as they started to differentiate and migrate to the different layers of the retina. In one cell type, however, Sox2 expression was retained after the cells have ceased division and migrated to their adult location. These cells formed two rows located on either side of the inner plexiform layer and were also positive for Neuropeptide Y, characteristics which indicate that they were a subpopulation of amacrine cells. The expression of Sox2 by only this population of post-mitotic neurones makes it possible to follow these cells as they migrate to their adult location and shows that they initially form a single row of cells which subsequently divides to form the double row seen in the adult tissue. We suggest that retained expression of Sox2 is involved in directing the differentiation of these cells and is an early marker of this cell type.

Amacrine Cells↗
PubMed2001

cSox3 expression and neurogenesis in the epibranchial placodes.

Epibranchial placodes are local thickenings of the surface ectoderm, which give rise to sensory neurons of the distal cranial ganglia. The development of these placodes has remained unclear due to the lack of any definitive marker for these structures. We show here that the chick transcription factor, cSox3, is expressed in four lateral patches at the rostral edge of the epibranchial arches and that these mark the epibranchial placodes. These patches of cSox3 expression arise by gradual thinning from broader areas of cSox3 expression with concomitant loss of cSox3 in nonplacodal regions. Cells leaving the epithelial placodes as they initiate neurogenesis, lose cSox3 expression and sequentially express Ngn1, NeuroD, NeuroM, and Phox2a, but do not express Ngn2. This is in contrast to studies in the mouse where it is Ngn2, rather than Ngn1, that is predominantly expressed in epibranchial-derived neuroblasts. Overexpression of cSox3 interferes with normal neuroblast migration and results in changes in ectodermal morphology. Thus, cSox3 provides a useful tool for the study of placode formation, and loss of cSox3 expression appears to be a necessary event in normal neurogenesis from the epibranchial placodes.

Animals↗
PubMed2001

Initiation and early patterning of the endoderm.

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Amphibians↗
PubMed1998

Granule cell development in the cerebellum is punctuated by changes in Sox gene expression.

Development of the vertebrate cerebellum is unusual compared to most other regions of the brain since it involves two germinal regions. Most cell types arise from the luminal, ventricular zone as in other brain regions, but granule cells arise from the second germinal layer, the external granular layer (EGL). Our analysis of the temporal and positional expression of three members of the Sox gene family of transcription factors in the cerebellum shows that granule cell development is unusual compared to most other neurons of the central nervous system (CNS). We show that granule cell precursors lose expression of cSox2 and cSox3 as they migrate to form the EGL. The EGL is the first example of a germinal layer in the CNS which does not exhibit expression of these genes. Throughout most of the CNS cSox11 expression is very low in the ventricular zone but increases dramatically as cells cease proliferation and migrate to form the subventricular zone. We also find that cSox11 expression increases when cells of the cerebellum migrate to form the EGL, but levels of expression as high as that in the subventricular zone are only seen when cells cease proliferation and migrate inwards to form the deep EGL. These observations demonstrate that cells of the proliferative superficial EGL differ qualitatively from cells of the ventricular zone in their expression of Sox genes whereas the post-proliferative cells of the deep EGL appear analogous, in their expression of Sox genes, to cells of the subventricular zone.

Animals↗
PubMed1998

Region-specific expression of chicken Sox2 in the developing gut and lung epithelium: regulation by epithelial-mesenchymal interactions.

In situ analysis of the chicken cSox2 gene, a member of the transcription factor family containing an Sry-like high-mobility group (HMG) box, demonstrated localized expression in the embryonic endoderm. Transcripts of cSox2 appeared before commencement of morphogenesis and cytodifferentiation in the rostral gut epithelium from the pharynx to the stomach. The caudal limit of cSox2 expression coincided with that of the region competent for proventricular differentiation and to the rostral limit of the domain of CdxA, a homologue of Drosophila caudal. During morphogenesis, the level of transcripts of cSox2 decreased in epithelia invaginating into surrounding mesenchyme to form glandular or tubular structures, such as the primordia of the thyroid and lung, glandular epithelium of the proventriculus, and secondary bronchus of the lung. Tissue recombination experiments demonstrated that cSox2 expression is regulated by the underlying mesenchyme as well as morphogenesis and cytodifferentiation. The results suggest that cSox2 plays pivotal roles in generating morphologically and physiologically distinct types of epithelial cells in the gut.

Animals↗
PubMed1997

Dynamic expression of chicken Sox2 and Sox3 genes in ectoderm induced to form neural tissue.

The chick genes, cSox2 and cSox3, are members of a large family of genes that encode transcription factors. Previous studies have shown that these genes are predominantly expressed in the central nervous system during embryonic development. We show that cSox3 is expressed throughout the ectoderm that is competent to form nervous tissue before neural induction. The expression of cSox3 is lost from cells as they undergo gastrulation to form nonectodermal tissues; the transcription factor, Brachyury, appears in cells about to undergo gastrulation a short time before cSox3 transcripts are lost. Therefore, Brachyury expression may act functionally upstream of cSox3 downregulation. cSox3 expression is also lost from non-neuronal ectoderm shortly after the neural plate becomes morphologically apparent. cSox2 expression increases dramatically in the central nervous system as neural ectoderm is established. The appearance of cSox2 in neural ectoderm represents one of the earliest molecular responses to neural induction documented thus far.

Animals↗
PubMed1997

Combination of non-isotopic in situ hybridisation with detection of enzyme activity, bromodeoxyuridine incorporation and immunohistochemical markers.

The advent of non-isotopic in situ hybridisation allows the possibility to detect the presence of both mRNAs and other markers in cells. We have established conditions for simultaneous analysis of gene expression and a variety of other immunohistochemical markers in tissue sections. We report the analysis of expression of a family of transcription factors (Sox genes) in combination with detection of: (1) protein antigens (using both monoclonal and polyclonal antibodies); (2) bromodeoxyuridine to mark cells which are proliferating; and (3) acetylcholinesterase activity. The approaches we describe, which demonstrate the compatibility of non-isotopic in situ hybridisation with a range of other treatments, should be generally applicable and open to many variations of probe, antibodies and colour detection systems.

Acetylcholinesterase↗
PubMed1997

cSox21 exhibits a complex and dynamic pattern of transcription during embryonic development of the chick central nervous system.

cSox21 is a novel member of the Sox gene family of transcription factors. This gene is a member of the subgroup B, which includes Sox1, Sox2 and Sox3. Although all of these genes are predominantly expressed in the nervous system, only cSox21 expression is positionally restricted within the CNS. Longitudinal stripes are seen in the spinal cord and a more complex pattern is seen in the brain. The timing and position in which cSox21 stripes of expression appear provides further insight into dorsoventral patterning of the CNS. The expression of cSox21, and other genes (such as Delta, Serrate and Pax genes), may play a part in defining the developmental fate of cells along the dorsoventral axis.

Amino Acid Sequence↗
PubMed1997

Non-isotopic in situ hybridization to detect chick Sox gene mRNA in plastic-embedded tissue.

In situ hybridization techniques have rapidly become widely used by the molecular biologist for the localization of specific nucleic acid sequences in individual cells or tissues. We describe the demonstration of Sox gene mRNA in chick tissue that has been embedded in the plastic methyl methacrylate to permit the preparation of sections for high-resolution light microscopy. Polymerization of the plastic was induced by using either N,N-dimethylaniline or N,N-3,5-tetramethylaniline. The in situ hybridization technique used was non-isotopic and used a digoxigenin-labelled probe detected with an antibody bound to alkaline phosphatase, which was then localized using X-phosphate-Nitro BT as a substrate-chromogen mix. Various pretreatments of the tissue sections were investigated, including the use of proteinase K, and heat-mediated techniques using a microwave oven and a pressure cooker. The best results were produced using pressure cooking on tissue in which the plastic had been chemically polymerized with N,N-3,5-tetramethylaniline. For the demonstration of Sox 11, this combination had a critical influence on the staining results, but for Sox 21 all protocols used produced good staining.

Aniline Compounds↗
PubMed1997

Preventing the loss of competence for neural induction: HGF/SF, L5 and Sox-2.

The response to neural induction depends on the presence of inducing signals and on the state of competence of the responding tissue. The epiblast of the chick embryo loses its ability to respond to neural induction by the organizer (Hensen's node) between stages 4 and 4+. We find that the pattern of expression of the L5(220) antigen closely mirrors the changes in competence of the epiblast in time and in space. For the first time, we describe an experiment that can extend the period of neural competence: when L5(220) expression is maintained beyond its normal time by implanting HGF/SF secreting cells, the competence to respond to Hensen's node grafts is retained. The host epiblast forms a non-regionalized neural tube, which expresses the pan-neural marker SOX-2 (a Sry-related transcription factor) but not any region-specific markers for the forebrain, hindbrain or spinal cord. Although HGF/SF secreting cells can mimic signals from Hensen's node that maintain L5 expression, they cannot rescue the ability of the node to induce anterior structures (which is normally lost after stage 4). The ectoderm may acquire stable neural characteristics during neural induction by going through a hierarchy of states: competence, neuralization and regionalization. Our findings allow us to start to define these different states at a molecular level, and show that the competence to respond to neural induction is not entirely autonomous to the responding cells, but can be regulated by extracellular signalling molecules.

Animals↗
PubMed1996

Transcription factors in early development of the central nervous system.

In studies of the central nervous system (CNS) few areas have progressed faster than the study of transcription factors and their role in controlling gene expression during development. Evidence for the pivotal roles of these factors in the formation of the CNS is reviewed; from neural induction to the maturation of neurons and the specification of cells according to their position within the CNS. In all of these processes, epigenetic factors affect the cells' developmental fate but it is transcription factors within the cells which function both to decode these incoming messages and then to effect changes in the expression of other genes. Soluble factors such as retinoic acid and the products of the Noggin and Sonic hedgehog genes induce changes in families of transcription factors such as the Hox, Sox, Pax and Pou gene products and these alter the expression of banks of downstream genes thereby controlling the developmental fate of those cells. Recent advances in understanding of the molecular events underlying normal neurogenesis might now lead to a clearer understanding of the molecular abnormalities underlying several developmental disorders of the CNS.

Animals↗
PubMed1996

The effects of allopurinol on the ultrastructure of ischaemic and reperfused large intestine of sheep.

OBJECTIVE: To test the possible inhibitory effect of allopurinol on reperfusion injury, caused by oxygen-derived free radicals, of sheep large intestine. DESIGN: An ultrastructural study on caecal tissues from control and treated groups. ANIMALS: Fifty sheep in four ischaemic and reperfused (treatment) groups and one control group. Three of the treatment groups were subdivided for half to be injected with allopurinol and the other half with its solvent, potassium hydroxide (KOH). PROCEDURE: Ischaemia of the caecum was induced in the four treatment groups for 60 minutes by clamping the apex. Allopurinol and its KOH solvent were injected intravenously in three treatment groups prior to ischaemia. Samples were collected before and 1 hour after induction of ischaemia and 1 min, 1 h and 8 h after reperfusion. Tissues were processed and examined with an electron microscope. RESULTS: Untreated and solvent injected sheep showed minor ultrastructural changes following ischaemia. With reperfusion, there was severe mitochondrial, goblet cell and basement membrane damage. Tissues from allopurinol-treated sheep were preserved and appeared similar to tissues from the control group. CONCLUSION: Pre-treatment with allopurinol prevented damage to tissues whereas untreated or allopurinol solvent-treated showed severe damage following reperfusion. It is believed that allopurinol, an analogue of hypoxanthine and xanthine, prevents reperfusion injury by competitively binding with xanthine oxidase. This reduces or inhibits the xanthine oxidase mediated conversion of hypoxanthine to xanthine thereby preventing the formation of oxygen-derived free radicals.

Allopurinol↗
PubMed1995

Embryonic expression of the chicken Sox2, Sox3 and Sox11 genes suggests an interactive role in neuronal development.

Three chicken Sox (SRY-like box) genes have been identified that show an interactive pattern of expression in the developing embryonic nervous system. cSox2 and cSox3 code for related proteins and both are predominantly expressed in the immature neural epithelium of the entire CNS of HH stage 10 to 34 embryos. cSox11 is related to cSox2 and cSox3 only by virtue of containing an SRY-like HMG-box sequence but shows extensive homology with Sox-4 at its C-terminus. cSox11 is expressed in the neural epithelium but is transiently upregulated in maturing neurons after they leave the neural epithelium. These patterns of expression suggest that Sox genes play a role in neural development and that the developmental programme from immature to mature neurons may involve switching of Sox gene expression. cSox11 also exhibits a lineage restricted pattern of expression in the peripheral nervous system.

Amino Acid Sequence↗
PubMed1994

Chick HoxB3: deduced amino-acid sequence and embryonic gene expression.

The deduced amino-acid sequence of chick HoxB3 shows 70% homology to the murine HoxB3. The major difference is that the chick sequence lacks a run of Gly residues present in mouse and human HoxB3. The basic pattern of expression of HoxB3 in the embryonic nervous system is conserved between mice and chicks.

Amino Acid Sequence↗