Mechanisms of action of interferon and nucleoside analogues.
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Antibody Formation↗
Biomedical subjects
Howard Thomas
Publications and source records attributed to Howard Thomas.
Quantitative versus morphological assessment of liver fibrosis: semi-quantitative scores are more robust than digital image fibrosis area estimation.
BACKGROUND/AIM: Digital image analysis (DIA) allows quantitative assessment of fibrosis on liver biopsy. Accurate determination of a threshold greyscale level representing fibrous tissue is critical. This method has not been fully evaluated in clinical practice. METHODS: Digital images of stained liver biopsy sections were captured by microscopy and converted to greyscale. A novel method of determining the threshold greyscale value at which to measure fibrosis area was developed (peak proportion area change (PPAC)). Reproducibility was tested in comparison with standard interactive thresholding and with semi-quantitative scoring using the Histological activity index (HAI) system by a histopathologist. Fibrosis areas for different sections from the same biopsy core were also compared by each method. RESULTS: Comparison between PPAC and interactive thresholding method demonstrated superior reproducibility of the PPAC method: r > 0.7, P < 0.001 compared with r = 0.19-0.64 (not all reaching significance). On a single section, reproducibility was similar for PPAC and the modified HAI system. When different sections from the same core were compared, the HAI system was more robust. CONCLUSIONS: The PPAC method is superior to standard interactive thresholding. However, variability in DIA scores between sections invalidates the technique for clinical use and semi-quantitative scoring systems remain the gold standard for fibrosis assessment.
Fibrosis↗
Defining senescence and death.
This article evaluates features of leaf and flower senescence that are shared with, or are different from, those of other terminal events in plant development. Alterations of plastid structure and function in senescence are often reversible and it is argued that such changes represent a process of transdifferentiation or metaplasia rather than deterioration. It may be that the irreversible senescence of many flowers and some leaves represents the loss of ancestral plasticity during evolution. Reversibility serves to distinguish senescence fundamentally from programmed cell death (PCD), as does the fact that viability is essential for the initiation and progress of cell senescence. Senescence (particularly its timing and location) requires new gene transcription, but the syndrome is also subject to significant post- transcriptional and post-translational regulation. The reversibility of senescence must relate to the plastic, facultative nature of underlying molecular controls. Senescence appears to be cell-autonomous, though definitive evidence is required to substantiate this. The vacuole plays at least three key roles in the development of senescing cells: it defends the cell against biotic and abiotic damage, thus preserving viability, it accumulates metabolites with other functions, such as animal attractants, and it terminates senescence by becoming autolytic and facilitating true cell death. The mechanisms of PCD in plants bear a certain relation to those of apoptosis, and some processes, such as nucleic acid degradation, are superficially similar to aspects of the senescence syndrome. It is concluded that, in terms of physiological components and their controls, senescence and PCD are at best only distantly related.
Apoptosis↗
Hepatic stem cells.
The liver in an adult healthy body maintains a balance between cell gain and cell loss. Though normally proliferatively quiescent, hepatocyte loss such as that caused by partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response to restore liver mass. This restoration of moderate cell loss and 'wear and tear' renewal is largely achieved by hepatocyte self-replication. Furthermore, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes can undergo significant clonal expansion, suggesting that hepatocytes themselves are the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential so-called oval cells within the lobules that can differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells can contribute to the albeit low renewal rate of hepatocytes, make a more significant contribution to regeneration, and even completely restore normal function in a murine model of hereditary tyrosinaemia. How these three stem cell populations integrate together to achieve a homeostatic balance is not known. This review focuses on two major aspects of liver stem cell biology: firstly, the identity of the liver stem cells, and secondly, their potential value in the treatment of major liver disease.
Animals↗
Ageing in plants.
Ageing in green plants differs in some fundamental ways from the process in animals. The seasonal cycle and persistence of a plant is governed by a combination of the determinate or indeterminate status of meristems (growth centres) and the cell death and disposal strategies employed by plants to generate well-adapted anatomies and morphologies. The degree of perenniality depends on the balance between exploratory growth and the wave of tissue death that succeeds it, and extremes of longevity can arise by relatively minor changes in the quantitative relationship between growth and death. The senescence and elimination of organs and tissues are related to the internal reallocation of resources but are programmed phases in the integrated development of the whole plant and do not represent a kind of ageing by stress or starvation. Meristems of long-lived plants accumulate genetic damage but selection mechanisms exist within the organism to control genetic load, and even to exploit somatic mutations that confer adaptive benefits. It is concluded that most plants do not age in the strict gerontological sense and that extremely long-lived forms like trees and clonal creeping perennials are sustained by selection and correction at the level of semi-autonomous cell lineages.
Aging↗
What stay-green mutants tell us about nitrogen remobilization in leaf senescence.
Leaf senescence has an important role in the plant's nitrogen economy. Chlorophyll catabolism is a visible symptom of protein mobilization. Genetic and environmental factors that interfere with yellowing tend to modify protein degradation as well. The chlorophyll-protein relationship is much closer for membrane proteins than it is for soluble or total leaf proteins. In stay-greens, genotypes with a specific defect in the chlorophyll catabolism pathway, soluble protein degradation during senescence may be close to normal, but light-harvesting and reaction centre thylakoid membrane proteins are much more stable. Genes for the chlorophyll catabolism pathway and its control are important in the regulation of protein mobilization. Genes for three steps in the pathway are reported to have been isolated. The gene responsible for the stay-green phenotype in grasses and legumes has not yet been cloned but a fair amount is known about it. Pigment metabolism in senescing leaves of the Festuca-Lolium stay-green mutant is clearly disturbed and is consistent with a blockage at the ring-opening (PaO) step in chlorophyll breakdown. PaO is de novo synthesized in senescence and thought to be the key enzyme in the chlorophyll a catabolic pathway. The stay-green mutation is likely to be located in the PaO gene, or a specific regulator of it. These genes may well be in the various senescence-enhanced cDNA collections that have been generated, but functional handles on them are currently lacking. When the stay-green locus from Festuca pratensis was introgressed into Lolium temulentum, a gene encoding F. pratensis UDPG-pyrophosphorylase was shown to have been transferred on the same chromosome segment. A strategy is described for cloning the stay-green gene, based on subtractive PCR-based analyses of intergeneric introgressions and map-based cloning.
Base Sequence↗