Biomedical subjects

Harald B Steen

Publications and source records attributed to Harald B Steen.

5 recordsLinked to original sources

PubMed2006

A stochastic model of cancer initiation including a bystander effect.

A stochastic model of cancer initiation is considered. The model is used to evaluate whether a bystander effect may be important in the pre-malignant and malignant stages of carcinogenesis, and furthermore, on the basis of epidemiological data, to estimate the mutation rates of genes involved in the development of oral leukoplakias. The bystander effect is defined here as the capability of oncogenic mutations to increase the mutation probability of neighbouring (bystander) cells, thus leading potentially to a cascade of neighbouring mutated and neoplastic cells as a pre-stage in the development to leukoplakias and cancer. We find that incidence data for oral cancer are indeed in accordance with a significant bystander effect, operating either alone or in combination with genomic instability in the early stages of carcinogenesis, i.e. the development of neoplasia. Simulations performed gave a picture of how mutations and neoplasia may spread in a tissue, to form characteristic leukoplakias with a core of neoplastic cells. The model also showed that the probability of finding at least one neoplastic cell in the tissue after a given number of years is more sensitive to changes in genomic instability within the cell itself than to changes in a bystander effect. Based on epidemiological data we also calculate the maximum number of oncogenic genes that may be involved in the bystander effect and development of genomic instability. Even if capable of explaining the initial development of oncogenic mutations towards neoplastic cells, the bystander model could not reproduce the observed incidence rates of leukoplakia without assuming a carcinogen mutation probability per cell per year of neoplastic cells practically equal to one. This means that the bystander effect, to be of substantial importance in the final development of neoplastic cells towards leukoplakias, requires a very significant increase in mutation probabilities for bystanders to neoplastic cells. Alternatively, additional mechanisms such as abnormal cell differentiation and uncontrolled proliferation and apoptotis in the neoplastic stage may be of major importance during the development to cancerization.

Bystander Effect↗

PubMed2004

Flow cytometer for measurement of the light scattering of viral and other submicroscopic particles.

BACKGROUND: Light scattering is an essential parameter in flow cytometry, facilitating functions such as size measurement, discrimination of cell types on the basis of shape and morphology, detection of fluorescence-negative cells, and gating of fluorescence measurements. Light scattering measurement of viruses is generally not feasible with current flow cytometers due to their small size. The problem is aggravated by the fact that the light scattering of particles in this size range (< 200 nm) falls off with roughly the sixth power of their linear dimensions. METHODS: A new optical layout using darkfield illumination and detection has been developed. A 532-nm laser was used for excitation, and scattered light was collected with large aperture optics. RESULTS: Light scattering histograms of polymer particles with diameters of 70-300 nm were recorded without gating by other parameters. By extrapolation, a detection limit of about 50 nm was obtained. Different species of virus with sizes of approximately 100 nm also were recorded. CONCLUSIONS: Flow cytometric light scattering measurement of submicroscopic particles, in a size range that includes many viral species, is now feasible. The results indicate that it may be practically impossible to measure by flow cytometry the light scattering of particles smaller than 40 nm.

Animals↗

PubMed2004

Automated counting of mammalian cell colonies by means of a flat bed scanner and image processing.

BACKGROUND: Clonogenic assays are used frequently to measure the cell killing and mutagenic effects of radiation and other agents. Clonogenic assays carried out manually are tedious and time-consuming and involve a significant element of subjectivity. However, several commercial automatic colony counters are available. Based on CCD video imaging and image analysis they are relatively expensive and can analyze only one petri dish at a time. METHOD: We have developed a cheaper and more efficient device, which employs a flat bed scanner to image 12 60-mm petri dishes at a time. Two major problems in automated colony counting are the clustering of colonies and edge effects. By using standard image analysis and implementing an inflection point algorithm, these problems were greatly diminished. The resulting system was compared with two manual colony counts, as well as with automated counts with the Oxford Optronix ColCount colony counter for cell lines V79 and HaCaT. RESULTS: Comparisons assuming the manual counts to be correct showed that our automatic counter was slightly more accurate than the commercial unit. CONCLUSIONS: As a whole, our automated colony counter performed significantly better than the commercial unit with regard to processing time, cost and accuracy.

Algorithms↗

PubMed2002

A sample injection device for flow cytometers.

BACKGROUND: The sample injection systems of flow cytometers employ either a pressure differential between the sample vial and the sheath fluid reservoir or volumetric injection of the sample from a syringe. The pressure differential method facilitates rapid and efficient flushing to eliminate carryover between samples, but does not allow accurate determination of the rate of sample flow and cell concentration. Volumetric injection, which comprises a valve for switching the sample flow, facilitates highly accurate measurement of the cell concentration, but requires a less efficient and more time-consuming flushing procedure. METHODS: Applying a removable syringe, which connects to the inlet of the sample tubing via a tight sealing, we eliminate the valve and obtain efficient flushing while maintaining the advantage of volumetric sample injection. RESULTS: This device gives highly constant sample flow rates strictly proportional to syringe velocity over the range 0.2-50 microl/min with a settling time of about 2 sec. CONCLUSION: This device has the same precision as the conventional sample injection system, whereas the speed and efficiency of flushing are improved greatly.

Flow Cytometry↗

PubMed2002

Dye exclusion artifact in flow cytometers.

BACKGROUND: Cells exclude their own volume of dye solution in the sample flow which carries them through the flow chamber of the flow cytometer, thereby affecting the otherwise constant signal arising from the fluorescence of this solution. Under certain conditions, this phenomenon may significantly influence the fluorescence signal of the cells. MATERIALS AND METHODS: Using the slit scan technique, we studied this phenomenon as observed for monodisperse polystyrene particles in fluorescein solution. RESULTS: The measurements show that dye solution accumulates just in front of the particle and just behind it, with a relative void in between. This phenomenon is most likely caused by the rapid constriction of the flow as it enters the orifice of the nozzle or flow chamber, giving rise to a pulse of fluorescence which adds to that of the particle or cell itself. The magnitude of this artifact depends on the design and dimensions of the nozzle/flow chamber as well as on the rate of sample flow. CONCLUSIONS: The dye exclusion artifact may affect measurements of cells when they are in a dye solution having a fluorescence per unit volume which is significant compared to that of the cells, especially at low sample flow rates.

Animals↗