| phase plate | <microscopy> The plate used near the back focal plane of a microscope objective lens (in conjunction with an annulus at the front focal plane of the condenser lens) to achieve phase contrast. The phase plate selectively shifts the phase of the waves diffracted by the specimen by a quarter wave and reduces the amplitude of the undeviated, direct beam. (05 Aug 1998) |
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| phase, resting | More appropriately called interphase. The interval in the cell cycle between two cell divisions when the individual chromosomes cannot be distinguished, interphase was once thought to be the resting phase but it is far from a time of rest for the cell. It is the time when DNA is replicated in the cell nucleus. (12 Dec 1998) |
| phase rule | An expression of the relationships existing between systems in equilibrium: P + V = C + 2, where P is the number of phases, V the variance or degrees of freedom, and C the number of components; it also follows that the variance is, V = C + 2 -P. For H2O at its triple point, V = 1 + 2 -3 = 0, i.e., both temperature and pressure are fixed. Synonym: Gibb's phase rule. (05 Mar 2000) |
| phase separation | <chemistry> The separation of fluid phases that contain different concentrations of common components. Occurs with partially miscible solvents used in many biochemical separation methods. Also temperature dependent phase separation occurs with some detergent solutions. With reference to membranes means the segregation of lipid components into domains that have different chemical composition. (31 Dec 1997) |
| phase shift | <microscopy> A change in the phase relationship between two alternating quantities of the same frequency. (05 Aug 1998) |
| phase variation | <microbiology> Alteration in the expression of surface antigens by bacteria. For example: Salmonella can express either of two forms of flagellin, H1 and H2, that are coded by different genes. Control of which form is expressed is brought about by inversion of the promoter for the H2 gene, which if functional (noninverted) is associated with the expression of H2 and the production of a repressor of the H1 gene. Inversion occurs about every 1000 bacterial divisions and is under the control of another gene, hin, that is within the invertable sequence. (31 Dec 1997) |
| chronic phase | Refers to the early stages of chronic myelogenous leukaemia. The number of immature, abnormal white blood cells in the bone marrow and blood is higher than normal, but lower than in the accelerated or blast phase. (12 Dec 1998) |
| clinical trial, phase I | A pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques based on a small number of healthy persons and conducted over the period of about a year in either the united states or a foreign country. (12 Dec 1998) |
| clinical trial, phase II | A pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques based on several hundred volunteers, including a limited number of patients, and conducted over a period of about two years in either the united states or a foreign country. (12 Dec 1998) |
| clinical trial, phase III | A pre-planned, usually controlled, clinical study of the safety and efficacy of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques after phase II trials. A large enough group of patients is studied and closely monitored by physicians for adverse response to long-term exposure, over a period of about three years in either the united states or a foreign country. (12 Dec 1998) |
| clinical trial, phase IV | Planned post-marketing studies of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques that have been approved for general sale after clinical trials, phases I, II, and III. These studies, conducted in the united states or a foreign country, often garner additional data about the safety and efficacy of a product. (12 Dec 1998) |
| clinical trials, phase I | Studies performed to evaluate the safety of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques in healthy subjects and to determine the safe dosage range (if appropriate). These tests also are used to determine pharmacologic and pharmacokinetic properties (toxicity, metabolism, absorption, elimination, and preferred route of administration). They involve a small number of persons and usually last about 1 year. This concept includes phase I studies conducted both in the u.s. And in other countries. (12 Dec 1998) |
| clinical trials, phase II | Studies that are usually controlled to assess the effectiveness and dosage (if appropriate) of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques. These studies are performed on several hundred volunteers, including a limited number of patients with the target disease or disorder, and last about two years. This concept includes phase II studies conducted in both the u.s. And in other countries. (12 Dec 1998) |
| clinical trials, phase III | Comparative studies to verify the effectiveness of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques determined in phase II studies. During these trials, patients are monitored closely by physicians to identify any adverse reactions from long-term use. These studies are performed on groups of patients large enough to identify clinically significant responses and usually last about three years. This concept includes phase III studies conducted in both the u.s. And in other countries. (12 Dec 1998) |
| clinical trials, phase IV | Planned post-marketing studies of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques that have been approved for general sale. These studies are often conducted to obtain additional data about the safety and efficacy of a product. This concept includes phase IV studies conducted in both the u.s. And in other countries. (12 Dec 1998) |