A scanning interference microscope arrangement

Quantitative Phase Imaging: Recent Advances and Expanding Potential in Biomedicine

Quantitative phase imaging (QPI) is a label-free, wide-field microscopy approach with significant opportunities for biomedical applications. QPI uses the natural phase shift of light as it passes through a transparent object, such as a mammalian cell, to quantify biomass distribution and spatial and temporal changes in biomass. Reported in cell studies more than 60 years ago, ongoing advances in QPI hardware and software are leading to numerous applications in biology, with a dramatic expansion in utility over the past two decades. Today, investigations of cell size, morphology, behavior, cellular viscoelasticity, drug efficacy, biomass accumulation and turnover, and transport mechanics are supporting studies of development, physiology, neural activity, cancer, and additional physiological processes and diseases. Here, we review the field of QPI in biology starting with underlying principles, followed by a discussion of technical approaches currently available or being developed, and end with an examination of the breadth of applications in use or under development. We comment on strengths and shortcomings for the deployment of QPI in key biomedical contexts and conclude with emerging challenges and opportunities based on combining QPI with other methodologies that expand the scope and utility of QPI even further.

A scanning microinterferometer with correction of errors caused by overlapping 'ghost' images

An experimental setup has been built to correct the errors caused by overlapping 'ghost' images when scanning and integrating measurements are carried out with the Jamin-Libedef interference microscope. First a reference field with the object, producing the 'ghost' image, is scanned and the values of the optical path difference (OPD) of each point in the reference field are stored in a computer. Subsequently, the OPD of the object to be measured is calculated for each point by adding the measuring result to the average OPD of five points, located around the corresponding point in the reference field. The applicability of the setup has been tested by measurements with Sepharose beads, Hela cells and the Zeiss microinterference refractometer. The difference between measurements on the same object with and without overlapping 'ghost' image was about 10%. The reproducibility was tested by repeated measurements on the same cell whereby a standard deviation of 1.1% was found.

Analysis of time dependent changes of lymphopoietic activity in organ cultures of embryonic chicken thymus

The time dependent development of lymphocytes in organ cultures of the thymus obtained from 10-day-old chick embryos was characterized by an initial phase of exponential increase of the number of lymphocytes per thymus followed by a plateau phase with no further increase in cell number. The proportion of cells in DNA synthesis dropped rapidly during the first 10 days of culture. Simultaneously the lymphocytes turned progressively smaller, as evidenced by both cell diameter and dry mass and constituted a homogeneous population of small cells at the end of the culture period. Thymic anlagen partially depleted of lymphoid precursor cells by a short hot pulse with 3H-TdR showed a prolonged exponential phase and reached normal plateau cell numbers 2-4 days later than usual. Furthermore, at least in the first part of the plateau phase, a reduction in the number of lymphoid cells per thymus resulted in a recovery in terms of the cell number which was associateed with increased DNA synthesis. These results are compatible with the regulation of thymic lymphopoiesis in organ culture through a mechanism operating via cell density.

I. Quantitative determination of the amount of DNA per nucleus by interference microscopy

A method for the determination of the DNA content of isolated nuclei of different ploidy has been developed. It is based on measurement of the nuclear dry mass, with an integrating microinterferometer, before and after DNase treatment. The values found are slightly low, because, as indicated by biochemical determinations, consistently 5% to 8% of DNA is not extracted by DNase under these conditions. The average DNA values thus obtained for diploid and tetraploid nuclei of adult rat liver are 7.7 and 15.6 pg (10(-12) g), respectively. Definite advantages of this procedure are: i) comparisons with biochemical determinations to give DNA values for each class of ploidy, ii) comparisons with histophotometry of the Feulgen dye-DNA complex to give absolute values instead of arbitrary units.

A comparison of nuclear dry weights determined by chemical and by interferometric methods

1. The nuclei of cells from the thymus of the calf were isolated by three different techniques; the citric acid, the sucrose-calcium chloride, and the non-aqueous. 2. The mean dry weights of the nuclei were determined by chemical methods and by microscopic interferometry. There was a close correlation between the results from the interferometric and chemical methods. 3. The range of values about that mean was determined in each sample: the nuclei isolated in aqueous media contained approximately 45 per cent less material than those isolated in non-aqueous media. 4. The variations in dry weight with varying nuclear type are discussed. 5. The possible relationship between DNA content and dry weight is discussed.