Laser scanning microscopy of surface spread polytene chromosomes

Analyzing the super-resolution characteristics of focused-spot illumination approaches

SIGNIFICANCE

It is commonly assumed that using the objective lens to create a tightly focused light spot for illumination provides a twofold resolution improvement over the Rayleigh resolution limit and that resolution improvement is independent of object properties. Nevertheless, such an assumption has not been carefully examined. We examine this assumption by analyzing the performance of two super-resolution methods, known as image scanning microscopy (ISM) and illumination-enhanced sparsity (IES).

AIM

We aim to identify the fundamental differences between the two methods, and to provide examples that help researchers determine which method to utilize for different imaging conditions.

APPROACH

We input the same image datasets into the two methods and analyze their restorations. In numerical simulations, we design objects of distinct brightness and sparsity levels for imaging. We use biological imaging experiments to verify the simulation results.

RESULTS

The resolution of IES often exceeds twice the Rayleigh resolution limit when imaging sparse objects. A decrease in object sparsity negatively affects the resolution improvement in both methods.

CONCLUSIONS

The IES method is superior for imaging sparse objects with its main features being bright and small against a dark, large background. For objects that are largely bright with small dark features, the ISM method is favorable.

Visualization of the microtubules of glutaraldehyde-fixed cells by reflection-enhanced backscatter confocal microscopy

Performing reflection-mode (backscatter-mode) confocal microscopy on cells growing on reflective substrates gives images that have improved contrast and are more easily interpreted than standard reflection-mode confocal micrographs (Keith et al., 1998). However, a number of factors degrade the quality of images taken with the highest-resolution microscope objectives in this technique. We here describe modifications to reflection-enhanced backscatter confocal microscopy that (partially) overcome these factors. With these modifications of the technique, it is possible to visualize structures the size-and refractility-of individual microtubules in intact cells. Additionally, we demonstrate that this technique, in common with fluorescence techniques such as standing wave widefield fluorescence microscopy and 4-Pi confocal microscopy, offers improved resolution in the Z-direction.

Potential clinical uses of laser scan microscopy

Compared with conventional light microscopy (LM), confocal laser scan microscopy (CLSM) is characterized in part by increased light sensitivity and higher spatial resolution. These characteristics permit the detection and discrimination of minor changes in cells and tissues in a more specific, sensitive, and accurate way. Potential clinical applications related to these characteristics of CLSM are discussed. Preliminary data indicate that the higher light sensitivity of CLSM allows detection of low amounts of proto-oncogene mRNA which are minimally detectable by LM. Evaluation of Grimelius-stained sections of lung cancer by CLSM with antiflex permits detection of previously undetected granula. The higher spatial resolution of CLSM may prove to be essential for accurate assessment of nuclear shape, which is of prognostic importance in tumors of a variety of organs. When applied to cancer research and diagnosis or other relevant fields, CLSM allows improved insight in tumor cell biology and may lead to improved diagnostic pathology. Consequently, the use of CLSM could lead to more refined therapeutic indications.

Laser scanning fluorescence microscopy

Fluorescence laser scanning microscopy (LSM) offers many advantages over conventional fluorescence microscopy. Very strong excitation light can be concentrated on small spots (0.5 microm) of the specimen, enabling the detection of low concentrations of fluorescent substances. The low levels of autofluorescence generated in the microscope objective and in the immersion oil in LSM provide images of great contrast, even with weakly fluorescent specimens. Confocal LSM permits the visualization of multiple focal layers of thespecimen and 3-D image reconstructions. Combination of images stored in computer memory allow the comparison of phase contrast and fluorescence images of the same area of the specimen enabling multiparameter analysis of cells.