University of Zurich objective offers versatile imaging of tissues

04 Apr 2023

Design based on Schmidt telescopes and the eyes of scallops could enhance several microscopy techniques.

A project at the University of Zurich has designed an objective for light microscopy based on the reflective principles used in Schmidt telescopes and also found in nature in the eyes of scallops.

The new objectives could assist high-resolution imaging of tissues and organs in a wider variety of immersion media than conventional immersion microscope lenses, according to the project.

Published in Nature Biotechnology, the research was targeted in particular at the imaging of large cleared samples, where some of the tissues that would normally inhibit imaging or block the passage of photons have been rendered transparent by suitable chemical transfusions.

Imaging of these samples often requires the use of immersion media with a range of refractive index, which can limit the performance of any one microscope objective or require highly complex and expensive objectives containing multiple individual lenses.

The Zurich project tackled the problem by designing an immersion objective based on a mirror instead of lenses. Reflection by the mirror is independent of the refractive index of the medium the mirror is in contact with, and light will be focused to the same point whether immersed in water or other fluids.

"Although, to our knowledge, such a design has not been previously applied in bioimaging, it is found in nature, where each of the hundreds of eyes of scallops contains a curved mirror to form an image," commented the project in its published paper.

"Each mirror is in direct contact with the liquid-filled space containing the photoreceptors. Each eye also contains a lens, which is not, however, the primary image-forming element. This combination of a lens and a mirror is reminiscent of the optical design of the Schmidt telescope, a telescope design with a spherical mirror and an aspherical correction plate, commonly used in astronomy since the 1930s."

Improved image quality compared to conventional objectives

As proof-of-concept the project designed and constructed a component suitable for use as a multi-photon objective, a relatively straightforward implementation of the optical principle, and applied it to the imaging of both cleared and living samples including mouse brains, tadpoles and chicken embryos. The objective was also able to analyze cleared human brain samples, and to image neuronal activity in larval zebrafish in vivo.

"In all cases, the image quality was equivalent to or even better than that achievable with conventional objectives, even though the Schmidt objective consists of only two optical elements," commented Zurich's Fritjof Helmchen.

Offering an alternative to expensive and complex multi-lens objectives, the design could be incorporated into several different imaging modalities, including wide-field and confocal microscopy. The project indicated that light-sheet microscopy may well represent the most promising initial area for multi-immersion Schmidt objectives, once the design is optimized.

The Zurich team predicts that its design will allow mesoscope objectives able to perform high-resolution imaging across centimeter-scale field of views and capable of extremely high-throughput imaging, for the examination of tumor tissues or the detection of neurological diseases.

"In this respect, scallops could show us the way to improved medical diagnostics," commented Helmchen.