Imaging the inner workings of cells is of vital importance to our understanding of them, but it has never been an easy task, as conventional optical microscopes are unable to examine organelles and cell membranes in fine detail due to their diffraction limit. Electron microscopes are capable of higher resolution, but are not well-suited to many biological applications. Therefore, surpassing the diffraction limit of optical microscopes to obtain nanoscale-resolution has been an area of key interest over the past few decades, from which the field of super-resolution microscopy has emerged.
In a recent free-to-read Topical Review published in Journal of Physics: Condensed Matter, Dr. Erdinc Sezgin summarises the main super-resolution microscopy techniques and shows how these techniques have revolutionised our understanding of cell biology, with a particular focus on the cell membrane. We asked him to discuss the importance of this area of research below:
Cells, the building blocks of our body, usually have a size of tens of micrometres. The cell accommodates several sub-compartments called organelles which have distinct biological functions. Mitochondria, for instance, are responsible for energy production for the cell, and the nucleus is the home of genetic material. The Golgi Apparatus and Endoplasmic Reticulum are responsible for generation and transportation of several structural components of the cell. The interior of the cell, where all the organelles reside, is separated from the exterior by a complex structure called the plasma membrane. This plasma membrane is a selective barrier between the inside and outside of the cell: i.e., it only permits certain molecules to pass through. It is fairly thin (5-8 nanometres) but large in area, in order to cover the whole cell. Organelles are also fairly small structures: mitochondria for instance have a size of 1-3 micrometres (see the figure). These structures inside the cell are composed of several different types of molecules (lipids, proteins, carbohydrates and nucleic acids) that typically have the size of a few nanometres.
The scales we have been talking about are extremely small compared to what we can see with our naked eyes. Therefore, we need microscopes to be able to study them. However, even microscopes have limits that stem from the wave nature of the light. In the simplest picture, light can be considered to travel as straight rays, but on microscopic scales, light propagation is much more complex and is also governed by the laws of wave optics. Due to the diffraction of light – a phenomenon light undergoes when light waves interact with fringes of intricate objects – the “resolution” of conventional microscopes is limited.
Resolution is the minimum distance between two objects at which an optical device such as a microscope can distinguish two objects as two separate objects. If the objects are closer to each other than the resolution of the microscope, they are seen as a single object (see the figure). In general, the resolution of an optical system is approximately equal to half the wavelength of the light we use to illuminate the object. For instance, the wavelength of visible daylight is around 500 nanometres, which limits the resolution of an optical systems using visible light to approximately 250 nanometres. This means that two small objects closer to each other than 250 nanometres cannot be distinguished as two separate objects. In other words, we cannot resolve these structures. However, there are smaller objects than 250 nanometres (e.g., viruses), therefore we need better resolution to investigate them. To this end, there has been an incredible effort to develop so called “super-resolution microscopy” techniques. These techniques, by providing incredible resolution down to a few nanometres, revolutionised the field of cell and molecular biology. We are now less blind to the nanoscale cosmos inside the cell thanks to the light shed on it by super-resolution microscopy.
Read Dr. Sezgin’s full Topical Review for free HERE
Erdinc Sezgin is an EMBO and a Marie Skłodowska-Curie fellow at the MRC Weatherall Institute of Molecular Medicine, University of Oxford since 2014. He carried out his PhD work in the group of Petra Schwille at the Technical University of Dresden and a short postdoctoral period in Kai Simons lab at Max Planck Institute of Cell Biology and Genetics in Dresden, Germany. His current research is focused on the role of membrane heterogeneity in immune system.
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