2017) allow the observation of a cryo-fixed specimen in a more advanced LM at better resolution and in an EM with high acceleration voltage. 2016) and Leica cryo-CLEM System (Hampton et al. In comparison, separated CLEM systems such as the CorrSight system equipped with a cryo module from Thermo Fisher Scientific Inc. The iCorr system thus can only have fluorescence signals at the green channel with a low resolution of about 500 nm (Wang et al. Within the iCorr system, however, only small light objective lens as 15×/0.5 NA can be integrated due to the limited space between the pole pieces of a standard Tecnai Spirit TEM. The experiments efficiency is greatly increased without repeated grid transfers (Agronskaia et al. The first integrated CLEM system called Tecnai with iCorr (Thermo Fisher Scientific Inc.) allows the stage to be tilted 90° to switch between the LM and EM mode for direct correlation of a specimen in situ, preventing the specimen from potential contamination and damage. 2010).įor the second type of CLEM method, two different designs are commercially available. While the first type of CLEM method provides convenience of use and better resolution of LM, the second type guarantees more accurate correlation and is more favored (Briegel et al. 2010 Schorb and Briggs 2014 Schorb et al. Special specimen stages designed for the related instruments are essential to preserve the cryo-fixed specimens (Li et al. Such a practice needs the cryo-fixed specimen to be maintained always in a humidity-free environment and below ~−140 ☌ during the whole CLEM process to prevent ice contamination on the specimen surface or recrystallization in the specimen. The sample is plunge frozen or cryo-sectioned first, then the grid is visualized under LM and EM sequentially with little disturbance of its structure, thus making the correlation more consistent. (2) Correlative LM/EM with freezing before FM imaging (Briegel et al. Therefore, it works better for specimen that is relatively stable and the fluorescently marked structure would not change in the seconds to minutes before vitrification. Various factors can be introduced to the specimen between the LM imaging and specimen vitrification, causing more difficulties for accurate correlation. After FM imaging, the sample needs to be frozen for further cryo-EM imaging. FM can have better resolution with high-powered, oil-immersion lenses with large numerical apertures. FM imaging of samples at room temperature could be done with any typical FM instrument. Currently, two types of methods are used for correlation: (1) correlative LM/EM with freezing after FM imaging (Briegel et al. Cryo-CLEM is thus developed in order to visualize vitrified specimens by cryo-EM (Bykov et al. 2014), vitrified biological specimens become more and more popular to avoid the potential artifacts and damages caused by the chemical fixation. 2016 Kong and Loncarek 2015 Loussert Fonta and Humbel 2015 Mourik et al. While the chemical fixed specimens are still the majority of targets investigated by CLEM (Hellstrom et al. 2012).ĬLEM technique was first reported in the early 1960s for adenovirus study with conventional chemical fixation EM specimen preparation (Godman et al. The advantages of specific labeling and localization in LM and high spatial resolution in EM can be combined, leading to the development of a powerful technique as the correlative light and electron microscopy (CLEM) (Anderson et al. Light microscopy (LM), more specifically, fluorescence microscopy (FM) is well developed to label specific molecules and localize them in a large viewing area with a resolution of several hundreds of nanometers. Furthermore, the large dimension of a cell and a rather small viewing area of EM at a high magnification render it more difficult to localize relatively sparse molecules in a specific cellular structure environment (Plitzko et al. The major hurdle lies in the fact that many different kinds of macromolecules are crowded within the cell and therefore are hard to be distinguished from each other simply by their shapes revealed in cryo-EM (Bauerlein et al. 2008 Oikonomou and Jensen 2017 Plitzko et al. However, it is still very challenging to identify and determine the structures of interested macromolecules in situ with cryo-electron microscopy (cryo-EM) (Lucic et al. In combination with the most recent hardware and software breakthroughs, cryo-fixed macromolecule complexes isolated from cells can be determined at near atomic resolution using the single particle reconstruction method (Ma et al. Vitrification of biological specimens in liquid nitrogen (LN2) temperature has been proved to be a powerful technique to preserve the native structures of macromolecules either in vitro or in situ.
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