Name: electron microscopy sample prep methods
Uploaded: 2023-04-30T12:01:16.000Z
Duration: 1 min 20 s
Channel: JoVE

Биологические образцы должны быть подготовлены для электронной микроскопии, чтобы защитить образец от повреждения мощным электронным пучком и низким давлением, создаваемым вакуумом.
Подготовка образца начинается с химической фиксации и полного обезвоживания. Химические фиксаторы, такие как глутаральдегид или формальдегид, сшивают белки, в то время как тетраоксид осмия часто используется для фиксации мембранных липидов.
Затем образец постепенно обезвоживают с увеличением концентрации этанола или ацетона, чтобы избежать усадки образца.
Для ПЭМ образцы погружаются в твердую эпоксидную смолу. Затем ультрамикротом разрезает образец на ультратонкие поперечные сечения, через которые могут проходить электроны.
Эти участки окрашены тяжелыми металлами, такими как уранилацетат. Молекулы, которые связывают металлы, кажутся более темными и обеспечивают контраст с изображением.
Для SEM образцы дополнительно высушиваются с использованием жидкого углекислого газа в процессе, называемом сушкой в критических точках. Затем образцы покрываются тонким слоем проводящего металла, такого как золото, что предотвращает термическое повреждение образца и обеспечивает лучшее качество изображения.

Подготовка образцов для электронной микроскопии

Preparation of Samples for Electron Microscopy

Для визуализации с помощью электронного микроскопа, просвечивающего или сканирующего, биологические образцы необходимо зафиксировать (стабилизировать), чтобы электронный луч не разрушил их, и тщательно высушить (обезводить), чтобы на них не влиял вакуум. Фиксацию необходимо производить как можно быстрее, поскольку свойства образца начнут меняться, как только он будет удален из естественной среды. Например, в образце ткани уровень кислорода начинает снижаться, вызывая изменение внешнего вида митохондрий. Достижения в подготовке проб ПЭМ Криофиксация В этом методе мелкие клетки и органеллы можно заморозить методом погружной (жидкостной) заморозки, а более крупные клетки и ткани можно заморозить под высоким давлением. Погружное замораживание достигается путем помещения небольшого количества образца на холодную ЭМ-решетку, а затем быстрого погружения в жидкий этан, заполненный в резервуаре и окруженный жидким азотом. Быстрое замораживание гарантирует, что вода не кристаллизуется. При замораживании под высоким давлением жидкий азот используется для быстрого замораживания образца и подвергается давлению ~ 2000 бар для замедления образования кристаллов льда. Криофиксированные образцы разрезаются на криоультрамикротоме, состоящем из замороженного алмазного ножа. Отбрасывание теней Вращающаяся платформа в вакуумной камере удерживает образец, который затем покрывают тяжелым металлом. Вращение образца обеспечивает металлическое покрытие с лицевой стороны и эффект тени с обратной стороны. Этот метод особенно полезен при изучении деталей ультраструктуры бактерий или вирусов, ДНК, РНК или изолированных белков. Негативное окрашивание Небольшие образцы, такие как рибосомы, молекулы ферментов, вирусы и т. д., можно визуализировать напрямую, без секционирования. Образцы смешивают с молибдатом аммония, уранилацетатом или фосфорновольфрамовой кислотой, которые являются электронно-непрозрачными. На сетке образуется металлический слой, за исключением места, где находится образец, создавая негативное изображение образца. Контраст достигается за счет характера распределения металлов. Подготовка высушенных и невысушенных образцов для СЭМ СЭМ может отображать три типа образцов, а именно: (i) высушенные и проводящие, (ii) высушенные и непроводящие и (c) невысушенные (влажные). Высушенные естественно проводящие образцы можно легко смонтировать и визуализировать. Однако, высушенные и непроводящие образцы необходимо покрыть проводящим слоем, чтобы защитить образец от перегрева и улучшить качество изображения. Эти образцы делают проводящими, покрывая образец тонким слоем золота с помощью устройства для напыления. Оно использует ионы аргона, чтобы выбить атомы золота из золотой пластины и покрыть поверхность образца золотом, делая ее проводящей для электронов. Влажные образцы могут выделять газы в вакууме микроскопа. Мокрый СЭМ, также называемый СЭМ окружающей среды или ESEM, должен контролировать сублимацию воды в образце во время визуализации. Это достигается путем предварительного покрытия образца тонким слоем мембранного полимера, прозрачного для электронных лучей и отделяющего влажный образец от вакуума. Затем температуру понижают, что снижает давление насыщенного пара. Эти условия позволяют работать с прибором при более высоком вакууме, что сохраняет ультраструктурные особенности образца и снижает шум. Deben Coolstage также можно использовать для визуализации влажных образцов путем их замораживания до -25 ° C, что сохраняет структуру образца и предотвращает выделение газа во время бомбардировки его электронами.

Для визуализации с помощью электронного микроскопа, будь то просвечивающего или сканирующего, биологические образцы должны быть зафиксированы (стабилизированы) таким образом, чтобы электронный луч ...

preparation-of-samples-for-electron-microscopy

Education

JoVE Core

Cell Biology

Клеточная биология

Unit 1

Раздел 1

Visualizing Cells, Tissues, and Molecules

Визуализация клеток, тканей и молекул

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

electron microscopy sample prep methods

Electron microscopy sample prep methods help biological samples survive imaging in TEM and SEM. Before a specimen can be viewed, it must be fixed, or stabilized, so the electron beam does not damage it. It also must be dried well, or desiccated, because the microscope uses a vacuum.

Fixation should happen as quickly as possible. Once a sample leaves its natural environment, its properties begin to change. In tissue samples, oxygen levels can drop and change the appearance of mitochondria. That is why early stabilization matters.

TEM sample prep can use cryofixation, shadow casting, or negative staining. In cryofixation, small cells and organelles are plunge frozen on a cold EM grid in liquid ethane surrounded by liquid nitrogen. Larger cells and tissues can be frozen by high-pressure freezing with liquid nitrogen and about 2000 bar of pressure. This slows ice crystal formation. Cryofixed samples are then cut in a cryo-ultramicrotome with a frozen diamond knife.

Shadow casting places the sample on a rotating platform in a vacuum chamber and coats it with heavy metal. The rotation helps cover the front side and creates a shadow effect on the back side. This method is useful for ultrastructural details in bacteria, viruses, DNA, RNA, and isolated proteins. Negative staining is used for small samples such as ribosomes, enzyme molecules, and viruses. The sample is mixed with an electron-opaque stain such as ammonium molybdate, uranyl acetate, or phosphotungstic acid, which leaves a metal layer around the sample and forms a negative image.

SEM sample prep depends on whether the sample is desiccated, conductive, non-conductive, or wet. Naturally conductive dry samples can be mounted and imaged directly. Dry but non-conductive samples need a thin gold coating from a sputter coater. Argon ions knock gold atoms off a gold plate and deposit them on the sample surface. This makes the surface conductive and helps prevent overheating while improving image quality.

Wet samples can outgas in the vacuum of the microscope. For these specimens, wet SEM, also called Environmental SEM or ESEM, controls water sublimation during imaging. One approach uses a thin membranous polymer that is transparent to electron beams and separates the wet specimen from the vacuum. The temperature is then lowered, which reduces saturated vapor pressure. A Deben Coolstage can also image wet samples by freezing them to -25°C, which preserves structure and limits outgassing while electrons strike the specimen.

Electron Microscopy Sample Prep Methods

imaging biological samples with optical microscopy

imaging-biological-samples-with-optical-microscopy

How Light Microscopes Bring Samples Into View

Optical microscopy uses light and lenses to make tiny biological samples visible. Antonie van Leeuwenhoek built the first compound optical microscope in the 17th century to view blood cells, bacteria, and yeast cells. In 1830, Joseph Jackson Lister developed an essentially modern light microscope. During the 20th century, microscopes gained better magnification and resolution. A sample is placed on a glass slide and clipped onto the stage, which is the flat platform of the microscope. The...

phase contrast and differential interference contrast microscopy

phase-contrast-and-differential-interference-contrast-microscopy

Microscope Contrast for Live, Unstained Cells

Phase-contrast and differential interference contrast microscopy help scientists see live cells without stains. Both methods boost contrast so thin, transparent structures become easier to study under the microscope. Phase-contrast microscopes use interference between light that passes straight through a cell and light that is refracted by cell parts. The microscope uses an annular stop in the condenser to make a hollow cone of light. The objective lens also contains a phase plate with a phase...

fixation and sectioning

Microscope Slide Prep and Staining

Microscope slide preparation includes wet mounts, fixation, and staining. These steps help make specimens easier to see under a light microscope. They also help preserve cells and highlight important structures. A wet mount is the simplest slide preparation. The specimen is placed in a drop of liquid on the slide. A liquid sample can be added directly with a dropper, and a solid sample, such as a skin scraping, can be placed on the slide before the liquid is added. The liquid is sometimes...

immunofluorescence microscopy

Fluorescent Antibody Staining in Cells

Fluorescent antibody staining uses a fluorescence microscope to make cells and microbes visible in bright colors against a dark background. The microscope works with fluorochromes, which are fluorescent dyes that absorb energy from a light source and then give off visible light. Some fluorochromes are natural, such as chlorophylls. Others are added to the specimen to improve contrast. Common examples of fluorochromes include Texas red and FITC. The method can also use nucleic acid dyes such as...

immunocytochemistry and immunohistochemistry

immunocytochemistry-and-immunohistochemistry

Antibody Staining in Cells and Tissues

Immunocytochemistry (ICC) and immunohistochemistry (IHC) use antibodies to detect specific proteins or antigens in samples. ICC focuses on individual cells, such as blood cells and stem cells. IHC is used for tissue samples. These methods were first published by Albert Coons in 1941, when they were used to detect pneumococcal antigen in mouse tissue sections from mice infected with Pneumococcus. Both techniques can produce either a colored signal or a fluorescent signal. Enzyme-conjugated...

confocal fluorescence microscopy

Confocal Microscopy for Sharp Cell Images

Confocal microscopy is an advanced imaging method used to capture sharp fluorescence images. Its main advantage is that it blocks out-of-focus light from the sample with pinholes. That helps produce high-resolution images with strong contrast. Unlike a standard optical microscope, a confocal microscope uses a focused laser beam to scan the sample surface at different z-planes. This makes it especially useful for thick specimens, such as biofilms. These samples can often be examined alive and...

protein dynamics in living cells

Fluorescent Methods for Protein Movement

Protein movement in living cells can be measured with fluorescence-based methods such as FRAP, FRET, and PET. These techniques help scientists track how proteins move, interact, and change distance inside the cell. Fluorescent recovery after photobleaching, or FRAP, measures protein movement rates within the cell. In this method, a small region is exposed to a strong laser beam. The beam permanently bleaches fluorophore-tagged proteins in that area. Over time, bleached proteins diffuse away...

total internal reflection fluorescence microscopy

total-internal-reflection-fluorescence-microscopy

Surface-Only Imaging with TIRF Microscopy

Total internal reflection fluorescence microscopy, or TIRF, is a microscope method used to view fluorophores near a solid surface with a higher refractive index, such as a glass coverslip. It is useful when scientists want to study signals close to the cell surface. TIRF works when light from a medium with a lower refractive index, such as air, hits the glass coverslip at a critical angle. Instead of entering the glass, the light undergoes total internal reflection. This reflection creates an...

atomic force microscopy

Atomic Force Microscopy for Surface Mapping

Atomic force microscopy is used to map the surface details of materials. It can examine ceramics, glass, polymers, and biological samples. The method gives far more resolution than an optical imaging system and produces three-dimensional surface profiles instead of flat 2D images. The probe is the key part of an AFM setup. It includes the cantilever and tip assembly, and it is also the part that is replaced most often. Constant contact with samples wears down the tip, so probe choice depends...

super-resolution fluorescence microscopy

Sharper Cell Images with Fluorescence Microscopy

Super-resolution fluorescence microscopy gives sharper images than standard fluorescence microscopy by reducing the point spread function, or PSF. The PSF is the light pattern from a point source that makes it look blurred. Because of this blur, each fluorescent spot appears larger than it really is. Nearby fluorophores can also overlap through their PSFs and make the image less clear. Several super-resolution methods have been developed to improve image detail. Photoactivated localization...

overview of electron microscopy

Electron Microscopy: TEM vs SEM

Electron microscopy uses electrons instead of visible light to make images with much higher detail than a light microscope. Visible light limits the resolution of light microscopes, which usually magnify up to 1000X, and sometimes up to 1500X. Electrons have a much shorter wavelength, about 0.005 nm, so they can produce resolution as fine as 0.05 nm. An electron microscope can also magnify images up to 2,000,000X and reveal subcellular structures and some molecular structures, including single...

scanning electron microscopy

How SEM Reveals Surface Details

A scanning electron microscope (SEM) reveals the surface details of a sample with an electron beam. The beam scans across the sample in a two-dimensional pattern. SEM can image areas from about 1 centimeter down to 5 micrometers wide. SEM is useful for studying bacteria, viruses, tissues, and larger samples such as insects. Conventional SEM can produce magnification from 20X to 30,000X. It also offers spatial resolution of about 50 to 100 nanometers. The process starts when the electron gun...

transmission electron microscopy

How TEM Reveals Tiny Structures

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope. In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam...

preparation of samples for electron microscopy

Electron microscopy sample prep methods help biological samples survive imaging in TEM and SEM. Before a specimen can be viewed, it must be fixed, or stabilized, so the electron beam does not damage it. It also must be dried well, or desiccated, because the microscope uses a vacuum. Fixation should happen as quickly as possible. Once a sample leaves its natural environment, its properties begin to change. In tissue samples, oxygen levels can drop and change the appearance of mitochondria. That...

immunogold electron microscopy

Immunogold Labeling in Electron Microscopy

Immunogold labeling in electron microscopy uses antibodies and tiny gold particles to find specific proteins in cells and tissues. The gold particles are very electron-dense, which means they show up clearly in electron microscope images. This gives researchers a high-resolution way to see where proteins are located and to compare their amounts under different stimulation conditions. The method can reveal proteins on the cell surface and inside cells. It has been used to study proteins...

cryo-electron microscopy

Cryo-EM for Native Biomolecule Imaging

Cryo-EM, or cryo-electron microscopy, lets scientists image biomolecules in their frozen, native state. It avoids the dehydration, fixation, and staining used in conventional electron microscopy, which can distort samples and create artifacts. It also reduces damage from the electron beam by using gentler imaging conditions. This method is useful when other structure tools have limits. X-ray diffraction needs a molecule to form a crystal, and that is not always possible. Even when...

electron microscope tomography and single-particle reconstruction

electron-microscope-tomography-and-single-particle-reconstruction

3D Imaging with Electron Microscopy

Transmission electron microscopy (TEM) can build 3D images of biological samples. It does this with electron microscope tomography and single-particle reconstruction. Tomography can show cell parts or small cells in vivo, while single-particle reconstruction is used for macromolecules and macromolecular complexes in vitro only. Electron tomography can be done in TEM or in STEM, which stands for scanning transmission electron microscopy. STEM is mainly used for thick biological specimens. It...

key terms and concepts

Learn how biological samples are fixed, dried, coated, or frozen for TEM and SEM, including cryofixation, negative staining, and wet sample imaging.

Electron Microscopy Sample Prep Methods | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

Electron Microscopy Sample Prep Methods

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