2.4 Electron microscopy

Although it’s not the focus of this course, electron microscopy was mentioned because of its higher resolution and ability to study ultrastructure in detail. You will now be briefly introduced to two important types of electron microscope (EM).

1. Transmission electron microscopy: a beam of electrons is accelerated in a transmission electron microscope (TEM) at high velocity through a sample, which is a very thin section (less than 10 nm). Samples can range from tissue sections to purified protein complexes. Electrons cannot pass through glass; instead, magnets are used as the ‘lenses’ that control and focus the path of the electron beam. The interior of the microscope is under vacuum to prevent scattering of the electrons by air molecules. Electrons that have passed through the sample reach the detector, where they activate a fluorescent screen, or are captured with a digital camera, forming the image (Figure 8a and b). Electron microscopes first became available in 1939 and assisted in the discovery of organelles like the Golgi apparatus (Figure 8c).

2. Scanning electron microscopy: a technique for studying the surface of intact cells using a scanning electron microscope (SEM). The sample is first coated with a thin metallic layer that deflects an electron beam onto the detector, giving a very fine detail of the surface features (Figure 8d).

Figure 8 (a) A transmission electron microscope in the imaging facility at The Open University. (b) Diagram illustrating the components of an electron microscope. (c) Transmission electron micrograph of a frog leukocyte (white blood cell). (d) Scanning electron micrograph of a HeLa cell, illustrating that cells are not flat and that their surface has many extensions.

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The figure consists of four parts, Figures 8a to d, that illustrate the typical arrangement of a transmission electron microscope and micrographs of a frog leukocyte and HeLa cell.

Figure 8a: a photo shows a tall cylindrical transmission electron microscope (TEM) with a binocular eyepiece near its base just above the detector. The transmission electron microscope shows the following labelled parts from top to bottom: electron source, sample, and detector.

Figure 8b: the typical arrangement of a transmission electron microscope has an electron source located at the top, emitting a beam of electrons that pass through a set of magnets acting as lenses, a sample positioned on a middle sample holder, and more magnets acting as lenses. The beam of electrons diverges and hits the fluorescent screen to form a highly magnified image, which is then recorded by the detector present at the bottom.

Figure 8c: a transmission electron micrograph of a frog leukocyte shows a large densely stained nucleus with a nucleolus in its core. The nucleus is surrounded by numerous small oval-shaped organelles labelled ‘mitochondria’ and a horizontal stack of tubules and vesicles labelled ‘Golgi apparatus’ residing in the lightly stained cytoplasm.

Figure 8d: a scanning electron micrograph of a HeLa cell shows dividing nuclei surrounded by the cell membrane. The cell membrane has several surface extensions. The scale bar reads 20 micrometres.

Figure 8 (a) A transmission electron microscope in the imaging facility at The Open University. (b) Diagram illustrating the components ...

Electron microscopy can only be used in fixed cells. During the sample preparation, heavy metals are used to increase the contrast in the samples, for example to clearly see cellular membranes.