A tour of the cell
A tour of the cell

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

A tour of the cell

Transmission electron microscopy

Transmission electron microscopy has been greatly refined since the first commercial electron microscope became available in 1939. TEM has allowed detailed examination of cell ultrastructure and assisted the identification and investigation of cell organelles such as the Golgi apparatus (Section 4.7), which had previously been seen only as indistinct subcellular structures using histochemical techniques and light microscopy.

As in light microscopy, samples must be fixed and processed before they can be viewed using a transmission electron microscope (Figure 2a) although the reagents used are different from those used for light microscopy. Preservation of the tissue is very important in this technique; it is crucial that the membranes of the cell organelles are preserved and not obscured by precipitates (insoluble deposits) formed during fixation. Glutaraldehyde, which fixes proteins, followed by osmium tetroxide, which fixes lipids, are the fixatives most often used. The fixed tissue is embedded in a very rigid resin, such as epoxy resin, which allows very thin sections to be cut on an ultramicrotome. 'Ultrathin' EM sections, which are used for study of cell organelles, are typically around 70 nm thick (compared with the 5-50 µm thick sections typically used for light microscopy). The image is obtained by passing electrons through the section, and focusing them on a fluorescent screen that emits visible light where electrons strike it. The interior of the microscope is under vacuum (to prevent scattering of the electron beam by air molecules) and the direction of the electron beam is controlled by magnets.

Described image
Figure 2 (a) A transmission electron microscope. (b) A transmission electron micrograph of a frog leukocyte (white blood cell). The nucleus and nucleolus (Section 4.3), mitochondria (Section 4.10) and Golgi apparatus (Section 4.7) can be seen. The dark area of the nucleus contains densely packed DNA.

Electrons pass readily through unstained tissue sections, because the cell components are made up of small atoms such as carbon, oxygen and hydrogen. To enable cellular components such as organelles to be viewed, the sections must first be 'stained' to increase contrast. However, unlike light microscopy, which uses coloured stains that absorb light, in TEM, the stains contain heavy metals, such as lead and uranium. These are large atoms that prevent electrons passing straight through the section. Uranium binds preferentially to nucleic acids and proteins, while lead binds preferentially to lipids. So, after staining, cell components that are rich in lipids and areas where proteins and DNA are concentrated prevent the passage of electrons and so appear relatively dark, or 'electron-dense', on the viewing screen. Areas in which proteins are less concentrated, such as the cytosol of animal cells, appear pale, or 'electron-lucent'. A typical TEM image, of a white blood cell, is shown in Figure 2b.

S294_1

Take your learning further371

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses372.

If you are new to university level study, we offer two introductory routes to our qualifications. Find out Where to take your learning next?373 You could either choose to start with an Access courses374or an open box module, which allows you to count your previous learning towards an Open University qualification.

Not ready for University study then browse over 1000 free courses on OpenLearn375 and sign up to our newsletter376 to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371