When it comes to scientific, technological, and medical progress, electron microscopy (EM) has proven invaluable. Its use has ably equipped scientists all over the world to broaden their understanding of life and the environment.

To date, it’s the only technique with an adequate resolution for localizing proteins to small membrane subdomains within the context of the cell. Procedural and technological advances have increased the power of EM as a cell-biological tool capable of producing a high-quality cell diagram.

In this article, we’ll illustrate how looking at the structure of cells is made possible with an electron microscope.

The Cell

Before proceeding, let’s first establish the importance of the cell.

The cell is the fundamental unit of all living organisms. They grow, adapt to their surroundings, and reproduce, all of which are processes that characterize life.  Cells also form groups that turn into complex structures. A good example is our body’s tissues which are made up of cells and the extracellular material they produce. Various types of tissues then come together to form organs. The cells in each of these organs communicate and collaborate to perform critical bodily functions.

The cell membrane, also known as the plasma membrane, serves as the cell’s borders. The nucleus and the cytoplasm surrounding the nucleus are the two main compartments of the cell. Organelles, cytosol, and inclusions are sub-portions of cytoplasm. Organelles are groups of specific macromolecules that work together to perform complex functions.

Cell Dimensions

The naked eye can only see cells that are massive in size, such as the human ovum, which has a diameter of 100 micrometers (µm).

When it comes to understanding the dimensions of cells and their components on a subcellular level, there are three main points to keep in mind:

• Most eukaryotic cells are 7-20 microns (µm) in diameter, while prokaryotic cells are smaller (0.2 – 5 µm).
• Red blood cells have an average diameter of 7.2   µm and can be used as a reference to approximate size.
• secretory granule has a diameter of about 1 µm.

Since not all cells can be seen with the unaided eye, we must use instruments, such as a microscope, to visualize cells in a tissue. Microscopic images can be produced by a light microscope with magnifications up to 400x or an electron microscope (with magnifications up to 500000x).

Given these capacities, light microscopes enable the visualization of cells and their more sizeable components, such as nuclei, nucleoli, secretory granules, lysosomes, and large mitochondria. Meanwhile, an electron microscope is required to see smaller organelles such as ribosomes, macromolecules, and macromolecular assemblies.

The Role of Electron Microscopy in Looking at Cell Structures

Electron microscopy is used to obtain high-resolution images of biological and non-biological specimens. In biomedical research, it is used to investigate the detailed structure of tissues, cells, organelles, and macromolecular complexes.

Using electrons (which have very short wavelengths) as the source of illuminating radiation results in the high resolution of EM images. Electron microscopy is used together with different ancillary techniques (e.g., thin sectioning, immuno-labeling, negative staining) to answer specific questions.

The resulting EM images provide critical information about the structural basis of cell function and cellular disease.

There are two types of electron microscopes: transmission EM (TEM) and scanning EM (SEM).

Transmission Electron Microscope

A TEM examines thin specimens (tissue sections, molecules, etc.) through which electrons can pass, generating a projection image. The TEM is similar to a conventional (compound) light microscope in many aspects. Some of its primary uses include:

• Imaging of the interior of cells (in thin sections)
• Structuring of protein molecules (as opposed to metal shadowing)
• Organizing of molecules in viruses and cytoskeletal filaments (as prepared by the negative staining procedure)
• Arranging protein molecules in cell membranes (through freeze-fracture).

Scanning Electron Microscopy

Conventional SEM is based on the emission of secondary electrons from a specimen’s surface. A scanning electron microscope is the EM equivalent of a stereo light microscope due to its extensive depth of focus. It can produce detailed images of the surfaces of cells and whole organisms that TEM cannot. It can also be used for particle counting, determining particle size, and controlling processes. The image is formed by scanning a focused electron beam onto the surface of the specimen in a raster pattern, hence the name scanning electron microscope.

Recent developments in SEM resulted in a wealth of new applications for cell and molecular biology and other related biological disciplines. Specifically, new sample preparation techniques partnered with the development of new instruments are some of the key factors in the emergence of this versatile research tool.

Specific Applications of SEM in the Field of Bioscience

SEM is used in cell and molecular biology to investigate the 3D structure (shape, size, structure, and form) of cells and tissues.

This instrumental method in microbiology allows for the study of viruses and bacteria, as well as their biological interactions. It’s also an essential tool for studying the effects of antibiotics on bacterial morphology.

SEM is essential in medicine, particularly in determining the nature and mechanisms of a particular disease to make an accurate diagnosis and develop appropriate medications.

Finally, an SEM can also be used to analyze trace evidence (such as hairs, fingerprints, blood, other biological substances, gunshot residue, fibers, and so on) and other factors to learn more about the events surrounding a specific crime and provide solid evidence for legal purposes.

Bottomline

Due to the light microscope’s limited resolution, analyzing the details of cell structure has necessitated the use of more powerful microscopic techniques, particularly electron microscopy.

Frequently Asked Questions

How are cell structures examined in electron microscopy?

A beam of electrons moves back and forth across the surface of a cell in an SEM, creating relevant information about cell surface characteristics. Meanwhile, in a TEM, an electron beam penetrates the cell and provides information about the internal structures of the cell.

What particular cell structures can be observed using an electron microscope?

The nucleus, cell wall, vacuoles, endoplasmic reticulum, Golgi Apparatus, ribosomes, and mitochondria are some of the easily visible structures of the cell when viewed through an EM.

What makes electron microscopes useful when looking at parts of a cell?

Since electrons have a much shorter wavelength than visible light, electron microscopes can produce higher-resolution images than regular light microscopes. These  microscopes can be used to examine whole cells but also the compartments and subcellular structures within them.