Cell Labeling

In order to understand the structures that make up a cell, it is vital that we can visualise and measure the life existing within cells. This is done through staining and labelling cells. By using fluorescent imaging or microscopy, for example, we are able to view a whole new world that exists all around us despite being invisible to the naked eye. So let's zoom in and have a look at cell labelling!

What is Cell Labelling?

Cells are the smallest units of life. They contain subcellular structures called organelles that carry out one or more specific jobs, similar to organs in a body.

Cells can be visualized using microscopes. There are different types of microscopes, the most popular ones being the light microscope and the electron microscope. These microscopes have different magnifications and resolving powers and are used with different goals in mind.

For instance, organelles visible under a light microscope include the nucleus, cytoplasm, cell membrane, chloroplasts, and the cell wall. Other organelles are smaller than the resolution of light microscopes, so they appear blurry and not well-defined under light microscopes.

Electron microscopes, on the other hand, are much stronger in magnification and resolution. This allows them to capture clearer images from the cells with much higher definition. Electron microscopes are essential for observing smaller organelles such as ribosomes, vesicles, granules, and filaments that cannot be seen directly with light microscopy. Therefore, electron micrographs of cells are detailed images of the cells in which organelles can be identified and labelled.

Cell Labelling Methods

Cells can be labelled in multiple ways. These include some of the most basic laboratory techniques like Gram staining, or more complex techniques like using fluorescent dyes, immunolabeling or fluorescent fusion proteins. These all give us ways to see the structures of a cell more easily.

We will have a look at the more advanced methods of cell labelling in this article, but you can check out our Gram-Staining article to learn more about this method!

Fluorescent Dyes as Cell Labelling Methods

Fluorescent dyes are biological molecules composed of at least one fluorophore. A fluorophore is a molecule that can emit light after getting excited by light energy. In other words, light reaches the fluorophore, which absorbs it and increases its energy. The fluorophore then emits light to liberate the extra energy it absorbed.

Fluorophores can be designed to bind to specific cell structures or components, and to absorb or emit light at a specific wavelength, allowing different types of fluorescent dyes to be combined in the same staining.

Fluorescent dyes can occur naturally in the living world, like the case of the Green Fluorescent Protein (GFP), or can be made synthetically. Synthetic dyes can be used to label biomolecules such as proteins, antibodies, peptides, nucleic acid, yeast and bacteria. These dyes include:

• Fluorescein IsoThioCyanate (FITC)
• Derivatives of rhodamine (TRITC)
• Coumarin
• Cyanine

Immunolabelling as a Cell Labelling Method

Fig. Shape of an antibody.

Cells are like cities. Similar to a city in which multiple sectors and departments work together for the city to thrive, cells also contain certain structures that carry out specific functions. These structures are called subcellular structures, and those subcellular structures that are membrane-bound are called organelles.

There are two main types of cells, eukaryotes, and prokaryotes. Prokaryotes, also known as bacteria, do not have any membrane-bound organelles. Their essential subcellular structures include the cell wall, plasma membrane, nucleoid (a circular DNA chromosome), and 70S ribosomes. Some bacteria contain more subcellular structures that are non-essential, such as flagella (singular flagellum), pili (singular pilus), plasmids, and a capsule. The table below summarises the structure and functions of these structures.

The rest of our discussion will be strictly on eukaryotes. These include animal, plant, and fungal cells. Eukaryotic cells are more advanced than prokaryotes and have more sophisticated subcellular structures. Going back to our previous analogy, comparing cells with cities, prokaryotes would be a small town while a eukaryote would be a large megacity. Eukaryotes are more than 100 to 10,000 times larger than prokaryotes and are much more complex.

All eukaryotes have a plasma membrane and some, such as plant cells, also have a cell wall surrounding them. The genetic material in eukaryotes is confined in the nucleus, which is a membrane-bound organelle. Eukaryotes also rely on ribosomes for protein synthesis, but the 80S eukaryotic ribosomes are larger than their 70S prokaryotic counterpart. Some other subcellular structures and organelles found in eukaryotic cells include mitochondria, chloroplasts, centrioles, rough and smooth endoplasmic reticulum, Golgi apparatus, and lysosome. The table below summarises the function of these structures.

All of these organelles can be stained using one or more of the methods described above, or another staining method not included in this article, like Gram-staining or DAPI staining.

Cell labelling diagram

Below are some examples of what cells labelled with fluorescent dyes might look like.

Hopefully, you now have more understanding of cell labelling and how useful it is!