Viewing tiny objects, like cells, under a microscope is a real game of hide-and-seek with the light. It follows that the specimen must be carefully prepared, or ‘mounted’ on a slide. Here we get a little closer to the eukaryotic cell. The building block of life itself…
Once There Was A Cell…
Cells emerged on Earth at least 3.5 billion years ago.
The almost improbable beginning of life as we know it – the humble cell…
The “Father of Microbiology” is no doubt Dutch scientist Antonie van Leeuwenhoek (1632-1723) who first teaches himself to make optical lenses, then constructs his own basic optical microscopes.
He explores the microscopic world, reporting his amazing findings to the Royal Society, and draws protozoa, such as Vorticella from rain water, and even bacteria from his own mouth.
When Robert Hooke discovers cells in cork, and living plant tissue, with an early compound microscope, he coins the term cell in his ‘Micrographia or Some Physiological Descriptions of Minute Bodies made by Magnifying Glasses with Observations and Inquiries thereupon’ (1665).
But it isn’t until 1839 that German physiologist Theodor Schwann and botanist Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, a common unit of structure and development.
Units of Life
The cell is the smallest unit of life, consisting of cytoplasm enclosed within a membrane, which contains bio-molecules: proteins and nucleic acids (DNA and RNA).
Cytoplasm is about 80% water and usually colourless. The membrane contain a variety of biological molecules, notably lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms. And inside the cell, a whole lot more is going on…
Be it prokaryotic or eukaryotic, the cell is seemingly the basic structural, functional, and biological unit common to all known living organisms. The basic unit of structure and the basic unit of reproduction.
Wall or No Wall?
Many types of cells also have a cell wall that acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials, like cellulose in plants.
Unlike plant cells and bacteria, however, animal cells have no cell wall to support them structurally. And this interesting fact is well worth remembering when ‘mounting’ your specimen on microscope slides because of a cellular process called osmosis.
Eukaryotic cells undergo two distinct types of division:
- mitosis – a vegetative division, whereby each daughter cell is genetically identical to the parent cell and
- meiosis – a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes.
Prokaryotic cells undergo a vegetative cell division: binary fission, whereby their genetic material is segregated equally into two daughter cells.
Regardless of organism, all cell divisions are preceded by DNA replication – the biological process of producing two identical replicas of DNA from one original molecule.
Replication is initiated at particular points in the DNA, known as “origins“. As DNA synthesis proceeds, the original DNA strands continue to unwind on each side of the bubble, forming a replication fork with two prongs.
Eukaryotic DNA strands have long linear chromosomes and initiate replication at multiple origins within these. Bacterial DNA has a single origin of replication on the circular chromosome, and their replication creates a “theta structure“.
Organisms can be classified as unicellular, i.e. consisting of a single cell. This includes bacteria.
They can be multicellular, like plants and animals.
While the number of cells in plants and animals varies from species to species, human beings contain over 10 trillion (1013) cells. In fact, humans are an excellent example of multi-cellular organisms because
WE are 10 trillion human cells…
100 trillion bacteria!
With dimensions between 1 to 100 micrometres (μm), most eukaryotic cells (plants and animals) are visible only under a microscope.
These are the very basics about the ‘humble’ (or rather ‘mighty’) cell.
The study of very small objects, like cells, with a compound light microscope, requires the object, or specimen under scrutiny, to be carefully prepared, and sometimes stained to increase the contrast between the object and its background.
We now look at mounting specimens on slides.
And why we need to do it that way.
Preparing Slides for the Microscope
This technique is only really suitable for looking at macroscopic objects: small insects, crystals, coin inscriptions, hallmarks…
Step 1 – Obtain the specimen to be used
What are you going to mount? Select the specimen to be used.
Specimens for use in Biology 101 often include: onion skin, Elodea leaves or epithelial cells…
Step 2 – Prepare thin sections
Prepare thin sections of plant or vegetable matter using a microtome or a very sharp cutting blade. Keep your thin sections well hydrated. You can try different techniques and see what works best, e.g. freezing the specimen, before slicing it. Most of the time, practice makes perfect.
Step 3 – Obtain a clean microscope slide
You can use the standard:
- microscope slide (25.4 x 76.2 mm) (1 – 1.2 mm thick)
- glass cover slip (22 x 22 mm) (0.13 – 0.16 mm thick)
Or you can procure concave depression slides if examining a drop of fluid.
Step 4 – Place a drop of liquid at the centre of the slide.
That’s the “wet” part of the wet mount. The liquid used depends on the type of cell being viewed (see below). Use tweezers to transfer the thin section into the liquid drop on the slide.
Step 5 – Stain the specimen to be used
If the specimen is transparent, a drop of iodine or methylene blue can improve the contrast.
Do not use stain if you’re viewing photosynthetic cells (already green with chlorophyll), or living organisms (because it will kill them).
Step 6 – Place a cover slip on
To avoid trapping air bubbles, set one edge of the cover slip on the large microscope slide, and let the rest of the cover slip drop to sandwich the specimen. As the cover slip drops from one side to the other, air will be pushed out and reduce the number of bubbles.
Now that you’ve got a slide ready for viewing, remember you are playing a game of hide-and-seek with light!
The reason why you need to make very thin sections is to allow as much light as possible to travel through the specimen. And that’s also why you need to use a glass microscope slide and cover slip.
Cells are often called the “building blocks of life“. This article may have just whet your curiosity about the invisibly small, and perhaps it will see you off on a personal voyage into microscopy. Then, it’s a good thing. The World needs more scientists. Because there are many more building blocks to life than meets the eye, Horatio…