Slide Preparation and Biological Drawings

Slide preparation is the process of mounting a thin, stained specimen under a cover slip so that it can be examined with a microscope, and a biological drawing is an accurate, labeled record of what is observed. In brief: a good slide has a thin specimen, a drop of liquid, a cover slip lowered at an angle to avoid air bubbles and, if needed, a stain such as iodine. A good drawing uses clear pencil lines, no shading, a title, labels with ruled lines and a stated magnification.

Observing cells is a core skill in biology, and both the preparation and the record must be done carefully. This guide shows how to make a wet-mount slide step by step, how stains help, how to draw what you see, how to calculate magnification of a drawing and how to avoid common errors.

How do you prepare a microscope slide?

A temporary wet mount is the usual method in school. The specimen must be thin enough for light to pass through, usually one cell layer thick.

1. Clean a glass slide and a cover slip, and handle them by the edges.
2. Place a small drop of water, or of stain, in the center of the slide.
3. Obtain a thin piece of specimen. For an onion, peel a very thin layer of the inner epidermis from a fleshy scale leaf using tweezers.
4. Lay the specimen flat in the drop, without folding.
5. Hold the cover slip at an angle of about 45 degrees with one edge touching the drop. Lower it slowly with a needle or a pencil so that air does not become trapped.
6. Remove extra liquid from the edge with filter paper.

Air bubbles appear as dark circles with bright centers and should not be confused with cells. To stain a slide that is already mounted, place a drop of stain at one edge of the cover slip and draw it across by touching filter paper to the opposite edge.

Specimens can also be permanent slides, which are fixed, dehydrated, stained and sealed in resin. Those are prepared by specialists and last for years. Sharp tools, glass and chemical stains require care, and eye protection should be used in a laboratory.

Why are stains used?

Most cells are almost transparent, so structures are hard to see. A stain binds to certain parts of the cell and gives contrast. Different stains highlight different structures.

• Iodine solution stains starch blue-black and gives the nucleus and cell wall a yellow-brown color.
• Methylene blue stains nuclei dark blue, and it is widely used for cheek cells.
• Eosin stains cytoplasm and many proteins pink or red.
• Gram stain separates bacteria into two groups, depending on the structure of the cell wall.

Stains are normally not used on living cells for long, because many are toxic, and they can change the appearance of the cell. The observer should keep in mind that stained structures may differ from the living ones.

How do you observe cells?

Begin with the lowest-power objective to find the specimen, then move up through the objectives, as explained in the article on the light microscope. Adjust the light, because too much light can hide transparent structures. Move the slide slowly and choose an area where the cells are clear and not overlapping. Note the magnification you are using, since it is needed for the drawing.

Look for the features that fit the type of cell. In plant cells, look for walls, the nucleus and the vacuole. In animal cells, look for the membrane outline and the nucleus. Compare what you see with the expected structures, but draw only what is actually visible.

How do you make a biological drawing?

A biological drawing records observations accurately. It is not an artistic picture and it is not a copy of a textbook diagram.

• Use a sharp pencil and a clean sheet. Make single, continuous lines. Do not sketch, shade or color.
• Draw large: fill at least half of the space available, so that details are clear.
• Draw a few cells, typically two to four, not the whole field. Show the real proportions of the cells.
• Label structures with straight ruled lines that do not cross and that touch the structure. Write labels in a column to the side, not on the drawing.
• Add a title that names the specimen and, if used, the stain.
• State the magnification, or add a scale bar, and the date if required.

Common stains and their uses

Worked example: onion cells and the drawing scale

A student prepares an onion epidermis slide with iodine and observes it at ×400. She draws one cell, and the drawing measures 48 mm in length. She also measures the real length of the cell using an eyepiece scale and finds 0.12 mm.

1. Magnification of the drawing: drawing size ÷ actual size = 48 mm ÷ 0.12 mm = ×400.
2. Check: This is equal to the microscope magnification, so the drawing is consistent with the view.
3. Write on the drawing: “Onion epidermal cells stained with iodine, ×400”. Label the cell wall, cytoplasm and nucleus.

Suppose instead that the student draws a cell 30 mm long and knows the actual length is 0.1 mm. The magnification of that drawing is 30 ÷ 0.1 = ×300. The drawing magnification does not have to match the microscope setting, and the calculation is given on the sheet. A scale bar solves this problem, since it remains correct if the drawing is copied or resized.

How did staining develop?

Staining of tissues began in the nineteenth century, when dyes from the textile industry were tried on biological samples. In 1884 Hans Christian Gram developed the stain that bears his name, and it is still used to classify bacteria. Santiago Ramón y Cajal used silver staining in the 1880s and 1890s to reveal the structure of nerve cells. Drawing was a main way to record what the microscope showed until photography became practical, and the careful drawings of Cajal are still admired.

Which mistakes are common?

Mistake 1: Using a thick specimen

Light cannot pass through layers of cells, and the image is dark and confusing. Take the thinnest piece possible.

Mistake 2: Trapping air bubbles

Lower the cover slip slowly at an angle. Bubbles are not cells.

Mistake 3: Shading and coloring a drawing

Biological drawings use clean outlines only. Shading hides detail and is not accurate.

Mistake 4: Drawing what you expect and not what you see

If a structure is not visible, do not add it. The drawing is a record of observation.