Do crime scene investigators need to stain their samples?

23 January 2012

In the CSI: Crime Scene Investigation episode, 4 x 4, a hit-and-run driver leaves behind his facial imprint in the vehicle airbag. To identify the suspect, the CSI unit collects trace samples for study, and delivers the particles to the lab. Grissom, the forensic entomologist, directly visualizes the specimens under a microscope and identifies the airbag remnants as "lard, pork, beef, chicken and some human flesh."

How can Grissom tell? I thought you're supposed to stain your samples before you look at them under the microscope so that you can tell different specimens apart...well at least that's what we've done in Biology. Does the script of the show contain a scientific flaw?

The simple answer is no. It has to do with the type of microscope Grissom used. Let's find out why...

The microscope you're probably most familiar with in the classroom is the light microscope. It aims light through a condenser lens, and focuses the beam onto a specimen placed on a stage. The light continues up through an objective, toward an eyepiece, and finally to the eye of the user.


Did You Know?
Electron microscopes use beams of electrons instead of light Light microscopy almost always requires the staining of samples. Only dense objects or coloured specimens are visible without stain, while pale-coloured samples (like lard or flesh) cannot be distinguished from the background because the cellular components absorb light to a similar degree as visible light. In other words, they have poor contrast.

Staining the specimen creates contrast, or the distinction between light and dark colours. Certain dyes bind particular cellular components. For example, the stain hematoxylin binds protein, while the stain fuchsin binds DNA.

But, the messy staining steps can be eliminated by using different microscopes!

For example, the dark-field microscope contains a patch under the condenser that blocks the central cone of light, producing a "dark" background and tiny "luminous" objects. This microscope is useful for visualization of things like bacteria in pond water.


Did You Know?
The first light microscope was invented in 1850 A cheap variation of the light microscope, the phase contrast microscope modifies the light path with a series of "rings". Waves pass either directly through the object or bend along a course determined by the properties of the specimen. For instance, the nucleus of a cell can easily be defined from its cytoplasm.

Phase contrast is frequently used in laboratories to study living material like cultured mammalian cells, but doesn't work well on thick objects. Unfortunately, this technique sometimes forms "halos" around tiny objects. The cells might look angelic, but the details are often obscured!

Grissom on the other hand chose the polarized light microscope (PLM), which uses two polarizers (above and below the specimen) to rotate the light into planes.


Did You Know?
Some sunglasses also have polarizers that allow the entry of vertical rays but block the glare from horizontal planes such as highways and lakes. In the PLM, directionally-dependent or anisotropic samples, meaning those with evenly spaced areas or crystal-like structures, produce two light waves (called birefringence) with different velocities. An analyzer (the second polarizer) recombines the light planes to form contrast. The PLM produces awesome images of crime scene artifacts like fibers and crystals.

Dark-field, phase, polarizing... who knew so many types of microscopes existed!? (And this is just a short list. Check out the links below to find out more). Plus, the microscopes save the investigator time! So the next time you watch CSI, check out the fancy microscopes they use in their crime fighting cases, and notice all the boring steps they skip!

Learn More!

How Stuff Works on How Light Microscopes Work

Wikipedia on Microscopy on CSI: Crime Scene Investigation

Wikipedia on CSI: Crime Scene Investigation

OLYMPUS Microscopy Resource Center

CRAM Science would like to thank Karla Badger for her help and expertise in answering this question. Karla is currently a PhD student at the University of Toronto where she studies leukemia using mice. Her favourite white blood cell to look at under a microscope is a neutrophil because it has a funky horseshoe-shaped nucleus. In her free time Karla enjoys yoga, rock-climbing, cooking and acting as an editor for a graduate student-run journal, Hypothesis.


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