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September 27, 2020 | Rome, Italy

Slices of life

By | 2018-03-21T19:04:24+01:00 February 8th, 2015|"Bio-Lingual"|
Light microscope image of mouse lung tissue. The dark purple boundaries are the edges of the alveoli (air sacs). A stain called hematoxylin produced the purple color. Photos courtesy of Lorien Menhennett.
I

still remember the bottomless awe. We were studying mitosis, the DNA dance of cell division. The intricate process was almost beyond comprehension. But what did me in were the multi-colored microscopic images. That mitosis happens amazed me. That we could see it so clearly left me speechless.

When I returned to school for a second bachelor’s degree five years ago, I knew I was in for science. I didn’t know I was in for art. Before long, I had poster prints of cells and tissues hanging in my living room — fluorescent green starbursts of our neurons, the magenta and violet rolling hills of our skin.

Cells aren’t really fluorescent green, or magenta and violet. They are mostly vague, colorless blobs, their characteristics indistinguishable, and their inner workings invisible. Scientists have developed a process called “staining” to help us highlight the details of tissues, cells and metabolic processes under the microscope. When treated with a fluorescent stain called DAPI, my beloved dancing DNA shows up blue under an ultraviolet microscope light.

Stained slides seemed to me like paintings. But you can’t paint cells. Staining involves reactions and attractions at a molecular level. Specific microscopic components turn a particular color to make distinctions clearer. Researchers and doctors use stained slides to find what they’re looking for. Call it explicable magic.

But it’s time-consuming. Before you can stain a sample, you need to get it on the slide. There are several kinds of “slide mounts,” or preparations. Section mounts — which I used in my research lab work — are arguably the most complex. They involve applying a thin cross-section of a tissue sample onto rectangular glass slide. First though, you must “fix” the sample with caustic chemicals like formalin to prevent decay. You must also dehydrate the sample and lace it with paraffin to strengthen the tissue. Once preserved, you slice the sample with a machine called a microtome (technicians did this in my former lab). I worked with lung tissue slices four microns thick. Let me put that number in perspective: 1 micron is 1 x 10-6 meters, or 1/1,000,000 of a meter. That’s one-millionth. This nearly invisible tissue slice is attached to the slide, and stained.

The best analogy for the staining I witnessed wasn’t painting but baking. Recipes exist, and specific ingredients, measurements, mixtures must all be meticulously applied using special tools. Timing is essential. Some stains work like complex dyes, coloring certain regions of the cell (but not others) based on molecular attractions and repulsions.

Another kind of staining is immunohistochemistry. “Immuno” refers to the immune system, with the staining process modeled on how immune cells detect invaders. “Histo” refers to tissue; “chemistry” refers to the chemical reactions involved.

The ease of immunohistochemistry shocked me. The staining itself requires precise repetition, though the specific protocols vary depending on the experiment. Steps may include immersing the slides in descending concentrations of alcohol — 100 percent then 95 percent — for several minutes each. Slides must then be rinsed in a special buffer solution, incubated with primary and secondary “antibodies” at distinct temperatures — room temperature, 4C, and rinsed again with more chemicals. As a final step, a thin, transparent, protective cover slip is glued over the sample.

You can then go to your microscope, which these days is connected to a digital camera and a computer, and take the artful images that are later analyzed visually or with scientific software.

But where do the colors come from? How do the cell parts turn purple or pink or green or blue? In immunohistochemistry, the key ingredients are antibodies. In the late 1930s, American doctor and scientist Albert Coons dreamed up the process, inspired by the human immune system. Immune cells called B lymphocytes produce what are called “antibodies,” a kind of protein. Think of antibodies as detectives. The antibodies recognize and attach to other protein markers called “antigens” on foreign cells. Each antibody recognizes only one antigen. In other words, each detective is after his own suspect. Coons realized that if he could attach a colored “tag” to an antibody, and have that antibody recognize an antigen involved in a particular cell part or cell process, he could then visualize that specific part or process under the microscope. Things would change colors, depending on whether the antigen being sought was there or not.

Now, more than 70 years later, we have scores of immunohistochemistry stains, from ones visible under a regular light microscope to fluorescent ones activated under ultraviolet light (immunofluorescence).

It’s nice to turn things pretty colors, or have them glow under UV. But what do immunohistochemistry and other staining methods accomplish? They allow the study of tissues and cells in new ways. Immunohistochemistry staining for a protein called E-cadherin, for example, can help pathologists differentiate between two kinds of breast cancers. Ductal carcinoma, which begins in the milk ducts, stains positive — via dark brown patches — for E-cadherin. Lobular carcinoma, which starts in the milk glands, usually stains negative for E-cadherin. Instead of brown patches, there are purple ones. The difference can help a doctor classify breast cancer type.

To me, slide staining is the accidental but joyous intersection of art, science, and medicine. Snap a picture. Set it on the mantle. Study the human body. Save someone’s life. It doesn’t get much better than that.

About the Author:

Lorien Menhennett
Lorien Menhennett wrote the "Bio-Lingual" column from 2014 through 2018

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