HISTOLOGY: A STAIN ON THE FIELD OF MEDICINE

Tavishi

Despite what the title of this post may communicate, I am actually very fond of histology; rather, I intend to make a pun about the staining process within histology. Histological staining procedures aren't something I'm too familiar with, as I've never actually done much, seeing as I don't work in any labs where I'd be using histology. But I do understand and know the procedures behind preparing samples for microscopy, as well as the difference between the most common stains used for histology. Histology is crucial for the understanding of physiology, but also very important in diagnosis of viral and bacterial infections, as well as cancerous cells. Histological examinations allow us to verify suspected diseases by looking for characteristic microscopic signs of the affliction. When a tissue sample is first taken, we have to first fix the tissue. This doesn't imply there is something wrong with the tissue. I do hope there is nothing wrong, but the aim of diagnostic histology is usually to identify if something is wrong. And so in this case, fixing tissue actually is just preservation. This allows us to be able to look at the tissue for a while without degradation. After 24 hours, if you don't fix a sample, the entire thing ceases to exist. (Joke) Fixation can occur through a dip in ethanol, alcohol, formalin, or even just heating a slide for bacteria and archaea.

After fixation, tissue samples it must be cut very, very fine. Generally, a microtome is used in laboratory settings to finely chop and slice. (Unfortunately, my knife skills are not comparable in the least to the precise work of a microtome.. this has caused issues in my at-home microscopy!) After some slicin' and dicin' from the microtome, our tissue should be only a few micrometers thick- preferably anywhere from 1-30 micrometers!

After some processing stuff, like dehydration via ethanol, and more. These steps are rather specific to the tissue itself. Now, we get down to the nitty gritty of the staining itself. The most common histological stain is, of course, hematoxylin and eosin, or HE! Hematoxylin and eosin are two different colored stains with different properties. We first stain a tissue with the blue hematoxylin. Hematoxylin is a basic dye, and thus stains acidic structures that are called basophilic (dyed by hematoxylin). After hematoxylin, the acidic pink dye eosin grabs all the basic, or eosinophilic structures. Structures that often end up blue, once the process is completed, are DNA, RNA, and other similar acidic structures. Structures that end up pink include the cytoplasm and other protein filled structures. Below is pancreas tissue stained with HE stain. The blue circles amongst the pink are the nuclei- where all the DNA and fun stuff is.

Next, the well known Papanicolaou stain! The name might not ring a bell till you hear the more common name: the Pap smear! The pap smear is a cervical cytology test done to check for the characteristic signs of cervical cancer, often from HPV. The Papanicolaou stain consists of hematoxylin, eosin, orange G, Bismarck brown, and light green SF.

The Pap stain first begins with a hematoxylin stain. Then, mixed with some ethanol, then goes in orange G, which gets the keratin in the cells. Finally, we have a concoction of eosin, light green SF (actually blue), and Bismarck brown. Eosin generally gets the cytoplasm of epithelial squamous cells, whereas the squamous and columnar cells are stained by light green SF.

Look at the abnormal, dark nuclei of those pink cells, and how irregularly shaped these cells are here. Those cells are, in fact, indicative of squamous cell carcinoma! Finally, the third stain I'll discuss in this post is Wright's stain which is used to differentiate the types of blood cells. Wright's stain contains eosin, azure B, and methylene blue. Different components of blood are detected by different stains. Platelets show methylene blue, and neutrophils azure B. Erythrocytes, or red blood cells, are pink from eosin. Eosinophils are... complicated. While you'd think they look entirely pink, they actually contain these characteristic pink granules which are eosinophilic. The nucleus, however, is purple from azure B or methylene blue. Those red cells are erythrocytes, and the big purple structure is the lovely neutrophil. The tiny little purple people are platelets.

And here, the purple guy with red dots is our eosinophil.

A part 2 will come soon, where we focus further on non-eukaryotic histology!