HISTOLOGY PT 2

Tavishi

In the last post, I discussed eukaryotic histology. And while it may seem that histology is fairly straightforward: color some cells and make certain bits stand out, histology is an incredibly complex field of biology. In non-eukaryotic histology, certain stains are used specifically for these organisms, and even non-living organismic things (dancing around defining a virus!)

The structural differences between your average prokaryote and eukaryote are taken advantage. Most notable is the cell wall.

Animals lack the presence of a cell wall point blank, but in different eukaryotic cells, cell walls are generally present, but with different characteristic components: for example, cellulose in plants, and chitin in fungi. In bacteria, cell walls are generally composed of peptidoglycan, with some funky lipids and other stuff in between. The gram stain takes advantage of all of these known characteristics of bacterial cells.

The gram stain consists of crystal violet, iodine, alcohol, and safranin. First, crystal violet and iodine are deposited on the cells. Then, alcohol strips away the cell wall, and finally, safranin is deposited on the cell.

Gram positive cells will remain purple, and gram negative cells the characteristic red from safranin. This occurs because the alcohol does not successfully break down the cell wall, preventing the crystal violet and iodine from exiting the cell. Gram positive cells generally have thicker peptidoglycan cell walls, and none of that fluffy stuff found in gram negative cells. Gram negative cells, instead, have their violet washed out, and instead, retain the red of a safranin stain.

Gram negative cells are generally more harmful, bearing the notable names of E. coli, Y. pestis, and S. enterica (one of the salmonella species!)

Our pink rod shaped bacteria, or bacilli, here are gram negative, and more likely to be harmful and pathogenic bacteria, whereas the cocci purple are more likely to be gram positive.

When detecting viruses, our non-living buddies, inclusion bodies show up on histopathological examinations. Inclusion bodies are sort of like clues to who was there last.

Think about a crime scene; if someone leaves behind the cap of a syringe by accident at the scene of a theft, it can be assumed that the person is in the medical field.

When a virus has infected someone's cells, after they replicate, they leave behind little bits and pieces that clue us into what they are. These are called inclusion bodies. Inclusion bodies are usually misfolded proteins, lipids, or organelle remnants.

Now, inclusion bodies aren't just characteristic of viral infections, but rather are just clues into diseases in general; for example, inclusion bodies like Lewy bodies in Parkinson's.

The most well known viral inclusion bodies are Cowdry bodies and Negri bodies. (I lie, I'm not sure if these are the most well-known inclusion bodies, but these are the ones I know about!)

Cowdry bodies are characteristic of a lot of viruses, including herpes, polio, and varicella. Cowdry bodies stain either eosinophilic or basophilic in a HE stain. There are two types of Cowdry bodies: type A and type B. Type A Cowdry bodies are typically more ovular, and type B are generally more spherical. Type A Cowdry bodies are present in herpes and varicella, and type B are present in rabies.

The first image below is tissue infected with herpes simplex virus, and the dark purple things are our Cowdry A bodies.

And above, here, the arrows point to Cowdry B bodies!

Even within the subject of inclusion bodies, you can infinitely magnify the topics even further; but I'm not a medical student, so I don't quite really want to know the histopathological implications of every single inclusion body.

And while these histopathological examinations do provide a clue into diagnosis, histopathology is only one piece of the puzzle. You can't ever really definitely call a disease something unless you have copious evidence and reason for thinking so; there's a variety of factors in every symptom.