Questions about stains and embryology

I already got an email with questions! I’m so happy – I love it when you guys ask questions. I’ll often post them here (without names of course), because if one person has a question, it’s likely that there are lots of students wondering the same thing. Here are the questions, along with my answers.

Q. For tissue specimens stained with hematoxylin and eosin, would you recommend students to become thoroughly familiar with what cell parts/organelles are negatively/positively charged? If so, do you have any resources you would recommend?

A. I do think it’s important to know which parts of a cell are negatively charged (and stain blue with H&E) and which are positively charged (and stain red with H&E), because this topic is something that comes up pretty frequently on boards (there was a question on this in last year’s board exam!).

However, there are just a few structures responsible for the staining patterns we typically see in cells – so there’s not really much to memorize (and no need to search for resources on this). Here’s a summary of what you need to know.

First, you should know about the two components of the H&E stain:

  • Hematoxylin is a basic dye. It is positively charged, and it stains negatively-charged structures blue.
  • Eosin is an acidic dye. It is negatively charged, and it stains positively-charged structures red.

Here are the cell structures you should remember:

  • Negatively-charged structures: DNA (in the nucleus) and RNA (in the cytoplasm)
  • Positively-charged structures: cytoplasmic proteins

That’s basically it! If anyone asks you about specific structures in an H&E stain, it will be about one of these three things (DNA, RNA, or proteins). That’s because these are the things that give a cell its typical staining pattern:

  • The nucleus stains blue (because it contains DNA)
  • The cytoplasm stains anywhere from pink (if it contains lots of proteins) to blue (if it contains a lot of RNA)

As we go through the course, we’ll learn about a few other cellular structures that have specific staining properties – but these structures are specific to particular tissues (like cartilage) or cells (like parietal cells in the stomach). So I wouldn’t recommend trying to memorize them now – it will stick better if we just talk about those things as they come up in lecture.

Q. Would it be correct in assuming that silver stains work very well for fungi and reticulin fibers because silver is best used for staining long, thin, stringy cells?

While it’s true that reticulin fibers and certain fungal organisms are long and stringy, that’s not what makes them stain with the silver stain. It’s actually the surface molecular structure of reticulin fibers and fungi that is responsible for the silver stain positivity!

Both reticulin fibers and fungi are “argyrophilic” (silver-loving) because they adsorb silver particles (in other words, their surfaces are able to bind silver particles). The silver particles actually bind to aldehyde groups on the surface of the fiber/fungus (this is probably more than you wanted to know…but I love stains so I can’t help myself!). Initially, the silver particles are in the form of silver salts, which are invisible – but if you apply a reducing agent, that will turn the silver salts into metallic silver, which is black and nicely visible.

Q. What happens to the primitive streak after epiblast cells are pushed downward to form the 3 germ cell layers?

A. It disappears! The primitive streak is just a transient structure that is formed at the beginning of gastrulation; its purpose is to serve as the place through which epiblast cells migrate and turn into the three germ cell layers. After that happens, the primitive streak disappears, and the top layer just becomes ectoderm.

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