Which stain demonstrates the rough endoplasmic reticulum (RER)?

• A. Methyl green-pyronin
• B. Hematoxylin
• C. Eosin
• D. Masson's trichrome

Answer: (A)

The key idea is that structures rich in RNA, like the rough endoplasmic reticulum with its ribosomes, stand out with stains that bind RNA. Methyl green-pyronin does exactly that: pyronin binds RNA and stains it pink/red, so the RNA-dense RER appears as a distinct pink/red network in the cytoplasm, while methyl green binds DNA in the nucleus, giving a contrasting green/blue to the nucleus. This specific RNA staining makes the RER visible where other stains don’t highlight it as clearly.

Hematoxylin mainly colors nuclei and basophilic structures, eosin stains cytoplasmic proteins but not RNA-rich organelles in a way that distinguishes the RER, and Masson’s trichrome targets connective tissue components like collagen and muscle fibers rather than cellular organelles.