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The science behind H&E staining: Why do cell nuclei turn blue?
Hematoxylin and eosin stain (H&E stain for short) is one of the most important tissue staining methods in histology. This staining technique has over time become a benchmark in medical diagnosis, particularly in the field of pathology. Pathologists often rely on this staining method when they look at biopsy samples for suspected cancer.
H&E staining consists of two main dyes: heme and eosin. Heme stains primarily the cell nucleus purple-blue, while eosin stains the cytoplasm and extracellular matrix pink, allowing pathologists to easily distinguish the nucleus from the cytoplasm of the cell. In addition, the stained color pattern can also reveal the structural layout and cell distribution of the tissue sample, thereby providing key histological information.
This dyeing technique has been widely used for its convenience and effectiveness since it was first introduced in 1877 by chemist Nicolaus Wissozky of the Kazan Imperial University in Russia.
Purpose of H&E
The H&E staining procedure has become the main staining method in histology because it is fast and cost-effective to perform, while being able to reveal a large number of microanatomical features. Not only can this staining technique be used to diagnose a variety of histopathological conditions, but it can also be adapted to the effects of a variety of different laboratory conditions and tissue fixatives, making it extremely valuable in practical applications.
Although H&E staining has broad applicability, in some complex cases a more specific staining method may be required to obtain clearer contrast.
Methods of application
There are many ways to prepare reagent solutions for H&E staining, and staining procedures may vary from laboratory to laboratory. Although no standard procedure exists, a rule of thumb is that the nucleus will usually be stained blue, while the cytoplasm and extracellular matrix will appear pink. This standardized process can maintain a high level of consistency across different laboratories.
After tissue samples are obtained, they are typically fixed, dehydrated, and embedded in molten paraffin, and then cut into thin sections using a microtome. In this process, the sample is attached to a microscope slide, its paraffin portion is removed with solvent, subsequently rehydrated, and finally prepared for staining.
Staining results
The heme dye primarily causes the cell's nucleus to appear blue or deep purple, while eosin stains the cytoplasm and some other tissues in up to five different shades of pink. This allows pathologists to effectively identify the internal structure of cells when observing. It is worth noting that red blood cells will be stained very bright red, which makes them easy to identify under a microscope.
The principle of staining the nucleus lies in the combination of a dye-metal complex formed by heme and a metal salt (usually an aluminum salt) and DNA.
Mechanism of staining
Although heme, the oxidized form of heme, is the primary source of color, the dyeing process is still named after heme. This is because, when heme is combined with a metal salt, it takes on properties similar to a positive dye, while eosin is a negative and acidic dye. The staining of cell nuclei mainly relies on the binding between color complexes and DNA. This process differs from the mechanism of nuclear staining using some positive dyes.
In addition, the yellowish-brown color that may appear in the sample is caused by endogenous pigments such as melanin, and the basement membrane requires PAS staining or some silver staining to clearly show it.
Examples of H&E staining
In the diagnostic process, H&E staining is widely used for the observation of different tissues, including tumors, inflammation and other pathological conditions. This not only shows the importance of this staining method, but also allows professionals to play its important role in real-life medical scenarios.
The color distribution of H&E and the representation of structures in the sample provide important insights into cytology and histology.
With the development of digital pathology, H&E staining not only remains the gold standard for on-site diagnosis, but also shows new potential in data analysis and image processing. Faced with such a widely used technology, the scientific mechanism behind it still raises many questions, making us wonder: How will this technology continue to evolve in medicine and provide more accurate diagnosis in the future?
