To expound upon my previous post regarding histology and pathology, there is an important step between conducting a biopsy and generating a diagnosis. The tissue in question must be “fixed” (preserved), sectioned (cut into slices), and stained [Origin: Old Norse steina (to paint)]. There are numerous stains and different ones are useful for different purposes. The classic stain that most students of histology are introduced to is the H&E stain. This stands for haematoxylin and eosin.

Origin: Greek, haima (blood) + xylos (wood)
Extracted from the logwood tree; depending on its preparation, its solution appears blood red

Origin: Greek, eos (dawn)
So named after the Greek goddess of the dawn, Eos, for its red/dawny color

To start, haematoxylin is oxidized into haematein and combined with aluminum ions. This forms the active dye-metal complex, and this complex is what binds to nucleic acids to stain nuclei and ribosomes purple. (The former contains DNA in the form of chromatin and the latter is composed of RNA.) It is thought that the highly negative phosphate backbone is what binds to the positively-charged haematein. We use the word basophilic [Gk. base + philia (love)] to refer to entities that bind to basic, i.e., positively-charged dyes. I will discuss the etymology of chemical terms such as “base” in a future post!


Eosin is used as the counterstain to haematoxylin. When eosin is dissolved in water or ethanol, its carboxylic acid and phenol are deprotonated to form a negative ion. This form of eosin binds to positively-charged entities such as the positive amino acids, e.g., arginine and lysine. Hence, the cytoplasm appears red because its proteins bind to the negatively-charged eosin. We use the word eosinophilic [Gk. eosin + philia (love)] to refer to entities that bind to acidic, i.e., negatively-charged dyes.

In the 1970s, the price of haematoxylin rose steeply, and it resulted in a search for a synthetic alternative. My schoolmate and former histology TA, Dr. Alexander Morgan, relayed an interesting aside to me: “European lust for haematoxylin based dye led to British logging in South America, and why there’s one tiny, mosquito-infested, swampy country in South America where they speak English [referring to Belize]. Many people died (dyed?) of malaria trying to harvest the wood to make the stain.”

These terms present a natural tie-in to immunology. There are two leukocytes that I am referring to: one is the eosinophil, which is colored quite intensely red, and the other is the basophil, which is colored quite intensely blue. I believe a picture is in order to better illustrate:

eosinophil basophil rev2

Another interesting stain is the silver stain. Camillo Golgi, a 19th-century Italian physician and Nobel laureate, developed silver staining to study the nervous system. (The cellular organelles known as Golgi bodies are named after him.) There are two classes of cells that take up silver: the argentaffins and the argyrophiles.

Origin: Latin, argentum (silver) + affinis (akin/affinity)
Affinity for silver; more specifically, cells that take up silver without a reducing agent

Origin: Greek, argyros (silver) + philia (love)
Love of silver; more specifically, cells that take up silver and need a reducing agent

There are various methods for preparing different silver stains. These stains can be used for a number of applications such as:

  1. Diagnostic microbiology (staining of fungi such as Pneumocystis jiroveci)
  2. Neuropathology (staining of neuritic plaques and neurofibrillary tangles in Alzheimer disease)
  3. Histology (staining of type III collagen and reticulin)

silver stains

Left: neurofibrillary tangles of Alzheimer disease. Right: membranous nephropathy

The last stain I will briefly touch upon is the periodic acid-Schiff (PAS) stain. This stains for polysaccharides, glycoproteins, glycolipids, and the like. Hence, it can be used to diagnose things like the glycogen storage diseases, adenocarcinomas (which often secrete mucin), Whipple’s disease, etc. The reason why I bring this up is that many people say it incorrectly. It is not periodic as in “recurring time intervals.” Rather, is per-iodic, i.e., of the periodiate ion (IO4)! Periodic acid oxidizes vicinal [L., vicinus (near)] diols to create a pair of aldehydes. These aldehydes then react with the Schiff reagent to produce a purple color.

Here is a clinical scenario: a patient comes to your office for a routine examination. While examining the skin, you notice an ill-defined and oddly-colored brown spot on the patient’s back. Or perhaps while palpating the thyroid gland, you palpate something that doesn’t feel quite right. What is the next step in this patient’s care? If you thought to perform a biopsy, you’d be correct. Indeed, to derive insights from what a patient is suffering from, clinicians obtain a sample of tissue via biopsy from the suspect organ for the tissue to be studied histologically.

Origin: Greek, bios (life) + opsia (a sight)
A sight of life

Origin: Greek, histos (web/tissue) + logos
The study of tissue

This tissue can initially be examined grossly [L., grossus (thick/coarse)], i.e., by the naked eye, but clinicians often require insights from a pathologist to more completely characterize and understand the tissue they obtained. What does a pathologist study? Let’s take a look:

Origin: Greek, pathos (suffering) + logos (word, taken to mean “study of”)
The study of suffering

This is a very generalized definition of pathology (which is absolutely wonderful to me), but let me offer my own comprehensive/digestible definition of pathology. Pathology is the molecular, histological, and gross study of disease processes and their clinical manifestations. Thus, staying true to its etymology, pathology is the systematic and scientific study of suffering. There are four main areas that pathologists explore to better understand disease: aetiology, pathogenesis, morphological changes, and clinical manifestations.

spitz papillary

Left: Spitz nevus (mole). Right: papillary thryoid carcinoma

Origin: Greek, aitos (cause) + logos
The study of the cause of a disease

Origin: Greek, pathos + genesis (origin/source)
The origin and development of disease

If all goes well, the pathologist will generate a diagnosis and a prognosis regarding the patient’s condition, and the clinician or the surgeon will proceed accordingly.

Origin: Greek, dia– (through) + gignoskein (to know) + –osis (condition of)
A condition of knowing through, i.e., a distinguishing

Origin: Greek, pro- (for/before) + gignoskein + –osis
A condition of foreknowledge, i.e., the likely outcome

What tools does a pathologist use to actually study the tissue in question and generate a diagnosis? You’ll have to stay tuned to find out!