Cytochemical staining procedures

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Introduction

  • Cytochemical staining is a laboratory technique used to identify specific chemical constituents present inside cells by means of selective chemical reactions.
  • These stains help demonstrate intracellular substances such as enzymes, lipids, glycogen, iron, and nucleic acids.
  • Unlike routine stains, cytochemical stains provide functional and biochemical information in addition to cellular morphology.
  • Cytochemical staining is widely used in hematology for differentiation of myeloid, lymphoid, monocytic, and erythroid cells.
  • The principle of cytochemical staining depends on the formation of a colored reaction product at the site of the chemical substance within the cell.
  • These procedures are particularly useful in diagnosing leukemias, bone marrow disorders, and storage diseases.
  • Commonly used cytochemical stains include myeloperoxidase, Sudan Black B, Periodic Acid-Schiff, and Perls’ stain.
  • Even with modern immunological methods, cytochemical staining remains important because it is simple, economical, and diagnostically valuable.

Cytochemical Stain

  • Cytochemical staining is a cornerstone of hematopathology, particularly for classifying and diagnosing various hematological disorders, including leukemias, lymphomas, and anemias.
  • Each staining technique targets specific cellular components, enhancing the identification of distinct cell types and their pathologies.

Myeloperoxidase Staining

Principle

Myeloperoxidase (MPO) is a lysosomal enzyme present in the primary granules of granulocytic cells and monocytes. It catalyzes the oxidation of a chromogenic substrate in the presence of hydrogen peroxide, producing a colored insoluble precipitate at the site of enzyme activity. Cells containing myeloperoxidase therefore show distinct colored granules within the cytoplasm.

The reaction can be represented as:

H2O2 + Chromogen → Myeloperoxidase →    Colored precipitate

Reagents

The commonly required reagents are:

  • Fixative – formalin, ethanol, or buffered formaldehyde
  • Hydrogen peroxide solution
  • Benzidine / o-dianisidine / 3,3′-diaminobenzidine (chromogenic substrate)
  • Buffer solution (pH 6.0–7.0)
  • Counterstain – hematoxylin or light green
  • Distilled water

⚠️ Modern laboratories prefer safer substitutes for benzidine because benzidine is carcinogenic.

Procedure

  1. Prepare a fresh peripheral blood smear or bone marrow smear on a clean glass slide.
  2. Air dry the smear completely.
  3. Fix the smear in formalin or suitable fixative for a few minutes.
  4. Wash gently with distilled water and dry.
  5. Flood the slide with freshly prepared MPO staining reagent containing chromogen and hydrogen peroxide.
  6. Allow incubation for 5–10 minutes at room temperature.
  7. Wash carefully with buffer or distilled water.
  8. Apply counterstain if required.
  9. Dry the slide and examine under microscope using oil immersion.

Results

Positive Reaction

  • Cytoplasm of myeloid cells shows brown to black granules.
  • Granules are usually coarse and clearly visible.

Negative Reaction

  • Lymphoid cells do not stain and remain negative.

Cells Showing Strong Positivity

  • Myeloblasts
  • Promyelocytes
  • Neutrophils
  • Eosinophils
  • Monocytes (weak positivity)

Cells Negative for MPO

  • Lymphoblasts
  • Mature lymphocytes

Uses

  • Differentiates acute myeloid leukemia (AML) from acute lymphoblastic leukemia (ALL).
  • Identifies granulocytic lineage in immature blast cells.
  • Helps classify leukemias in hematology laboratories.
  • Useful in bone marrow examination when blast morphology is unclear.
  • Assists in diagnosis of myeloproliferative disorders.

Clinical Importance

Positive MPO strongly supports diagnosis of:

  • Acute myeloid leukemia

Negative MPO suggests:

  • Acute lymphoblastic leukemia

Sudan Black B Staining

Principle

Sudan Black B is a fat-soluble dye that stains intracellular lipids, phospholipids, neutral fats, and sterols present in the primary and secondary granules of granulocytic cells. Because myeloid cells contain abundant lipid-rich granules, the dye dissolves in these lipids and produces a dark-colored deposit within the cytoplasm.

The stain mainly demonstrates lipid components of:

  • primary granules
  • secondary granules
  • phospholipid-containing membranes

This property makes Sudan Black B useful for identifying cells of myeloid origin.

Reagents

The commonly required reagents are:

  • Sudan Black B powder
  • 70% ethanol or absolute ethanol
  • Fixative (formalin vapor or ethanol)
  • Counterstain – safranin or nuclear fast red
  • Distilled water

Sudan Black B Staining Solution

Usually prepared by dissolving Sudan Black B in ethanol and filtering before use.

Procedure

  1. Prepare a fresh peripheral blood smear or bone marrow smear on a clean glass slide.
  2. Air dry the smear completely.
  3. Fix the smear using formalin vapor or ethanol for a few minutes.
  4. Dry the slide properly.
  5. Flood the smear with Sudan Black B staining solution.
  6. Allow staining for 15–30 minutes.
  7. Wash gently with ethanol to remove excess stain.
  8. Rinse with distilled water.
  9. Apply counterstain such as safranin.
  10. Dry the slide and examine under microscope.

Results

Positive Reaction

  • Cytoplasmic granules appear blue-black to black.

Strong Positivity Seen In

  • Myeloblasts
  • Promyelocytes
  • Neutrophils
  • Eosinophils

Weak Positivity

  • Monocytes

Negative Reaction

  • Lymphoblasts remain unstained or negative.

Microscopic Appearance

  • Positive cells show black granules within cytoplasm against a lightly stained nucleus.

Uses

  • Differentiates Acute myeloid leukemia from Acute lymphoblastic leukemia.
  • Identifies myeloid lineage in blast cells.
  • Useful when morphology alone cannot classify leukemia.
  • Helps confirm granulocytic differentiation.
  • Serves as an alternative to myeloperoxidase stain because both usually show similar positivity.

Diagnostic Importance

  • Positive Sudan Black B strongly suggests myeloid leukemia.
  • Negative stain supports lymphoid leukemia.

Esterase Staining

Principle

Esterase staining is a cytochemical method used to detect esterase enzymes present within leukocytes. These enzymes hydrolyze ester substrates to release products that react with diazonium salts, producing an insoluble colored precipitate at the site of enzyme activity inside the cytoplasm.

The reaction can be represented as:

Ester substrate → Esterase → Colored precipitate

Two major types of esterase stains are commonly used:

  • Specific esterase stain
  • Non-specific esterase stain

Specific esterase identifies granulocytic cells, whereas non-specific esterase is mainly used for monocytic cells.

Reagents

For Specific Esterase Stain

  • Naphthol AS-D chloroacetate substrate
  • Diazonium salt (Fast Blue B or Fast Red Violet)
  • Buffer solution
  • Fixative (formalin vapor or citrate-acetone-formaldehyde)
  • Distilled water
  • Counterstain

For Non-Specific Esterase Stain

  • Alpha-naphthyl acetate or alpha-naphthyl butyrate substrate
  • Diazonium salt
  • Buffer solution
  • Fixative
  • Counterstain

Procedure

  1. Prepare a fresh peripheral blood smear or bone marrow smear on a clean slide.
  2. Air dry the smear thoroughly.
  3. Fix the smear with suitable fixative for a few minutes.
  4. Wash gently and dry.
  5. Apply freshly prepared esterase staining reagent.
  6. Incubate for 15–30 minutes at room temperature.
  7. Wash with distilled water.
  8. Apply counterstain if required.
  9. Dry the slide and examine under microscope.

Results

Specific Esterase Stain

Positive Cells

  • Neutrophils
  • Myeloid precursors

Appearance

  • Bright red granular cytoplasmic staining

Negative Cells

  • Monocytes
  • Lymphocytes

Non-Specific Esterase Stain

Positive Cells

  • Monocytes
  • Monoblasts

Appearance

  • Diffuse reddish-brown cytoplasmic staining

Negative Cells

  • Granulocytes usually weak or negative

Tartrate Inhibition Test

Non-specific esterase activity in monocytes is inhibited by sodium fluoride, which helps confirm monocytic origin.

Uses

  • Differentiates granulocytic and monocytic leukemias.
  • Identifies monocytic differentiation in acute leukemias.
  • Helps classify Acute myeloid leukemia subtypes.
  • Particularly useful in diagnosing acute monocytic leukemia.

Clinical Importance

Specific esterase positive in:

  • granulocytic leukemia

Non-specific esterase positive in:

  • monocytic leukemia

Associated disease:

  • Acute monocytic leukemia

Periodic Acid-Schiff Staining

Principle

Periodic Acid–Schiff (PAS) staining is a cytochemical technique used to demonstrate intracellular glycogen, neutral mucopolysaccharides, glycoproteins, and glycolipids.
The stain is based on oxidation of carbohydrate-containing substances by periodic acid, which converts adjacent glycol groups into aldehydes. These aldehydes then react with Schiff reagent to produce a bright magenta or reddish-purple color.

The reaction can be shown as:

Carbohydrate → Periodic acid→Aldehyde→Schiff reagent→Magenta color

Thus, PAS positivity indicates the presence of carbohydrate-rich substances within cells.

Reagents

The main reagents required are:

  • Periodic acid solution
  • Schiff reagent
  • Fixative – methanol or formalin
  • Distilled water
  • Sulfurous acid rinse or running water
  • Counterstain – hematoxylin or light green

Procedure

  1. Prepare a fresh peripheral blood smear or bone marrow smear on a clean glass slide.
  2. Air dry the smear completely.
  3. Fix the smear with methanol for a few minutes.
  4. Wash and dry the slide.
  5. Flood the slide with periodic acid solution for 5–10 minutes.
  6. Wash with distilled water.
  7. Apply Schiff reagent and keep for 10–15 minutes.
  8. Wash thoroughly in running water until color develops.
  9. Apply counterstain if required.
  10. Dry the slide and examine under microscope.

Results

Positive Reaction

  • Cytoplasm shows bright magenta or reddish-purple granules or blocks.

Strong PAS Positivity Seen In

  • Lymphoblasts
  • Erythroblasts
  • Megakaryocytes

Weak Positivity

  • Granulocytes may show faint diffuse staining.

Negative Cells

  • Some mature lymphocytes remain negative.

Characteristic Pattern

  • Lymphoblasts often show coarse block positivity.

Uses

  • Helps differentiate Acute lymphoblastic leukemia from Acute myeloid leukemia.
  • Detects glycogen accumulation in blast cells.
  • Useful in identifying erythroid precursors in erythroleukemia.
  • Assists in diagnosis of storage disorders involving carbohydrates.

Clinical Importance

PAS strongly positive in:

  • acute lymphoblastic leukemia
  • erythroleukemia

PAS weak or variable in:

  • myeloid leukemias

Associated disease:

  • Acute lymphoblastic leukemia

Toluidine Blue O Staining

Principle

Toluidine Blue O is a basic thiazine dye used in cytochemical staining to demonstrate acidic tissue components, especially sulfated mucopolysaccharides and granules of mast cells. The stain has a special property called metachromasia, in which certain cellular substances stain a color different from that of the original dye.

Normally, Toluidine Blue O is blue, but when it binds to highly acidic substances such as heparin-containing granules, the color changes to reddish-purple.

The staining reaction is based on ionic binding between the basic dye and acidic cellular components.

Metachromatic reaction

Toluidine Blue (blue)+Acidic granules→Purple / red color

Reagents

The main reagents required are:

  • Toluidine Blue O powder
  • Buffer solution (usually acetate buffer, pH 4–5)
  • Distilled water
  • Fixative – methanol or formalin
  • Glass slides

Procedure

  1. Prepare a fresh smear of peripheral blood, bone marrow, or tissue section on a clean slide.
  2. Air dry the smear completely.
  3. Fix the smear with methanol for a few minutes.
  4. Wash gently and dry the slide.
  5. Flood the slide with Toluidine Blue O staining solution.
  6. Allow staining for 2–5 minutes.
  7. Wash carefully with distilled water.
  8. Air dry the slide.
  9. Examine under microscope.

Results

Positive Reaction

  • Mast cell granules show reddish-purple or violet metachromatic granules.

Background

  • Nuclei and other cellular structures appear blue.

Characteristic Finding

  • Strong metachromasia is seen in cells rich in acidic mucopolysaccharides.

Uses

  • Demonstrates mast cells in blood, bone marrow, and tissue sections.
  • Detects heparin-containing granules.
  • Helps identify mast cell disorders.
  • Used in diagnosis of mucopolysaccharide-rich lesions.
  • Useful in hematology and histopathology laboratories.

Clinical Importance

Positive staining is seen in:

  • Mastocytosis

It is also useful for studying:

  • basophils
  • connective tissue mast cells

Reticulocyte Staining

Principle

Reticulocyte staining is a supravital staining technique used to identify immature red blood cells known as reticulocytes, which still contain residual ribonucleic acid (RNA) and ribosomal material in their cytoplasm. These residual intracellular structures are not visible with ordinary Romanowsky stains but become visible when stained with supravital dyes.

The dye precipitates intracellular RNA to form a characteristic blue reticular network or granules inside young erythrocytes.

The reaction can be represented as:

Residual RNA+Supravital dye→Reticular blue networkResidual\ RNA + Supravital\ dye \rightarrow Reticular\ blue\ network

Reagents

The commonly used reagents are:

  • New methylene blue solutionor
  • Brilliant cresyl blue solution

Other materials:

  • Fresh anticoagulated blood sample
  • Test tubes
  • Glass slides
  • Pasteur pipette
  • Microscope

Procedure

  1. Take equal volumes of fresh anticoagulated blood and supravital stain in a test tube.
  2. Mix gently and incubate for 10–15 minutes at room temperature.
  3. After incubation, prepare a thin smear on a clean glass slide.
  4. Allow the smear to air dry completely.
  5. Examine under oil immersion microscope.

Results

Positive Finding

  • Reticulocytes appear as red blood cells containing blue-stained reticular network, granules, or filaments.

Appearance

  • Mature red blood cells remain pale without internal network.
  • Reticulocytes show dark blue reticulum inside cytoplasm.

Counting

Usually counted per 1000 red blood cells and expressed as percentage.

Normal Reticulocyte Count

  • Adults: 0.5% to 2.5%

Uses

  • Evaluates bone marrow erythropoietic activity.
  • Assesses response to treatment in anemia.
  • Helps differentiate regenerative and non-regenerative anemia.
  • Monitors marrow recovery after therapy.
  • Useful in hemolytic states and blood loss.

Increased Reticulocyte Count Seen In

  • Hemolytic anemia
  • acute blood loss
  • recovery after anemia treatment

Decreased Reticulocyte Count Seen In

  • Aplastic anemia
  • marrow suppression
  • severe nutritional anemia

Iron Staining (Prussian Blue Staining)

Principle

Iron staining, commonly called Prussian Blue staining, is a cytochemical method used to demonstrate ferric iron (Fe³⁺) present in cells and tissues. The stain is based on the reaction between ferric iron and acidified potassium ferrocyanide, which produces an insoluble blue-colored compound known as Prussian blue (ferric ferrocyanide).

This reaction specifically detects stored iron in the form of hemosiderin inside macrophages, erythroblasts, and bone marrow cells.

The reaction can be represented as:

Fe3++K4[Fe(CN)6]→Ferric ferrocyanide (blue)

Reagents

The commonly required reagents are:

  • Potassium ferrocyanide solution (2%)
  • Hydrochloric acid solution (2%)
  • Distilled water
  • Counterstain – nuclear fast red or safranin
  • Fixative – methanol or formalin

Working Prussian Blue Reagent

Prepared freshly by mixing equal parts of:

  • potassium ferrocyanide
  • hydrochloric acid

Procedure

  1. Prepare a fresh bone marrow smear, peripheral blood smear, or tissue section on a clean slide.
  2. Air dry the smear completely.
  3. Fix the smear with methanol if required.
  4. Flood the slide with freshly prepared Prussian blue reagent.
  5. Allow staining for 10–20 minutes.
  6. Wash thoroughly with distilled water.
  7. Apply counterstain such as nuclear fast red.
  8. Wash, dry, and examine under microscope.

Results

Positive Reaction

  • Iron-containing granules appear bright blue or deep blue.

Background

  • Nuclei and cytoplasm take the counterstain color.

Microscopic Appearance

  • Hemosiderin granules in macrophages appear blue.
  • Iron granules in erythroblasts may form ring pattern around nucleus.

Ring Sideroblast

When iron granules surround the nucleus of erythroblast, it forms a ring sideroblast.

Uses

  • Demonstrates bone marrow iron stores.
  • Detects hemosiderin in tissues and marrow.
  • Helps diagnose iron deficiency and iron overload states.
  • Identifies ring sideroblasts in bone marrow.
  • Assists in classification of refractory anemias.

Clinical Importance

Absent marrow iron seen in:

  • Iron deficiency anemia

Increased iron stores seen in:

  • Sideroblastic anemia

Ring sideroblasts suggest:

  • sideroblastic disorders
  • marrow dysplasia

Cytochemical Markers in Acute and Chronic Leukemias

Different cytochemical stains are routinely used to characterize various types of leukemia based on specific cytological features:

  • Acute Myeloid Leukemia (AML): Typically shows positivity for MPO, SBB, and CAE. PAS may show mixed results depending on the subtype.
  • Acute Lymphoblastic Leukemia (ALL): Generally negative for MPO and SBB, with possible PAS positivity depending on the subtype.
  • Chronic Myeloid Leukemia (CML): Characterized by the presence of myeloid cells; cytogenetic studies confirm the Philadelphia chromosome.
  • Chronic Lymphocytic Leukemia (CLL): Flow cytometry is often used alongside cytochemical staining to confirm the diagnosis based on specific surface markers.

Summary of Cytochemical Staining Techniques

Staining Technique Purpose Key Features
Myeloperoxidase (MPO) Differentiate myeloid vs. lymphoid lineage Positive in myeloid cells (AML)
Sudan Black B (SBB) Identify lipid content Positive in myeloid cells (granulocytes)
Esterase Staining Differentiate myeloid and monocytic lineage CAE positive for myeloid; NBE positive for mono
Periodic Acid-Schiff (PAS) Identify glycogen and polysaccharides Positive in certain myeloblasts and lymphoblasts
Toluidine Blue O Identify mast cells Stains mast cells blue
Reticulocyte Staining Assess erythropoiesis Reticulocytes stain blue due to RNA
Iron Staining (Prussian Blue) Identify iron deposits Positive indicates iron overload

 

Clinical Implications of Cytochemical Staining

  • Cytochemical staining plays an important role in identifying the biochemical composition of blood cells when routine morphology alone is insufficient.
  • It is especially valuable in differentiating Acute myeloid leukemia from Acute lymphoblastic leukemia, because myeloid blasts usually show positivity with myeloperoxidase and Sudan Black B, whereas lymphoblasts are negative.
  • Periodic Acid–Schiff (PAS) stain helps identify lymphoblasts and erythroid precursors by demonstrating glycogen accumulation, which is useful in leukemia classification.
  • Esterase staining is clinically important for detecting monocytic differentiation and is particularly useful in diagnosing Acute monocytic leukemia.
  • Prussian Blue staining demonstrates iron stores in bone marrow and helps diagnose Iron deficiency anemia, sideroblastic anemia, and iron overload states.
  • Reticulocyte staining evaluates bone marrow erythropoietic activity and helps determine whether anemia is regenerative or non-regenerative.
  • Toluidine Blue staining is useful for identification of mast cells and supports diagnosis of Mastocytosis.
  • Cytochemical stains remain clinically valuable because they are rapid, economical, and useful in laboratories where advanced immunophenotyping is not readily available.
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