9.4 Lipid Histochemistry: Oil Red O, Sudan Dyes & Frozen-Section Requirements

9.4 Lipid Histochemistry: Oil Red O, Sudan Dyes & Frozen-Section Requirements

ASCP HT Exam Focus: Lipids are a named procedure area in the ASCP BOC Staining content outline, and Oil red O appears in the Summary of Stains. Expect items that pair a lipid target with the preparation requirement, not just the dye name.

Every other special stain in this guide begins with a deparaffinized section. Lipid histochemistry cannot, because the reagents that make a paraffin block possible - graded alcohols, xylene, and molten wax - are the same reagents that dissolve neutral fat. Demonstrating lipid is therefore a specimen-handling problem first and a staining problem second.

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Lysochromes: Solubility, Not Chemistry

Lysochromes (Greek lysis, dissolution) are intensely colored, fat-soluble, essentially water-insoluble dyes. They do not react with a functional group and they form no covalent bond. They simply have greater solubility in tissue lipid than in the solvent that carries them, so on contact they partition out of the solution and concentrate in the lipid droplet.

Three consequences follow directly and are all testable:

1. The dye must be nearly saturated in a poor solvent. A working solution that is too dilute, or made in too good a solvent, keeps the dye dissolved and nothing stains.
2. The working solution is unstable. Diluting an alcoholic stock with water pushes the dye toward precipitation. Oil Red O working solution must be filtered immediately before use and discarded within a few hours; precipitated dye deposits as extracellular red granules that mimic fat.
3. Alcohol and xylene reverse the stain. Once the section re-enters a good lipid solvent, the dye and the lipid both leave.

Oil Red O versus Sudan IV: Oil Red O is the more intensely colored and is preferred for small fat droplets such as microvesicular steatosis and fat emboli. Sudan black B is the most sensitive of the group because it also binds phospholipid, but its blue-black product is harder to separate from melanin, formalin pigment, and carbon.

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Getting Lipid to the Slide

There are exactly three acceptable preparations, and the exam expects all three:

1. Formalin-fixed frozen sections. Fix in 10 percent neutral buffered formalin or formal-calcium (formalin with 1 percent calcium acetate/chloride, which stabilizes phospholipid), then cut on the cryostat. Never process through the tissue processor. Sections are usually cut somewhat thick (8 to 10 micrometers) because lipid droplets are three-dimensional.
2. Carbowax (polyethylene glycol) embedding. Water-soluble wax replaces the alcohol-xylene-paraffin route entirely, so lipid remains in situ and permanent-looking sections are possible. Sections must be floated on a chilled non-aqueous or glycol-containing bath because Carbowax dissolves in water.
3. Air-dried smears, touch preparations, and cytospins, fixed briefly in formal-calcium.

Osmium tetroxide is the exception to all of this. Osmium adds across carbon-carbon double bonds of unsaturated fatty acids and is reduced in place to black osmium dioxide, forming an insoluble product that survives dehydration and paraffin embedding. This is exactly why osmium is the routine post-fixative for electron microscopy: it fixes lipid membranes and simultaneously supplies electron density.

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Procedure, Counterstain, and Mounting

A routine Oil Red O procedure runs as follows:

1. Rinse the formalin-fixed frozen section in distilled water, then in 60 percent isopropanol (a pre-rinse that prevents the working solution from being diluted by water carryover).
2. Stain in freshly filtered Oil Red O working solution for 10 to 15 minutes.
3. Differentiate briefly in 60 percent isopropanol to strip loosely held dye from the background.
4. Rinse in distilled water.
5. Counterstain in Mayer hematoxylin (aqueous, progressive, non-alcoholic) for 1 to 2 minutes, then blue in running tap water.
6. Mount in an aqueous medium - glycerin jelly, Aquamount, or warmed gelatin - and seal the coverslip edges with nail varnish for storage.

Never dehydrate and never clear. A resinous mount requires absolute alcohol and xylene, which extract both the dye and the lipid and leave an empty section.

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Clinical Applications

• Fat embolism syndrome. Frozen sections of lung with Oil Red O demonstrate intravascular fat globules after long-bone fracture; the diagnosis is impossible on routine paraffin sections.
• Hepatic steatosis and donor liver assessment. Frozen-section Oil Red O separates macrovesicular from microvesicular fat and is performed intraoperatively during transplant procurement.
• Liposarcoma and lipid-rich tumors. Confirms intracytoplasmic neutral lipid in lipoblasts.
• Neonatal and metabolic disease. Muscle and liver biopsies for lipid storage myopathies are snap-frozen; Oil Red O plus enzyme histochemistry is the standard panel.
• Myelin breakdown. Sudan black B demonstrates degraded myelin lipid in macrophages in demyelinating lesions, complementing the Luxol fast blue methods covered in the next chapter.