Fixation and permeabilisation concepts

Fixation is a chemical commitment. After it, some epitopes are still there and some are not, and the clone's sheet is the record that matters. The research use on this page is intracellular staining for a cytokine or a nuclear protein, read on a cytometer or a microscope, and the limit of the claim is that a lost signal can be chemistry rather than biology. How the cytometer scores the cells that remain is in how a flow cytometer reads a cell. How a fixed slide can glow for the wrong reason is in fluorescence microscopy without the myths.

Instrument classes are in the scientific instruments catalogue. The experiment's context is the molecular biology pathway. Fixative-compatible optics and sample format belong in a quote request.

A concrete use and its limit

You have treated cultured cells, and you want to know whether a transcription factor has moved into the nucleus, or whether a cytokine is being held inside a blocked secretory pathway. The cytometer can tell you what fraction of a gated population stains above a control. The microscope can tell you whether the stain sits in the nucleus or in the cytoplasm, within the resolution of the objective. Neither instrument can tell you that the epitope survived a fixative the clone hates. A negative after the wrong permeabilisation is a methods result. Say so, or test the sheet's method against the method you inherited.

Cross-linking fixation

Formaldehyde-class fixatives cross-link proteins and stabilise the cell so later detergent does not simply dissolve it. Laboratories often use a freshly diluted low-percent paraformaldehyde in buffered saline, cold or as the sheet specifies, for a short incubation. Exact percent and time belong to the protocol you are following. Over-fixation can increase autofluorescence and can hide epitopes. Under-fixation lets cells fall apart when detergent arrives, which looks like a lost population on the cytometer and like debris on the slide.

Aldehyde fixation itself adds background glow on a microscope, which is why an unstained fixed control belongs in the imaging session. Quenching free aldehydes is a microscopy habit for some amine-reactive dyes and some tissues. Do not invent a quench the antibody sheet forbids. Fluorescent proteins sometimes survive formaldehyde and sometimes do not. If the panel includes a fluorescent protein, test survival before you interpret a dim channel.

Permeabilisation classes

Detergent permeabilisation inserts into membranes and lets antibody through. Saponin is a milder, often reversible member of this class: many cytokine protocols keep a little saponin in the stain and wash buffers because the pores close when the detergent leaves. Stronger detergents, such as the Triton class, punch more thoroughly and are common when you need to reach a nuclear epitope or clear a cytoplasm for imaging. They are also better at destroying surface staining you meant to keep.

Alcohol permeabilisation, typically methanol or ethanol, dehydrates and precipitates. It can reveal phospho-epitopes that cross-linking hides, and it can destroy other epitopes and some fluorophores, including some fluorescent proteins and some tandems. Alcohol both fixes and permeabilises, so a separate formaldehyde step may or may not be in the sheet. Follow the sheet for that clone. A phospho-flow panel copied onto a cytokine clone, or the reverse, is a frequent false negative.

The order is part of the chemistry. Surface antibodies that cannot tolerate fixative go on first. A fixable viability dye goes on before the fixative, in a buffer the dye's sheet allows. Intracellular antibody goes on after permeabilisation, in the buffer that keeps the pores open. Skipping the viability dye because the cells "looked fine" before the fix leaves dead-cell binding in the intracellular channel, where nonspecific stickiness is already high.

Branch when the control is blank

If the intracellular positive control is blank, check the order and the class before you conclude the treatment failed. A cytokine clone run in an alcohol protocol, or a transcription-factor clone run in saponin alone, is a methods mismatch. Run the sheet's method on a known positive sample. If that works, the experimental negative is more believable. If the known positive is also blank, the antibody, the fluorophore, or the buffer is the problem.

If surface markers collapse only after the intracellular step, stain them before fixation or choose a clone the sheet marks as tolerant. If background rises on the microscope after aldehyde fixation, compare the unstained fixed slide at the same exposure. A brighter unstained field means you cannot call weak specific stain. The exposure and filter discussion in MicroscopyU is general optics, not a fixative recipe.

A split that tells chemistry from biology

Take one known-positive sample and divide it across the two chemistries you are tempted to treat as equivalent: formaldehyde plus the mild detergent class, and the alcohol class another clone asked for. Stain both with the same antibody lot. If only one is positive, the epitope is method-dependent and the sheet was the right authority. If both are positive and the microscope location agrees, you have a reason to choose the method that also preserves the surface markers. If neither is positive, check the antibody before you edit the biological conclusion. Record which split you ran. A later reader should not have to guess why the intracellular channel exists in only half the files.

Safety and the research limit

Formaldehyde is a chemical hazard with its own laboratory rules. Permeabilisation buffers can contain detergents and alcohols that also need handling rules. None of that chemistry is an automatic biosafety inactivation for every agent. The institution decides, with the WHO Laboratory biosafety manual and the CDC BMBL as references. This page does not approve a fixed sample for a shared instrument, and it does not approve skipping fixation because it is inconvenient. A cytokine percentage is not a clinical immune assay. Reporting practice is discussed by the International Society for Advancement of Cytometry.

What to send with an enquiry

State whether the read-out is a cytometer or a microscope, whether you need an intracellular step, and which fixative class the clones already require. Ask if the instrument and the room can accept that material after fixation, as a facilities question rather than a promise. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A microscope family name does not tell you that the epitope will survive.