๐ฆ๐๐ผ๐ฝ ๐ณ๐ถ๐
๐ฎ๐๐ถ๐ป๐ด ๐ผ๐ป ๐ณ๐ถ๐
๐ฎ๐๐ถ๐ผ๐ป.
There. I said it. The single-cell field has developed a collective obsession with PFA-based fixation as the answer to one of our most persistent operational headaches: how do you decouple sample collection from cell capture?
It's a real problem. Anyone who has run a multi-site clinical trial, managed a core facility with unpredictable sample flow, or tried to batch longitudinal timepoints knows the pain. Samples degrade. Biology drifts. The clock starts ticking the moment tissue leaves the body.
So, the field reached for the most familiar tool in the histology drawer โ formaldehyde โ and declared the problem solved. Except it isn't solved. It's traded.
Here's what nobody wants to say out loud: PFA fixation solves your logistics problem by destroying half your biology.
Let me be specific, because vague claims don't move science forward:
โ PFA cross-links cause RNA fragmentation. That's why probe-based hybridization had
to replace poly(dT) capture in fixed-cell workflows. That's not innovation โ that's a workaround for chemistry-induced damage.
โ PFA alters protein conformation through methylene bridging. Surface epitopes get masked. If you're trying to do high-fidelity protein quantification alongside transcriptomics โ the very thing that makes single-cell multiomics transformative โ you've compromised your starting material before you even load the instrument.
โ PFA-protein-DNA cross-links obliterate native chromatin architecture. ATAC-seq on
PFA-fixed material? Incompatible. Multiomic ATAC gene expression? Incompatible.
That entire epigenomic axis โ gone.
โ Full-length TCR/BCR profiling requires intact, full-length mRNA. PFA fragments it.
Paired chain immune repertoire recovery from fixed material? Not happening.
โ And because fragmented RNA needs probe-based rescue, you're locked into pre-designed panels for human and mouse only. Working with NHPs, rat models, organoids from non-standard species? Out of luck.
So what are we actually preserving here โ the sample, or just the RNA-seq data?
This is the question the field needs to confront honestly. If your experiment only needs gene expression from fixed cells, PFA-based workflows are perfectly adequate. I won't pretend otherwise. They work. The data is good. But "adequate for scRNA-seq" is not the same as "ready for the future of single-cell
biology."
And the future...well, the future is unambiguously multiome. The papers being published today โ the ones reshaping our understanding of tumour microenvironments, autoimmune mechanisms, vaccine responses, neurodegeneration โ they don't profile one analyte. They integrate transcriptomes, surface proteomes, intracellular signalling states, chromatin accessibility, and immune repertoires. From the same cells. Simultaneously. You cannot do that from PFA-fixed material. This is not a limitation that better engineering will fix. It's a thermodynamic consequence of cross-linking chemistry. The
information is destroyed at the point of preservation.
The alternative already exists. Non-crosslinking cryopreservation approaches โ and I'll disclose my interest here, I'm referring specifically to BD OMICS-Guardโข CRYO โ take a fundamentally different path. No methylene bridges. No protein denaturation. No RNA fragmentation by design. Cells are preserved in a near-native state, cryostored for months, and when you're ready to process them, they behave like fresh cells. That means every downstream modality remains open: WTA, CITE-seq, ATAC-seq, TCR/BCR profiling, intracellular protein detection, flow cytometry. All from one preserved aliquot. And here's the part that should make every core facility director pay attention: because the cells come out in a near-native state, they're not locked to one platform. You preserve once, then choose your instrument โ whatever fits your question. Platform-agnostic preservation. One reagent. Every modality. Every platform.
Thoughts? Pushback?