Researchers label cultured cells with a passage number, and patients rarely learn what it means. Cell passage number for mesenchymal stem cells counts how many times a population has been expanded in a lab, not how old the donor is. This guide explains the term and the evidence.
TLDR: A passage is one round of lifting cells off a culture surface and reseeding them at lower density. The number counts expansions, not donor age or calendar time. Research reports falling population doublings, shorter telomeres, and eventual proliferation arrest as it rises. Expansion also changes how the FDA classifies a product. Banked adipose tissue sits at passage zero. No adipose-derived product is FDA approved, and banking does not guarantee eligibility, access, or clinical benefit.
Important Disclaimer: Save My Fat is a tissue preservation connector and does not provide FDA-approved treatments or cures. Passage number is a laboratory measurement, not a measure of whether any product will help a patient. Save My Fat does not guarantee eligibility, access, or clinical benefit. This article is provided for educational purposes only, and readers should consult their own licensed healthcare professionals.
Read almost any mesenchymal stem cell paper and you will hit a line like “cells were used at passage 4.” Nothing in that sentence explains itself. Patients often assume the number describes the donor’s age, or how long a sample sat in storage. It describes neither.
The term matters for two separate reasons. It predicts how a cell population behaves in the lab, and it helps determine how the FDA classifies a finished product.
What a Passage Actually Is
Cells grown in a dish do not multiply forever in the same space. They spread until they run out of room, and crowded cells stop dividing. To keep the population growing, a technician lifts the cells off the surface, counts them, and reseeds a smaller number into fresh dishes.
That cycle is one passage. Do it once and the cells are at passage 1. Do it four more times and they are at passage 5. The number tallies how many times a population has been split and expanded.
Passage Number Is Not the Donor’s Age
A young donor’s cells can reach passage 12 in a few months, and an older donor’s cells can sit at passage 0 indefinitely. The number tracks handling, not biography. It does not track shelf time either, because frozen cells are not dividing.
What Cell Passage Number Means for Mesenchymal Stem Cells
Every passage is a round of division, and division carries a cost. Researchers tracking mesenchymal stem cells across serial passages report a consistent direction: growth slows, telomeres shorten, and the population drifts from the profile it started with.
A 2006 study in BMC Cell Biology followed cultures through ten passages. Bonab and colleagues reported the average number of population doublings decreased from 7.7 to 1.2 in the 10th passage, mean telomere length decreased from 9.19 kbp to 8.7 kbp in the 9th passage, and differentiation potential dropped from the 6th passage on. Population doubling counts how often a population doubles between splits, so 1.2 is near standstill.
Senescence Starts Earlier Than Most People Expect
A 2008 analysis in PLoS One traced the same arc in more detail. Wagner and colleagues found that within 43 to 77 days of cultivation, covering 7 to 12 passages, MSC demonstrated morphological abnormalities, enlargement, attenuated expression of specific surface markers, and ultimately proliferation arrest. Adipogenic differentiation potential decreased while the propensity for osteogenic differentiation increased.
The cells grew less inclined to form fat and more inclined to form bone in laboratory assays. That paper also undercuts any idea of a safe early window, describing replicative senescence of MSC preparations as a continuous process starting from the first passage onwards.
A Passage by Passage Timeline From the Published Research
These findings come from different laboratories using different cell sources, so they do not line up perfectly. Read together, they still sketch a rough timeline, placing each reported observation at the passage where a study recorded it.
| Passage | What the research reports | Source |
|---|---|---|
| 0 | No expansion yet. Cells have never been lifted and reseeded. Preserved, uncultured tissue sits here. | Definition, not a finding |
| 1 to 2 | Senescence already underway, continuous from the first passage onwards | Wagner 2008 |
| 2 to 4 | Infrapatellar fat pad cells stayed stable | Radhakrishnan 2019 |
| 3 | Stable proliferation and morphology for at least three passages post-thaw, then major transcriptional shifts | Johansen 2026 |
| 6 | Differentiation potential dropped from the 6th passage on; nucleostemin lost and doubling time rose beyond passage 6 | Bonab 2006, Radhakrishnan 2019 |
| 7 to 12 | Morphological abnormalities, enlargement, attenuated surface markers, proliferation arrest, within 43 to 77 days | Wagner 2008 |
| 9 | Mean telomere length fell from 9.19 kbp to 8.7 kbp | Bonab 2006 |
| 10 | Average population doublings fell from 7.7 to 1.2 | Bonab 2006 |
The last two rows sit inside the 7 to 12 window, where most of the decline concentrates. None of this describes a clinical outcome or predicts what any sample will do.
The Adipose Specific Picture
Two studies used fat-derived cells directly. Radhakrishnan and colleagues worked with cells from the infrapatellar fat pad, the cushion of fat behind the kneecap. Those cells were stable at passages 2 to 4. Beyond passage 6, the team reported loss of the proliferative marker nucleostemin and an increased population doubling time.
Why the Passage 3 Inflection Matters
A 2026 report in Stem Cells Translational Medicine examined a GMP-manufactured adipose product. Johansen and colleagues found ASC maintained stable proliferation and morphology for at least three passages post-thaw, and that major transcriptional shifts occurred between passage 3 and later passages. A transcriptional shift means the pattern of genes a cell actively reads has changed, even when the cells still look ordinary.
Why Cell Culture Expansion Changes Regulatory Classification
Passage number is not only a biology question. The cell culture expansion regulatory classification issue turns on how much processing altered the cells. The definition at 21 CFR 1271.3 treats processing of cells or nonstructural tissue as minimal when it “does not alter the relevant biological characteristics of cells or tissues.” Serial passage is designed to change what a population does.
All four criteria at 21 CFR 1271.10(a) must be met before a product is regulated as tissue alone under Section 361 of the Public Health Service Act. Products missing any criterion are generally handled as drugs or biologics under Section 351, requiring an investigational new drug application. FDA’s tissue product questions and answers page covers that split, and the minimally manipulated standard is worth reading in full.
Why Passage Zero Matters for Banked Tissue
Preserved adipose tissue sits at passage zero, because nothing has been lifted and reseeded. That is a factual description of the material, not a claim about what it can do, and it is the honest reason cell age is worth understanding. Every change described above happens in a laboratory, after tissue leaves the body.
Whether that matters to any future research protocol depends on the protocol, and nobody can say today what a future protocol will require. Readers wanting the operational picture can see how adipose tissue biobanks handle intake, and how cryopreservation and viability get measured.
No adipose-derived product is FDA approved. Preserving tissue does not create a treatment pathway, does not enroll anyone in a study, and does not guarantee eligibility, access, or clinical benefit. Individual results cannot be predicted.
Frequently Asked Questions
What does passage number mean when researchers describe cultured cells?
It counts how many times a cell population has been lifted off its culture surface and reseeded at lower density. Passage 4 means that cycle ran four times. The number describes laboratory handling only, not the donor’s age or time in storage.
Why does culturing and expanding cells change their FDA regulatory classification?
Because expansion is processing, and processing is what the minimal manipulation definition measures. Under 21 CFR 1271.3, processing of cells is minimal when it does not alter their relevant biological characteristics. Serial passage changes how a population grows, so expanded products commonly fall outside that definition.
Does Save My Fat culture or expand banked cells?
No. Save My Fat is a connector linking patients and providers to a United States tissue bank. It does not collect, process, store, expand, or treat. L2 Bio handles laboratory processing and storage. Preserved tissue is not cultured, so banking generates no passage number.
Do cells change characteristics as passage number increases?
Yes, and the published record is consistent. Population doublings fall, telomeres shorten, differentiation potential declines, surface marker expression becomes attenuated, and proliferation eventually stops. Wagner and colleagues describe senescence as continuous from the first passage onwards, so change is gradual rather than sudden.
How does passage number relate to the Section 351 versus Section 361 pathway distinction?
Passage number is evidence about processing, not a rule in itself, and no regulation names a cutoff. Products meeting all four criteria at 21 CFR 1271.10(a), including minimal manipulation, may be regulated as tissue under Section 361. Culture expanded products generally fall under Section 351.
Does banking tissue at passage zero guarantee it will be usable later?
No. Passage zero describes the condition of the material, not its future. No adipose-derived product is FDA approved, future protocols and their acceptance criteria do not exist yet, and regulatory status can change. Banking does not guarantee eligibility, access, or clinical benefit.
Key Takeaways
A passage is one round of lifting cells off a surface and reseeding them, so the number counts expansions rather than donor age or freezer time. Bonab and colleagues reported population doublings falling from 7.7 to 1.2 by the 10th passage, telomere length falling from 9.19 kbp to 8.7 kbp by the 9th, and differentiation potential dropping from the 6th on.
Wagner and colleagues documented morphological abnormalities, attenuated surface markers, and proliferation arrest across passages 7 to 12, plus a shift from adipogenic toward osteogenic potential, and framed senescence as continuous from the first passage. Adipose-specific work puts a transcriptional inflection between passage 3 and later passages. Expansion also bears on the minimal manipulation criterion. Preserved tissue sits at passage zero, which describes the material and promises nothing.
Save My Fat operates as a tissue preservation service, not a medical practice or treatment provider. Stem cell and regenerative medicine regulations vary by state, including specific informed-consent and disclosure requirements in Florida, Utah, and Nevada governing tissue and stem cell services. Banking adipose tissue does not connect patients to any treatment pathway, and any future use depends on FDA regulatory status, physician guidance, and the availability of approved or investigational pathways at that time.
Patients comparing preservation programs can review current pricing, and questions go to the contact page.
Save My Fat partners with L2 Bio for laboratory processing and storage.
This article is for educational purposes only and does not constitute medical or legal advice. Legal and medical review including neurology and neurosurgery input is required before publication. Please consult your neurologist or neurosurgeon before making any decisions about banking, treatment, or research participation.
About the author: Oscar Tellez is the founder and CEO of Save My Fat. He holds a Bachelor of Science in Exercise Science and Health Promotion from Florida Atlantic University. He has spent more than a decade in the regenerative medicine industry across product distribution, laboratory and vendor relationships, and provider training. He is not a licensed clinician, and this article is educational, not medical advice.
Related guide: the stem cell science glossary.
