Reviewed by Dr. Ankeet Choxi, and Jarred Mait, MD
In conversations around regenerative medicine, patients often hear about cell counts, cell types, and treatment protocols. Less commonly discussed – but equally important – is the concept of stem cell passage.
It’s a technical term, but it plays a direct role in how cells are prepared, how they behave, and how they may function once introduced into the body.
Understanding what passage means provides a clearer view into how regenerative therapies are developed – and why not all cell-based treatments are biologically equivalent, even when they appear similar on the surface.
Starting With the Basics: What Is Cell Passage?
At its core, cell passage refers to the number of times cells have been grown and re-cultured in a laboratory environment.
Cells used in regenerative medicine are often expanded outside the body through a process known as Cell Culture. During this process, cells are placed in controlled conditions with nutrients, temperature regulation, and an environment designed to support growth.
As the cells multiply, they eventually reach a point where they need more space and resources. At that stage, they are divided and transferred into new culture environments. This step is called a passage.
Each time this process occurs, the cell passage number increases.
So when you hear references to early or late passage cells, it’s describing how many cycles of expansion those cells have undergone.
How Stem Cells Are Expanded in the Lab
To understand passage more clearly, it helps to visualize how cells grow over time.
A small initial population – sometimes derived from donor tissue or a specific cell subset – is placed into culture. With the right conditions, those cells begin to divide. Over time, that population increases.
Once the culture becomes dense, the cells are split and re-seeded. This is where the concept of In Vitro Expansion comes into play.
Each cycle of splitting and re-growing contributes to the overall expansion of the cell population.
From a production standpoint, this process is efficient. It allows laboratories to generate large numbers of cells from a relatively small starting sample.
But as expansion continues, something else happens alongside growth – the cells begin to change.
Why Passage Number Matters
Cells are not static. As they divide and replicate, they undergo subtle biological shifts.
Early passage cells – those that have gone through fewer expansion cycles – tend to retain characteristics that are closer to their original state. These may include:
- Higher replicative capacity
- More consistent differentiation potential
- Greater responsiveness to environmental signals
As passage number increases, cells may begin to show signs of adaptation to the lab environment. This can include changes in:
- Gene expression
- Signaling behavior
- Functional lifespan
One of the key concepts here is Cellular Senescence. Over repeated divisions, cells can gradually lose some of their ability to function as they did earlier in the expansion process.
This does not happen all at once, and it does not render cells unusable. But it introduces a variable that can influence how cells behave in a therapeutic setting.
The Balance Between Expansion and Function
In regenerative medicine, there is often a balance between cell quantity and cell quality.
Expanding cells through multiple passages increases the total number available for treatment. This can make therapies more scalable and accessible.
At the same time, earlier passage cells may retain biological properties that are more aligned with how those cells function naturally in the body.
This creates a practical consideration:
Is it better to have more cells, or cells that have undergone fewer expansion cycles?
The answer depends on how the therapy is intended to work.
In some cases, the primary goal is cell signaling – influencing the local environment through the release of biologic factors. In these scenarios, higher passage cells may still provide meaningful activity.
In other cases, where integration, adaptation, or longer-term interaction with tissue is expected, the functional characteristics of the cells may carry more weight than total volume.
Stem Cell Types and Passage Considerations
Different types of cells may respond differently to expansion.
For example, Mesenchymal Stem Cells are commonly used in regenerative medicine and are often expanded in culture before use. Their behavior can shift gradually across passages, particularly in terms of signaling and differentiation.
Similarly, MUSE Cells are sometimes discussed in the context of stress tolerance and selective differentiation. While passage is only one factor influencing their behavior, expansion conditions still play a role in how these cells are prepared and maintained.
The key point is that passage is not an isolated variable – it interacts with cell type, processing method, and intended use.
How Passage Relates to Potency and Viability
Two additional concepts often come into play when discussing passage: potency and viability.
Stem Cell Potency refers to what the cells are capable of doing – how they respond, signal, and potentially differentiate.
Cell Viability refers to how many of the cells are alive and intact at the time of use.
While these concepts are related, they are not the same.
A high number of viable cells does not automatically mean those cells retain the same functional characteristics as earlier passage cells. Passage number can influence potency even when viability remains high.
This distinction is one reason why passage is considered alongside, rather than replaced by, other metrics.
Why Passage Isn’t Always Discussed
Despite its relevance, passage is not always a central part of patient-facing discussions.
One reason is that it is more complex to explain than straightforward metrics like total cell count. Another is that passage interacts with multiple variables, making it less suitable as a single-point comparison.
Instead, it tends to be part of a broader set of considerations that include:
- Cell sourcing
- Processing methods
- Delivery technique
- Clinical intent
When taken together, these factors provide a more complete picture of how a therapy is designed.
Looking at Passage in Context
Understanding passage is not about assigning a fixed value to a specific number. It’s about recognizing that how cells are expanded influences how they behave.
In practice, this means considering:
- What stage of expansion the cells represent
- How that stage aligns with the goals of treatment
- How the therapy is being delivered and monitored
In many cases, particularly with targeted, image-guided procedures, the behavior of cells within a specific environment becomes more relevant than the total number alone.
This shifts the focus from quantity to function.
A Behind-the-Scenes Variable With Real Impact
Stem cell passage is not something most patients see, but it is a meaningful part of how regenerative therapies are developed and delivered.
It reflects the process of expansion – how cells are grown, multiplied, and prepared for clinical use. And with that process comes a balance between scalability and biological behavior.
As passage number increases, cells continue to grow in quantity, but they may also undergo changes that influence how they function. Understanding that balance helps bring more clarity to how therapies are structured.
In regenerative medicine, where outcomes depend on both biology and precision, these underlying variables matter.
They are part of the larger picture – one that goes beyond numbers and into how treatment is designed to work.
FAQ
What does stem cell passage mean?
It refers to the number of times cells have been grown and re-cultured during laboratory expansion.
Why are stem cells expanded in the lab?
To increase the number of cells available for treatment through controlled growth conditions.
Do higher passage cells work the same as early passage cells?
They may behave differently, as repeated expansion can influence responsiveness and functional characteristics.
Is a higher passage number better or worse?
It depends on the treatment goal. Higher passage increases quantity, while earlier passage may retain certain biological properties.
Should patients ask about passage?
Understanding how cells are prepared can provide additional context when evaluating treatment approaches.
The information provided in this article is for educational and informational purposes only and is not intended as medical advice. Treatments and outcomes described may not be appropriate for every individual. Always consult a licensed healthcare provider to determine the best course of care for your specific needs.
Certain regenerative medicine procedures discussed – such as stem cell therapy, exosome therapy, or other biologic treatments – may be considered investigational or not FDA-approved for all conditions. Florida law requires that we disclose this status. While these procedures are offered in accordance with state and federal guidelines, their safety and efficacy have not been fully established by the U.S. Food and Drug Administration.
Results vary, and no guarantee of specific outcome or benefit is implied. All medical procedures involve potential risks, which should be discussed with your treating provider prior to treatment.
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