This article in 30 seconds
- Conditioned medium is collected as the whole liquid. The target components come mixed together with animal-derived material such as bovine serum, with supplemented factors, and with cell debris.
- Secretome is designed never to mix them in. Rather than removing them afterward, it is made with a medium and a process that never bring unwanted components in — a liquid of nothing but what the cells secreted.
- The difference shows up in how easily it can be explained. Fewer elements vary from lot to lot, and the origin of every component can be accounted for from the manufacturing process itself.
1. The bottom line — remove it afterward, or never let it in
The conclusion first. Conditioned medium is the culture liquid collected as a whole after stem cells have been cultured. Alongside the components the cells secreted, it holds ingredients of the medium used for culture (animal-derived material such as bovine serum, and supplemented factors) together with cell debris and ammonia generated during culture.
Secretome, by contrast, is composed of only the components the cells secreted. What separates the two is not purification technology applied after collection. It is the design of the medium and the culture process, which never brings unwanted components in to begin with.
The key point
To put it in cooking terms, conditioned medium is skimming unwanted material off a finished pot, while secretome is building the dish from clear stock alone. Skimming cannot lift out what has already dissolved into the liquid.
2. What is mixed into conditioned medium
There is one thing to grasp. Under conventional manufacturing, whatever is added to grow the cells is carried straight through to the product.
Conventional culture of mesenchymal stem cells (stem cells that can differentiate into bone, fat, and other tissues) uses a medium containing fetal bovine serum (FBS) to support cell growth. When the conditioned medium is collected, the following come along with the secreted components you actually want.
- Medium-derived factors: proteins, RNA, and other constituents of bovine serum
- Supplemented factors: ingredients added to promote cell growth
- Unwanted cell-derived material: dead cells, cell debris, and ammonia as a metabolic byproduct
None of these are target components of the product. Because their amounts fluctuate with culture conditions and from lot to lot, they are also a source of compositional variability.
3. The option of removing them afterward, and its limits
The conclusion first: removal after collection is a step forward, but it has an inherent ceiling — components close in character to the target cannot be fully separated out.
Unwanted components can indeed be removed after collection, using techniques such as ultrafiltration and size-exclusion chromatography (a method that sieves components apart by size), which separate on differences in molecular weight. Compared with untreated conditioned medium, this is a genuine step forward in terms of safety.
Complete separation, however, is not straightforward. RNA derived from fetal bovine serum has been reported to interfere with the analysis of extracellular RNA derived from cultured cells [1]. The closer the unwanted components are in character to the target components, the harder they are to separate.
The key point
Separating them afterward is like picking grains of salt out of white sand. Things alike in appearance and alike in size cannot be fully sorted apart, however carefully the work is done. The remove-it-afterward approach has an inherent ceiling. This problem is examined in detail in Bovine serum RNA cannot be fully removed after the fact.
Set the three approaches side by side and they compare as follows.
| Item | Conventional conditioned medium | With removal processing | Stem cell secretome |
|---|---|---|---|
| Safety | 1 of 3 (medium-derived components remain) | 2 of 3 (reduced by removal) | 3 of 3 (never present in the first place) |
| Medium-derived factors | High | Removed after culture | None |
| Supplemented factors | High | Removed after culture | None |
| Bovine serum | Used | Used (removed later) | Not used |
| Design philosophy | — | Remove afterward | Never let it in |
Safety is rated on three levels, △ < ○ < ◎, by how much medium-derived and supplemented material remains.
4. The six conditions we require before calling it a secretome
We use the word “secretome” only for preparations that satisfy all six of the following. They also work as a checklist to run through in order.
-
Composed solely of components secreted by cells or tissue
A liquid that also carries components originating in the medium falls outside this definition.
-
No cytokines (proteins that carry signals between cells) or growth factors remain in the collection medium
If the collection medium itself contains growth factors (proteins that stimulate cell growth and repair), there is no way to tell whether the growth factors in the product were secreted by the cells or were simply in the medium from the start. What the figures on a composition report stand for becomes ambiguous.
-
Contains proteins, peptides, nucleic acids, and membrane vesicles such as exosomes (tiny membrane-bound sacs released by cells)
The requirement here is that it be the full body of what the cells secreted.
-
Free of harmful substances such as ammonia and heavy metals, and of impurities such as dead cells
Metabolic byproducts generated during culture, and unwanted material of cellular origin, are kept out of the product.
-
No bovine serum used
As long as bovine serum is used, no amount of removal processing can completely rule out residual serum-derived components. Choosing not to use it at all is what keeps the product simplest to account for.
-
Composition backed by data (protein quantification, measurement of key cytokines, exosome analysis, and so on)
Not merely claiming the definition, but being able to show it in measured data.
The two that do the most work in practice are conditions 2 and 5. Both bear directly on whether the figures on a composition report can be explained by where they came from.
We have reworked these six into a list of questions to put to suppliers in Six Points to Confirm with Your Supplier. It may be useful when comparing products.
5. What changes for the buyer
In day-to-day use, the difference in manufacturing shows up as less variability and easier explanation. Three points in particular.
Less compositional variability
With no medium-derived components entering the product, there are fewer variables to fluctuate between lots. We measure cytokine levels and the miRNA content of exosomes for every lot and disclose the results.
Every component can be traced to its origin
“Where did this component come from?” can be answered from the manufacturing process itself. Wherever handling has to be recorded and explained, being able to trace a component back to its origin is worth a good deal.
A simpler answer on animal-derived components
Because no bovine serum is used, concerns about animal-derived components can be met not with a removal rate but with “we do not use it.”
The definition of conditioned medium itself and how to read a specification are covered in What Is Stem Cell Conditioned Medium; the wider picture of the secretome concept in What Is a Secretome; and the varieties of “leave it out” medium design in Medium Design by Exclusion.
Frequently Asked Questions
Is secretome a type of conditioned medium?
In the broadest sense, both are obtained from the liquid in which cells have been cultured. We, however, reserve the term secretome for preparations that meet all six of the conditions set out in this article. Because the same word is sometimes used more loosely in the market, we recommend checking the definition and the availability of compositional data before purchasing.
How does it differ from products containing isolated exosomes only?
Secretome contains soluble factors (components that act in dissolved form) such as cytokines and growth factors in addition to exosomes. Products consisting of isolated exosomes alone do not contain soluble factors.
Do you disclose compositional data?
We measure cytokine levels and the miRNA content of exosomes for every manufacturing lot, and provide a Certificate of Analysis and compositional data on request. See Safety & Quality for details.
This article describes how the product is manufactured and quality-controlled. Our stem cell secretome is supplied for research use only and is not intended to demonstrate efficacy against any specific disease.
References
- [1] Wei Z, Batagov AO, Carter DR, Krichevsky AM. Fetal Bovine Serum RNA Interferes with the Cell Culture derived Extracellular RNA. Sci Rep. 2016;6:31175. doi:10.1038/srep31175
View Stem Cell Secretome specifications Request compositional data