One is a living cell. The other is a vesicle a cell secretes. That distinction changes how each product works — and what should be tested for.
The difference is categorical, not just a matter of degree: stem cells are living cells, with a nucleus, the capacity to divide, and in many cases the capacity to differentiate into other cell types. Exosomes are extracellular vesicles — small, membrane-bound packages of molecular cargo that a cell secretes into its surroundings. They have no nucleus, cannot replicate, and are not, in the biological sense, alive.
| Feature | Stem Cells | Exosomes |
| Cellular status | Living cells | Acellular vesicles |
| Typical size | 10–30 microns | 30–150 nanometers |
| Key release test | Viable cell count / viability % | Particle concentration and size distribution |
| Mechanism | Direct cellular activity, paracrine signaling, possible differentiation | Paracrine signaling only, via delivered molecular cargo |
A viability percentage is meaningless for an exosome product — there are no cells to be viable. Conversely, a particle count and size distribution is the wrong test to ask for on a cell product. Documentation should match the product category: cell products should report a lot-specific viable cell count (see How Stem Cell Counting Works), and exosome products should report particle concentration and size, typically within the 30–150 nm range that distinguishes exosomes from larger vesicle types.
This page is educational reference material for licensed healthcare providers and does not constitute medical, legal, or regulatory advice. HCT/P products are not FDA-approved.