In Vitro vs In Vivo

In vitro means “in glass”: an experiment run outside a living body — a substance in a tube, a monolayer of cultured keratinocytes, a reconstructed epidermis grown on a membrane. Ex vivo sits in between, using donated human skin kept alive for a short time. In vivo means on living people.

Each format answers its own question well. A tube can rank antioxidants by how fast they quench a radical. A cell culture can show that a molecule changes what a fibroblast does. A reconstructed-epidermis model can flag a corrosive formula without an animal, which is why validated laboratory methods now carry a large share of the safety work.

What none of them can do is settle whether a finished product works on a face. Between a good result in a dish and a change in the mirror sit the formula itself, whether the molecule survives in it, whether it crosses the stratum corneum at all, how much arrives, and what the rest of the ingredient list is doing. A cell in a well has no barrier over it: the main obstacle the product has to negotiate has been removed from the experiment.

So “clinically proven” and “shown in laboratory studies” are not synonyms, and neither one is a lie. The useful habit is to ask which of the two a claim rests on, and whether anyone has since checked it on skin.