In vitro.

Experiments performed in cells or test tubes, outside a living organism.

In vitro means in glass, and refers to experiments done on isolated cells, tissues, or purified biochemical systems outside a living organism: cell cultures in a dish, enzyme assays in a tube, receptor binding measured on a plate. These experiments are the workhorse of mechanism research because they are fast, cheap, controllable, and let you change one variable at a time. Much of what is known about which receptor a peptide binds and what signalling it triggers comes from exactly this kind of work. The gap to reality is wide, and it is worth naming precisely. In a dish, the compound is applied directly to the cells at a concentration the experimenter chooses, so absorption, distribution, enzymatic breakdown, kidney clearance, and the barrier problem that dominates peptide delivery all simply do not exist. Cultured cells are frequently immortalized lines that behave differently from cells in tissue, and they lack the surrounding tissue, blood supply, immune cells, and hormonal context of a body. Concentrations that work in a dish are often unreachable in a living organism. The misunderstanding worth correcting is treating an in vitro finding, especially a promotional line like shown to stimulate collagen production, as though it described what happens in a person.