When researchers want to know whether a compound speeds up or slows down how cells move, the scratch wound healing assay is usually the first experiment they reach for. It is deliberately low-tech. Open a gap in a sheet of cells, then watch how fast the cells crawl in to close it. Everything described here happens in a dish, with cultured cells — for research use only, not a claim about anything that happens in people. If you are researching how a compound influences cell movement, this is one of the methods you will see cited over and over, so it pays to understand what the numbers actually mean.
Below, we walk through what the assay is, how it is run, how migration becomes a number, where it breaks down, and how newer platforms are shoring up its weak spots.
What is a scratch wound healing assay?
In plain terms, the scratch assay measures directional cell migration by making a gap in a confluent layer of cells and watching the cells close it over time. Grow cells until they carpet the bottom of a dish, scrape a clean lane through them, and the cells at the edges start migrating inward. Track how fast that lane vanishes and you have a readout of how mobile the cells are.
The method was formalized in a widely cited 2007 Nature Protocols paper by Liang, Park, and Guan at the University of Michigan, and it remains one of the earliest and simplest methods for studying directional cell migration in vitro. The "wound healing" in the name is an analogy. The closing gap loosely resembles the way cells repopulate a scraped patch of tissue during repair — picture tiles regrowing across a bare stripe on a floor. Imperfect as a metaphor, but it captures the idea.
Its appeal is practical. The assay is inexpensive, runs on an ordinary light microscope, plays well with live-cell imaging, and is especially good for asking how cell-to-cell and cell-to-surface interactions shape movement. That mix is why it has stuck around in labs for nearly two decades while flashier techniques came and went.
How a scratch assay is run, step by step
The workflow is short, but every step carries a control that matters.
Grow a confluent monolayer
Everything starts with a complete, even carpet of cells. In one representative study using human dermal fibroblasts, cells were grown to full confluence in multi-well plates before any scratch was made, because confluence is the critical requirement that keeps wells comparable. Uneven starting layers give you uneven results.
Make the scratch
The classic technique is to draw a sterile pipette tip across the monolayer, leaving a straight, cell-free lane. That is the "wound," and everything that follows measures how quickly its two edges meet again.
Image the gap over time
Researchers photograph the lane at the start, then again at fixed intervals — in the fibroblast example, every four hours across a 24-hour window, with plates held at 37 °C and 5% CO2. Consistent timing and consistent imaging positions are what make one timepoint comparable to the next.