Development and characterization of FI 7500: a novel near-infrared dye for in vitro and in vivo cell labeling
This paper presents the development and comprehensive characterization of FI 7500, a novel near-infrared dye that serves as a versatile and reliable alternative to bioluminescence for visualizing cellular morphology in 2D/3D cultures and tracking tumor growth and metastasis in vivo with long-term stability and minimal impact on cell viability.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Scientists have long relied on a simple trick to see the invisible: they paint cells with glowing dyes. By attaching a fluorescent tag to a living cell, researchers can watch that cell move, divide, and interact with its neighbors under a microscope. For decades, these paints have been limited to visible light, like the colors of a rainbow, which scatter easily when trying to look deep inside a living body. To see deeper, scientists turned to a different part of the spectrum: near-infrared light. This invisible light passes through skin and tissue much more easily than visible colors, acting like a flashlight that can shine through a thick fog. However, using this light inside a living animal has traditionally required a complicated workaround. Researchers usually had to genetically engineer cells to produce their own light, a process that involves inserting foreign genes and then feeding the animals a special chemical substrate every time they wanted to take a picture. This method is expensive, time-consuming, and can alter how the cells behave.
A team of researchers set out to find a simpler way. They wanted to test a new, ready-to-use dye called FI 7500, which glows in the near-infrared range without needing any genetic changes to the cells. The goal was to see if this dye could stick to cells, stay bright inside a living animal, and allow scientists to track where those cells went, all without the hassle of genetic engineering or repeated chemical injections. If successful, it would offer a straightforward alternative for watching how cells move through the body, from the surface of the skin to deep inside organs.
The researchers began by testing the dye's behavior in a petri dish. They mixed the dye with various types of human and mouse cells, watching carefully to ensure it did not harm them. They found that at a specific, low concentration, the dye was safe and began to glow within thirty minutes. Under the microscope, the dye did not just coat the outside of the cell; it settled specifically inside the endoplasmic reticulum, a network of tubes and sacs that acts as a factory and transport system within the cell's cytoplasm. Because the dye filled this internal network, it lit up the entire shape of the cell, including long, thin extensions, while leaving the central nucleus dark. This provided a clear, detailed map of the cell's structure. The team also tested the dye on three-dimensional balls of cells, known as spheroids, which mimic the way cells clump together in real tissues. In live, unaltered spheroids, the dye could see deep enough to show the internal structure of small clusters. When the spheroids were fixed and treated with a special clearing agent to make them transparent, the dye could penetrate even deeper, revealing the insides of larger clusters.
Moving from the dish to the living world, the team injected cells stained with this dye into mice to see how long the glow would last. In one experiment, they placed the stained cells under the skin of mice. The dye remained bright and visible for three weeks, allowing the researchers to track the growth of the cells as they formed tumors. Interestingly, the glowing area did not grow larger as the tumor grew; instead, the light stayed concentrated in the original spot while the tumor expanded around it. This told the scientists that while the dye was excellent for tracking where the cells started, it could not be used to measure the size of a growing tumor simply by looking at the light. However, the dye did not stop the cells from growing, proving it was safe for the animals.
In a second experiment, the team injected the stained cells directly into the bloodstream of mice to see if they could track where the cells traveled. These cells were known to naturally settle in the lungs. Within hours, the researchers could clearly see the glowing cells accumulating in the lungs, confirming that the dye stayed attached and bright enough to follow the cells' journey. This worked just as well as the traditional method of using genetically engineered cells, but without the need for complex genetic modifications or repeated injections of chemicals. The study concludes that this new dye is a reliable tool for watching cells move and settle inside a living body. It offers a practical, less invasive way to study cell migration and metastasis, removing the barriers of genetic engineering and giving researchers a clearer, more direct view of how cells behave in real time.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.