[rgejman]

Many things that are done in biology (and medicine to some extent) are way simpler than they seem because much of it was figured out a long time ago using tools that scientists/doctors had at hand.

We grow cells in what is essentially sugar water + some growth factors that we get from cow serum (b/c America has lots of cows). Google tissue culture 101 videos on youtube to get an idea of how it all works. You can't (and don't want) to inject the animals with all that fluid + protein (it can be liters), so you transfer the cells into tubes and spin them in a centrifuge (~300 x g). Spinning them will bring them to the bottom of the tube and they will form a pellet. The pellet sticks to the bottom as you literally dump the fluid out. Now you can resuspend the cells in a few microliters of a saline solution and inject them into the mice.

For example, cells growing in an incubator usually need to be at a density of 200k-1e6/ml so that they have enough nutrients and waste doesn't build up too quickly. For 5e6 cells, that would be 5-25ml of fluid, which is WAY too much to inject into a mouse, so you have to concentrate the cells into a much smaller volume before you can inject. I will typically inject mice with 100µl of fluid containing 5e6 cells.

In terms of injection, you can google subcutaneous injections in mice to see how it's done. It's the same as if you've ever had a tuberculosis skin test or a minor operation where they injected you with lidocaine locally (or what your dentist does). The cancer cells are injected into the space right under the skin.

[dmix]

Right, I should have figured you meant centrifuge when talking about spinning. And interesting re: subcutaneous injections, I watched a video, seems pretty straight forward.