If you need more details contact me. Tamara, We use Osmium to stain culture cells in agar, before we embed, then you can cut out the most concentrated area of cells to put in plastic. I needed to stain agar several years ago. Rather than stain it, I found it better to incorporate a colored microsuspension into it. Its a high mw polysacharride, much like agar and is virtually innocuous.
Sigma carries it.
This staining survives subsequent infiltration into methacrlate resins very well. Trouble is, if you are going through acetone, fast green is really not to solulble in acetone. I have not found a very good stain to use with acetone dehydrations, although Alizarin red isn't bad. Don't know what it might do to antigens, though. Doesn't work well with acetone.