This is something me and my friends have been curious about for a while. Its not the serious, but if anyone knows what they do with the breast tissue and fat I'd love to know. Do they just throw it away or is there another use for it?

Thrown out or incinerated, possibly sent to a lab first. When I had a Phalloplasty length reduction, a small amount of tissue was sent to the lab for testing (my surgeon was curious about the effects of urine on the neo-urethra) and the rest was simply thrown in the trash.