I'm sorry you are feeling as though no one cares for you. I would just like to say though, that the majority of the population doesn't directly deal with extremes in mood or suicidal ideations. And while it may seem as though they are being insensitive they might either a) not realise you are being serious b) feel uncomfortable and their laugh is a reflection of this, or c) Not know what to do.

Also, it depends on how close you are with them. If these coworkers of yours are intimate friends that you share details with, and they laughed when you said you were suicidal... well then that is a little scary. But if these people are more so acquaintances then close friends, then chances are, they don't know a whole lot about you... and in that case, you can't really expect them to look into this a whole lot. They're probably not on the lookout for you to be talking about things with this degree of magnitude with them if you've never had similar conversations before.

If you feel as though you need to talk with one of them about it though, maybe you could bring it up in a different way.