How do I specify CSS classes for specific rows in a GridView?

I am creating a SharePoint web part in C# and part of it outputs a GridView control to the page. While I can get fairly extensive control over the way it is displayed by setting the CSS class of the GridView itself, what I would really like to do is be able to specify classes to certain specific td elements. I’m not sure how to go about doing this, or if it would be done at the time that the GridView is being populated with rows, or at the time the GridView is added to the page.