Controlling cell sizes in a Datagrid which contains expanders

Hi,

I'm having a problem with a datagrid where each cell contains an expander which in turn contains an RTFControl. The problem is that the expander ignores the size of the datagrid cell it is contained in and doesn't add a vertical scroll bar when it should. The table populates from a container which is provided from the view model so can be 0 or more items.