How to lock aspect ratio for Grid element (make it square)

I'll jump right to the chase:

I'm trying to create a silverlight application with a 3x3 grid.  Here are the row definitions:

RowDefinition rowDef1 = new RowDefinition();
rowDef1.Height = new GridLength(5, GridUnitType.Pixel);
RowDefinition rowDef2 = new RowDefinition();
rowDef2.Height = new GridLength(1, GridUnitType.Star);
RowDefinition rowDef3 = new RowDefinition();
rowDef3.Height = new GridLength(75, GridUnitType.Pixel);

Now, I want the three columns to be like this:

ColumnDefinition colDef1 = new ColumnDefinition();
colDef1.Width = new GridLength(5, GridUnitType.Pixel);
ColumnDefinition colDef2 = new ColumnDefinition();
colDef2.Width = ??????? // equal to rowDef2.Height
ColumnDefinition colDef3 = new ColumnDefinition();
colDef3.Width = new GridLength(1, GridUnitType.Star);

How do I do this? I want to keep the middle element of the grid square, to be determined by the Row 2's height. I also want it to stay square after resizing, as if it's bound to Row 2's height.

Any help would be appreciated. I've searched the internet, but I can't really find anything to help. Also, if it's possible to do this in straight-up xaml, that'd be fine to. There's no real reason to do it in C#.

If it makes any difference, a grid is going to be the content of that middle grid location.

Thanks!