How to open a WPF page in a Grid?

How can I open a page into a Grid? I can open the WPF page in a new window, but I need to open it in the a Grid. Hvow do I do that?

Code works for open a page in a new window

      NavigationWindow _navigationWindow = new NavigationWindow();
      _navigationWindow.Height =this.Height;
      _navigationWindow.Width =      this.Width;
      _navigationWindow.Show();
      _navigationWindow.Navigate(new ProductBacklog());

But how can I open a page into my MainGrid?

<Grid x:Name="MainGrid" DockPanel.Dock="Bottom"></Grid>