How do I get the header content for a DataGridColumnHeadersPresenter control template?

Question

<dg:DataGridColumnHeadersPresenter>
    <dg:DataGridColumnHeadersPresenter.Template>
        <ControlTemplate>
            <Border>
                <ContentPresenter ContentSource= ??? />
            </Border>
        </ControlTemplate>
    </dg:DataGridColumnHeadersPresenter.Template>
</dg:DataGridColumnHeadersPresenter>

Thanks for your help!