Silverlight组合框问题

我试图在datagrid中使用一个组合框,但是遇到了一个问题。代码如下:

<sdk:DataGrid AutoGenerateColumns="False" Margin="6,71,6,0"  Name="dgridDeptNums" Loaded="dgridDeptNums_Loaded" Height="226" VerticalAlignment="Top">
            <sdk:DataGrid.Columns>
                <sdk:DataGridTemplateColumn Header="Speciality" Width="*">
                    <sdk:DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <ComboBox SelectedValue="{Binding Department, Mode=TwoWay}">
                                <ComboBox.Items>
                                    <ComboBoxItem>Information Technology</ComboBoxItem>
                                    <ComboBoxItem>Human Resources</ComboBoxItem>
                                    <ComboBoxItem>Engineering</ComboBoxItem>
                                 etc...
                                </ComboBox.Items> 
                            </ComboBox>
                        </DataTemplate>
                    </sdk:DataGridTemplateColumn.CellTemplate>
                </sdk:DataGridTemplateColumn>
                <sdk:DataGridTextColumn Header="# of Supervisors" Binding="{Binding Providers, Mode=TwoWay}" Width="*"/>
                <sdk:DataGridTextColumn Header="# of Managers" Binding="{Binding Midlevels, Mode=TwoWay}" Width="*"/>
            </sdk:DataGrid.Columns>
        </sdk:DataGrid>

问题是我想把部门数据绑定起来,但我得到的只是System.Windows.Controls.ComboBoxItem. 我确信我在这里遗漏了一些东西,所以非常感谢您的帮助。