WPF

public class DepartmentViewModel
{
  public ObservableCollection<GroupViewModel> Groups { get; set; }
  // Other properties here...
}

public class GroupViewModel
{
  public ObservableCollection<TeamViewModel> Teams { get; set; }
  // Other properties here...
}

public class TeamViewModel
{
  public ObservableCollection<EmployeeViewModel> Employees { get; set; }
  // Other properties here...
}

public class EmployeeViewModel
{
  // Other properties here...
}

public class TreeNodeUI
{
    public TreeNodeUI(string sLabel,string sNodeKey,object oTag)
    {
        NodeLabel = sLabel;
        NodeKey = sNodeKey;
        NodeTag = oTag;
        ChildNodes = new ObservableCollection<TreeNodeUI>();
    }

    public string NodeLabel { get; set; }
    public string NodeKey { get; set; }
    public object NodeTag { get; set; }
    public ObservableCollection<TreeNodeUI> ChildNodes { get; set; }
}

public void LoadTree()
{
    NodeList.Clear();
    gUI.TreeNodeUI oNode;
    FurnishCategoryList.OrderBy(x => x.Category);

    foreach (FurnishCategoryDB oDB in FurnishCategoryList)
    {
        oNode= new gUI.TreeNodeUI(oDB.Category, string.Format("/{0}/", oDB.FurnishCategoryID), oDB);
        LoadNode(oNode, oDB.FurnishCategoryID);
        NodeList.Add(oNode);
    }
}

    private void LoadNode(gUI.TreeNodeUI oParent, int iCategoryID)
    {
        List<FurnishSubCategoryDB> lSC = VML.FurnishSubCategoryVMStatic.FurnishSubCategoryList.Where(x => x.CategoryID == iCategoryID).ToList();
        oParent.ChildNodes.Clear();
        foreach (FurnishSubCategoryDB oDB in lSC)
        {
            oParent.ChildNodes.Add(new gUI.TreeNodeUI(oDB.SubCategory, string.Format("/{0}/{1}/", oDB.CategoryID, oDB.SubCategoryID), oDB));
        }
    }