adding objects to treeview as expanded

Question

I have xml file contains data, I serialize this file and set to List and adding all patient items to treeview within for loop All items looks fine but there is no expand arrow... I need these items with expand button for all and first item expanded in first view. How can I do this

namespace WpfApplication1
{
    [Serializable, XmlRoot("patients")]
    public class patients
    {
        [XmlElement("patient")]
        public List<patient> patients_list { get; set; }

    }
    public class patient
    {
        [XmlElement("firstname")]
        public string name { get; set; }
        [XmlElement("lastname")]
        public string surname { get; set; }
        [XmlElement("age")]
        public int age { get; set; }
        public string gender { get; set; }
        [XmlArray("exams"), XmlArrayItem("exam")]
        public List<exam> exam { get; set; }
    }
    public class exam
    {
        [XmlElement("id", IsNullable = false)]
        public int id { get; set; }
        [XmlIgnore]
        public DateTime date { get; set; }

        [XmlElement("date")]
        public string DateString
        {
            get
            {
                return XmlConvert.ToString(date, XmlDateTimeSerializationMode.Utc);
            }
            set
            {
                // You will need to check whether the dates and times in your XML file are in universal time or local time!
                date = DateTime.Parse(value, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal);
            }
        }
        [XmlElement("comment")]
        public string comment { get; set; }
    }
}

and the code piece that I add nodes to tree(all items contains sub items and sub list items..)

DataReader readXML = new DataReader();
            List<patient> items = readXML.ReadXML();

            foreach(patient item in items)
            {
                trvMenu.Items.Add(item);
            }