There are also some issues involved when using mesenchymal stem cells derived from the umbilical cord. The isolation and culture time can take quite long, from several days up to several weeks. Once enough cells have been isolated, they need to be expanded to achieve the desired cell number. Because this process can take several weeks, it needs more handling steps (medium change, monitoring, etc.) which increases the change of contamination. In addition, the stem cell culture can not get too confluent or it will loose its potential. They will start to differentiate into a (unwanted) cell type or stop proliferating at all. If this happens, all the time and work will have been in vain.