Question

How are stem cells preserved?

Answer 1

if you remember, stem cells are the undifferentiated cells that are capable of growing into any type of specialized cells of the body. These are most abundant during the early embryonic stages. In adults, the pluripotent cells are found in the bone marrow where they produce the different types of blood cells (RBCs, WBCs etc.) hence they are called hematopoietic stem cells.

Complete answer:
Depending upon the source of the stem cells, the procedure of stem cell collection and preservation may vary a little. Example
(a) Harvested embryos: in vitro fertilization, IVF results in the generation of multiple embryos only one of which is used for implantation in the patients. Following the consent of the donor, the remaining embryos can be harvested and allowed to grow into a blastula. At this point, the inner cell mass can be isolated and cultured indefinitely to produce and maintain a line of stem cells if provided suitable nutrition and chemical environment.
(b) Amniotic stem cells: These cells are collected from the amniotic fluid (I.e. the fluid surrounding the growing fetus inside the womb that protects the fetus from impact and facilitates the exchange of gases and nutrients). These can be collected during a routine amniocentesis procedure without any kind of risk to the mother or the baby. They are collected before birth and around the time of the second trimester.
The other two categories of stem cells are collected at the time of the baby’s birth and they are
(c) Umbilical cord fluid: The cells in the umbilical cord blood have a high concentration of hematopoietic stem cells that can grow into different blood cells as well as Mesenchymal stem cells that can grow into different tissues.
(d) Umbilical cord: in addition to hematopoietic and mesenchymal cells, the cord tissue also has endothelial stem cells that have immense potential in regenerative therapies.
Following the birth, the blood and cord tissue is collected and transferred to the lab for further processing. In the lab, the stem cells are collected, purified before being preserved through cryopreservation (preservation by cooling to subzero temperatures) in the following steps.
(1) The cells are first allowed to proliferate and divide to some degree to obtain a sizable store.
(2) These cells are transferred to cryovials. A suitable cryoprotection agent (generally 10%DMSO or DiMethyl SulfOxide) is added to prevent the formation of ice crystals inside the vials when cooled.
(3) The prepared vials are then cooled gradually in the presence of liquid nitrogen at the rate of -1 to -3 degree Celsius per minute until the temperature of -196 degree Celsius is reached.
(4) At this temperature the metabolic activities in the cells are drastically reduced, the cell growth and divisions are inhibited and the stem cells can now be preserved and maintained.

Note: Since stem cell banking itself is a recent phenomenon (~25 years or so) there is yet no scientific evidence as to how long a line of stem cells can be preserved. However, the scientists believe that cryogenically preserved cells have no expiration dates and therefore in theory your stem cells can be preserved for centuries through a combination of cold temperature and careful culturing.