Stem cells are unique cells with the ability to self-renew and differentiate into various specialized cell types. These properties allow them to form new cells and repair damaged tissues. Their potential to treat diseases has led to significant interest in how these cells are obtained for therapeutic applications.
Understanding Stem Cells and Their Origins
Stem cells are categorized by their origin and differentiation potential. Adult (somatic) stem cells are found in specific adult tissues like bone marrow, fat, and blood. These cells are multipotent, differentiating into a limited range of cell types relevant to their tissue of origin. For example, hematopoietic stem cells form blood cells, while mesenchymal stem cells become bone, cartilage, or fat cells.
Embryonic stem cells originate from the inner cell mass of a blastocyst, an early-stage embryo. These cells are pluripotent, capable of developing into almost any cell type in the body. This allows them to form all specialized cells and organs. The distinct potencies and natural occurrences of these stem cell types influence their collection methods and therapeutic uses.
Harvesting from Bone Marrow
Harvesting stem cells directly from bone marrow is a traditional method, typically involving hematopoietic stem cells. This surgical procedure occurs in an operating room. Donors receive general, regional, or local anesthesia to ensure comfort.
The posterior iliac crest, at the back of the hip bone, is the most common extraction site. A thin, hollow needle is inserted multiple times through the skin and into the bone to extract the marrow, a thick, red liquid containing the stem cells. Typically, 700 to 1500 milliliters of marrow-rich blood are collected, a volume the body usually replaces within four weeks. Donors may experience soreness or bruising at the harvest site, and pain medication is provided to manage discomfort.
Collecting Stem Cells from Peripheral Blood
Peripheral blood stem cell collection (apheresis or leukapheresis) is a widely used non-surgical method. The process begins with daily injections of granulocyte colony-stimulating factor (G-CSF) for 4-5 days. G-CSF is a growth factor that stimulates the bone marrow to release a higher number of stem cells into the bloodstream.
Once enough stem cells are mobilized into the peripheral blood, collection begins. Blood is drawn from a vein in one arm and flows through an apheresis machine, which separates and collects the stem cells. The remaining blood components are returned to the donor through a vein in the other arm. The procedure typically lasts 3 to 5 hours, sometimes requiring multiple sessions over 1-3 days. Donors might experience mild bone or muscle aches from the G-CSF injections.
Retrieving Stem Cells from Umbilical Cord Blood and Tissue
Umbilical cord blood and tissue offer a unique stem cell source, collected non-invasively after birth without risk to mother or baby. Following the clamping and cutting of the umbilical cord, a needle is inserted into the umbilical vein to draw blood from the cord and placenta into a sterile collection bag. The volume of cord blood collected typically ranges from 50 to 150 milliliters.
This cord blood is rich in hematopoietic stem cells, which can develop into various blood cell types, similar to those found in bone marrow. A segment of the umbilical cord tissue can also be collected, containing mesenchymal stem cells, particularly in Wharton’s Jelly. These mesenchymal stem cells can differentiate into bone, cartilage, and fat cells. Advantages of this source include its ready availability, painless collection, and a lower risk of graft-versus-host disease compared to other stem cell types.
Generating Induced Pluripotent Stem Cells
Induced pluripotent stem cells (iPSCs) are laboratory-generated, not harvested from a donor. This involves reprogramming specialized adult somatic cells (e.g., skin or blood cells) into an embryonic-like pluripotent state. Shinya Yamanaka’s 2006 discovery showed that specific transcription factors could achieve this.
These “Yamanaka factors” (Oct3/4, Sox2, Klf4, and c-Myc) are introduced into somatic cells using methods like viral vectors (e.g., retroviruses, lentiviruses, Sendai virus) or non-viral approaches (e.g., plasmids, mRNA). Once reprogrammed, iPSCs share many characteristics with embryonic stem cells, including the ability to differentiate into nearly any cell type. This technology provides a patient-specific source of pluripotent cells, offering potential for disease modeling, drug development, and regenerative medicine without the ethical considerations of embryonic stem cells.