Doctors get stem cells from a handful of sources in the body, with the three most common being bone marrow, circulating blood, and umbilical cord blood collected after birth. Beyond these workhorses of transplant medicine, stem cells can also be harvested from fat tissue, created in a lab from ordinary skin cells, or derived from donated human embryos. The source a doctor chooses depends on what the stem cells are needed for, whether the patient can use their own cells, and how quickly treatment needs to happen.
Bone Marrow
Bone marrow is the oldest and most established source of stem cells. The spongy tissue inside large bones is rich in blood-forming stem cells and a smaller population of cells that can develop into bone, cartilage, and other connective tissues. Doctors harvest marrow from the hip bone, specifically the large pelvic crest at the top. If the patient is lying face up, the needle goes into the front of the hip. If lying face down, it goes into the back. The procedure uses a special needle to draw out liquid marrow, which is then processed in a centrifuge to concentrate the useful cells and separate out red blood cells and platelets.
The actual stem cells make up a tiny fraction of what comes out. After processing, the progenitor cells account for roughly 0.001% to 0.01% of the marrow sample. That small number is why lab expansion is often necessary before treatment, particularly for therapies targeting cartilage repair or other regenerative applications. Bone marrow tends to produce fewer colonies of stem cells per sample than some other tissue sources, but each colony grows larger, yielding robust cell populations once cultured.
Peripheral Blood
For many transplants today, doctors skip the bone marrow needle entirely and collect stem cells from the patient’s bloodstream instead. Under normal circumstances, very few stem cells circulate in the blood. To change that, patients receive injections of a growth factor drug for several days beforehand. This medication stimulates the bone marrow to release large numbers of stem cells into the bloodstream, a process called mobilization. Some patients, especially those with lymphoma or myeloma, don’t respond well to the growth factor alone. In those cases, a second drug can be added that loosens the grip holding stem cells inside the bone marrow, boosting the number released into circulation.
Once enough cells are circulating, collection happens through a process called apheresis. The patient is connected to a machine through intravenous lines. Blood flows out of one line, enters a centrifuge that spins it into layers, and the stem cell-rich layer is siphoned off into a collection bag. The remaining blood components are returned through the second line. Each session takes about two to four hours and processes seven to ten liters of blood, roughly twice the body’s total blood volume. An anti-clotting agent keeps the blood flowing smoothly through the machine. Some patients need more than one session to collect enough cells.
Umbilical Cord Blood
Cord blood is collected in the moments right after a baby is born. After the umbilical cord is clamped and cut, a healthcare provider inserts a needle into the cord and drains the remaining blood into a collection bag. The whole process takes a few minutes and poses no risk to the mother or baby. It doesn’t affect labor or delivery in any way.
This blood contains blood-forming stem cells similar to those found in bone marrow, with one key advantage: they’re immunologically immature. That means they’re less likely to trigger an immune rejection in the recipient, which widens the pool of potential matches for transplant patients. The FDA has approved multiple cord blood products for transplant use. Families can bank cord blood privately for potential future use by a family member, or donate it to public banks where it becomes available to anyone who needs a transplant match.
Fat Tissue
Adipose tissue, the fat beneath your skin, turns out to be a surprisingly rich source of stem cells. Doctors collect it through a minor liposuction procedure, then isolate the stem cells by breaking down the fat mechanically and, in most protocols, treating it with enzymes that digest the tissue and release a mixture of cells called the stromal vascular fraction. Newer techniques skip the enzyme step to be gentler on the cells. The stem cells from fat tissue can develop into bone, cartilage, and other connective tissues, much like bone marrow stem cells.
Clinical applications for fat-derived stem cells are expanding. They’re being used or studied for treating osteoarthritis, healing chronic wounds including diabetic ulcers, repairing damaged heart tissue after a heart attack, and various anti-aging and skin regeneration therapies. One practical advantage of fat as a source is volume: most adults have plenty of it, and the collection procedure is minimally invasive.
Lab-Created Stem Cells
In 2006, researchers demonstrated that ordinary adult cells, initially skin cells called fibroblasts, could be reprogrammed back into a stem cell state. These are called induced pluripotent stem cells, or iPSCs. The technique works by introducing four specific genes into an adult cell, which essentially rewind its developmental clock. The cell’s internal structure and gene activity shift from a specialized state back to one resembling an embryonic stem cell, capable of becoming virtually any cell type in the body.
Since that initial breakthrough, researchers have successfully reprogrammed multiple cell types beyond skin cells, including smooth muscle cells. The process involves inserting the reprogramming genes using various delivery methods, from viruses to protein transfer. iPSCs are primarily a research and experimental therapy tool right now, not a routine clinical source, but they hold enormous potential because they can be made from a patient’s own cells, sidestepping immune rejection entirely.
Human Embryonic Stem Cells
Embryonic stem cells come from the inner cell mass of a human embryo at the blastocyst stage, about five days after fertilization. These embryos are donated from fertility clinics by individuals who created them through IVF for reproductive purposes but no longer need them. Under NIH guidelines, strict ethical requirements govern this process. Donors must give voluntary written consent specifically for research use. No payment of any kind can be offered for the embryos. The decision to donate must be completely separate from fertility treatment decisions, and the treating physician and the stem cell researcher should ideally be different people.
Donors retain the right to withdraw consent up until the embryos are actually used to derive stem cells or until identifying information is removed. They must be told that any stem cell lines created may be kept indefinitely, that the research won’t benefit them directly, and that they cannot direct who ultimately receives medical benefit from the cells. These cells are the gold standard of pluripotency, capable of becoming any cell type, but their use remains ethically sensitive and heavily regulated.
Your Own Cells vs. a Donor’s
One of the most important decisions in stem cell treatment is whether to use the patient’s own cells (autologous transplant) or cells from a donor (allogeneic transplant). The choice depends on several factors: the type of disease, the patient’s age, whether a suitable donor match exists, and whether the patient’s own marrow is free of disease.
Using your own cells is simpler in many ways. There’s no need to find a matched donor, no risk of the transplanted cells attacking your body (a serious complication called graft-versus-host disease), and no need for immune-suppressing drugs afterward. The downside is that if you have a blood cancer, your own harvested cells might contain cancer cells that could cause a relapse.
Donor cells come cancer-free and carry an additional benefit: the donor’s immune cells can recognize and attack any remaining cancer in the recipient’s body. This immune effect lowers the risk of the disease coming back. However, allogeneic transplants carry higher risks of complications and are generally reserved for younger patients in good overall health. In practice, autologous transplants are used more often for lymphoma, myeloma, and solid tumors, while allogeneic transplants are the standard for leukemias and related bone marrow diseases.
Growing Enough Cells for Treatment
Regardless of where stem cells come from, there often aren’t enough in the initial sample to treat a patient. Lab expansion bridges that gap. Starting from a limited bone marrow sample, clinical protocols can generate up to 500 million to one billion stem cells. The key variable is how densely cells are seeded onto culture surfaces at the start, which dramatically affects how quickly they multiply.
Timelines vary by source. Stem cells from fat tissue typically need about 10 days of culture to reach usable numbers. Bone marrow cells grown in specialized bioreactor systems, which slowly feed fresh nutrients and mimic conditions inside the body, can reach target numbers in 8 to 12 days. Advanced closed-system bioreactors can expand stem cells 320 to 360 times over, matching the output of traditional open culture methods while reducing contamination risk. This expansion step is what makes many regenerative therapies possible, turning a small tissue sample into a therapeutic dose.