How Are Stem Cells Harvested? A Look at the Methods

Stem cells are harvested from several different body tissues, and the method depends on which type of stem cell is needed. Bone marrow aspiration from the hip, mobilization and collection from circulating blood, extraction from umbilical cord blood at birth, liposuction of fat tissue, and even reprogramming ordinary skin or blood cells in the lab are all established routes. Each method has its own procedure, recovery profile, and set of trade-offs for the donor or patient.

Bone Marrow Aspiration

Bone marrow harvesting is the oldest method in clinical use, with the first bone marrow transplant performed in 1956.¹ It remains a common way to obtain blood-forming stem cells for transplant and mesenchymal stem cells for orthopedic and regenerative therapies. The procedure targets the iliac crest, the curved ridge of the hip bone, because it contains a dense pocket of marrow that is relatively easy to reach.²

Under general or regional anesthesia, a physician inserts a specialized needle through the skin and into the bone, then draws marrow into a syringe. Needle gauge and aspiration depth vary by clinical goal. In one study of adults, an 11-gauge needle was inserted to about 7 centimeters, and small volumes of marrow were pulled at multiple depths and angles along the iliac crest to maximize the concentration of stem cells in the sample.³ Drawing from several spots rather than one deep pull tends to yield a richer cell population because each new puncture site taps into a fresh pocket of marrow rather than diluting the sample with peripheral blood.

For transplant donors giving a large volume of marrow, the procedure can involve repeated needle insertions on both sides of the pelvis. That is a bigger deal physically. A National Marrow Donor Program trial found that bone marrow donors were more likely to experience moderate-to-severe side effects at one week, and pain that persisted at the one-month mark, compared to donors who gave stem cells through the blood. About 3% of marrow donors still had not fully recovered at 24 weeks.⁴ Pre-donating and later reinfusing a unit of the donor’s own blood has been shown to reduce these complications and speed recovery.⁵

Peripheral Blood Stem Cell Collection

Most stem cells normally stay anchored inside bone marrow. To collect them from the bloodstream instead, doctors first coax them out using medication, a process called mobilization. The standard drug is a growth factor called G-CSF, which stimulates the marrow to overproduce stem cells until they spill into the circulating blood. The donor typically injects G-CSF under the skin for four or five days before collection.

When G-CSF alone does not push enough stem cells into the blood, a second drug called plerixafor can be added. Plerixafor works differently: it blocks the molecular handshake that keeps stem cells tethered to the marrow, effectively loosening their grip and releasing them into the bloodstream.^{6 7} This combination is especially useful for patients who have already received chemotherapy, which can damage the marrow’s ability to mobilize cells on its own.

Once enough stem cells are circulating, the donor or patient is connected to an apheresis machine. Blood is drawn from one arm, spun through the device to separate out the stem-cell-rich fraction, and the remainder is returned through the other arm. The whole session can last several hours, and collection efficiency varies by person. Research modeling the process has found that the sex of the donor is a significant factor influencing how many cells can be collected per liter of blood processed, with female donors tending to yield fewer cells per pass.⁸

Umbilical Cord Blood

Cord blood is collected immediately after a baby is born, making it the only stem cell source that involves no procedure on the donor at all. Once the umbilical cord is clamped, a needle is inserted into the cord’s vein and blood drains by gravity into a collection bag. The timing and technique matter for how much blood you get. A clinical trial comparing collection before versus after the placenta is delivered found that both approaches can yield usable samples, provided the cord is clamped about a minute after birth and the collection meets a minimum weight threshold of around 85 grams.⁹

The collection technique itself also affects volume. A study comparing three approaches found significant differences: collecting after the placenta was placed on a flat surface (the “plateau” method) yielded a median of about 70 milliliters, while collecting while the placenta was still inside the uterus yielded about 56 milliliters, and using a funnel setup collected only about 36 milliliters.¹⁰ These numbers matter because cord blood units are small to begin with, and a higher volume generally means more total stem cells available for transplant.

Once collected, the cord blood must be processed and frozen for storage. The standard approach uses a cryoprotectant called DMSO to prevent ice crystals from destroying the cells during freezing. Conventional protocols use 10% DMSO, but newer research suggests that lower concentrations, around 5%, combined with additives like trehalose or ascorbic acid, can preserve cell viability just as well or better while reducing the toxicity that DMSO causes when the unit is eventually thawed and infused into a patient.¹¹ Interestingly, whether you use a controlled-rate freezer or simply place the unit in a mechanical freezer at the right temperature does not seem to make a significant difference in cell quality after thawing.¹²

Adipose Tissue and Liposuction

Fat tissue turns out to be a surprisingly rich source of mesenchymal stem cells. The collection method will sound familiar to anyone who has heard of cosmetic liposuction: a thin cannula is inserted under the skin, typically around the abdomen or thigh, and fat is suctioned out. The resulting lipoaspirate is then processed in the lab, usually with an enzyme called collagenase that breaks down the fat tissue and releases the stem cells from the surrounding scaffold. Protocols exist for processing volumes well over 100 milliliters of lipoaspirate, making it one of the higher-yield sources available.¹³

Compared to bone marrow stem cells from the same donors, fat-derived stem cells grow faster in culture and are more reliable to expand in the lab regardless of the patient’s age or health status.¹⁴ That practical advantage matters for therapies that need a large number of cells. The trade-off is biological: donor-matched comparisons show that while fat-derived cells are better at proliferating and forming fat tissue, bone marrow cells are significantly better at forming bone and cartilage.¹⁵ The choice of source depends heavily on what the cells will be used for.

An important regulatory wrinkle applies here. In many countries, using enzymes to isolate cells from fat tissue is classified as “more than minimal manipulation,” which subjects the product to the same regulatory review as a drug. To sidestep that burden, mechanical methods have been developed that process the tissue without enzymatic digestion, producing a product that stays within the “minimally manipulated” category and can sometimes be used in the same surgical session.¹⁶ The trade-off is that mechanically processed fat contains a less purified cell population than the enzymatically digested version.

Embryonic Stem Cells and Lab-Made Alternatives

Embryonic stem cells come from the inner cell mass of a blastocyst, the ball of cells that forms a few days after fertilization. Isolating them is a laboratory procedure, not a clinical one performed on a living donor. Traditionally, the inner cell mass was freed using a technique called immunosurgery, which uses antibodies to dissolve the outer cell layer. That approach is slow and introduces animal-derived materials, raising contamination concerns for any future clinical use. Microsurgical alternatives now exist that use fine mechanical tools under a microscope to physically cut away the outer layer and extract the inner mass without animal products.¹⁷

Because embryonic stem cell derivation destroys the embryo, it remains ethically contentious. That controversy helped drive the development of induced pluripotent stem cells, or iPSCs, which are made by reprogramming ordinary adult cells back into a stem-like state. The original and still most common starting material is fibroblasts from a small skin biopsy. These cells are isolated from the tissue sample and then exposed to a set of key genes that rewind their developmental clock.¹⁸ The resulting iPSCs behave much like embryonic stem cells, able to become almost any cell type in the body, but they come from the patient’s own tissue.

The field has been expanding the menu of starting materials. Blood cells from a simple draw, cells shed in urine, and even cells from extracted wisdom teeth have all been successfully reprogrammed into iPSCs.¹⁹ Each source has its own practical pros and cons. Blood and urine are easier to collect than skin, which requires a biopsy. But the reprogramming efficiency and speed can vary with cell type, so the choice often comes down to what is most practical for a given patient and clinical goal.

Dental Pulp Stem Cells

The soft tissue inside teeth, called dental pulp, contains mesenchymal stem cells that have attracted research interest for bone regeneration and nerve repair. Third molars (wisdom teeth) are the most studied source, since they are routinely extracted and would otherwise be discarded. Once a tooth is removed, the pulp can be accessed either by splitting the tooth open through the crown or by pushing the tissue out through the root end. A randomized comparison of these two approaches found no significant differences in cell viability or the cells’ ability to differentiate into hard tissue.²⁰

In the lab, the extracted pulp is processed using one of several techniques: digesting it with enzymes to release individual cells, culturing intact pulp pieces and letting cells migrate out on their own, or a combination of both.²¹ The enzyme digestion method tends to yield cells faster, while the outgrowth method is gentler and avoids potential enzyme-related damage. Dental stem cells remain a niche source, but the appeal is clear: they come from tissue that is already being thrown away, and the collection adds no extra burden to the patient beyond the extraction they were already having.

How Do You Know the Cells Are Actually Stem Cells?

Regardless of how stem cells are harvested, confirming their identity is a critical step that is easy to overlook in popular discussions. The standard method uses a technology called flow cytometry to check which surface markers the cells carry. For mesenchymal stem cells, the expected profile includes positive expression of markers like CD44, CD73, CD90, and CD105, and negative expression of markers associated with blood cells, like CD34 and CD45.²²

The trouble is that the marker profiles are not always clean-cut. Research comparing stem cells from Wharton’s jelly, adipose tissue, bone marrow, and placental tissue with ordinary fibroblasts found that many of the commonly used markers were expressed at virtually identical levels in all cell types. Statistically significant differences showed up only for a subset of markers, including CD105, CD106, and CD146.²³ This means a lab relying only on the basic marker panel could mistake fibroblasts for stem cells, or vice versa. The practical implication for patients considering stem cell therapies: ask what quality-control testing the provider uses, because simply calling something a “stem cell product” does not guarantee the cells in the vial are what the label says.

Long-Term Safety for Donors

A persistent worry, especially for people donating peripheral blood stem cells, is whether the mobilization drugs could increase the risk of cancer years later. The concern is understandable: G-CSF forces the bone marrow into overdrive, and any drug that stimulates cell growth invites the question of whether it might trigger something malignant.

The best long-term data come from a Swedish registry study that tracked over a thousand healthy peripheral blood stem cell donors for a median of nearly 10 years. The cancer rate among donors was not meaningfully different from that of age- and sex-matched controls from the general population. Even when hematological cancers were looked at specifically, the slightly higher rate seen among donors was not statistically significant.²⁴ A separate study following pediatric donors who received G-CSF found no cases of cancer at a median follow-up of nearly five years, and two pregnancies that occurred after donation resulted in healthy births.²⁵

These findings are reassuring, though researchers acknowledge that truly rare late effects could take even longer to show up. Registries continue to track donors for this reason. The current evidence, however, does not support the idea that stem cell donation raises cancer risk.

Keeping Cells Alive After Collection

Harvesting stem cells is only half the battle. Getting them to the patient or the lab in good condition requires careful temperature management. For freshly harvested peripheral blood stem cells, research has shown that the best temperature range for overnight storage and transport is between 2 and 8 degrees Celsius. Cells stored at room temperature or higher show a measurable drop in viability.²⁶ Bone marrow samples, interestingly, appear to tolerate temperature variation somewhat better.

For mesenchymal stem cells from other sources, the preservation medium matters as much as the temperature. Synovial stem cells stored in human serum at 4 to 13 degrees Celsius maintained their viability, surface marker expression, and ability to form cartilage, while cells stored in standard saline solutions under the same conditions lost much of their function.²⁷ These details rarely make it into patient-facing discussions, but they are one of the reasons that established transplant centers tend to produce better outcomes than smaller clinics: the logistics chain between harvest and use is where a lot of cell quality is won or lost.

Organoids and the Frontier of Stem Cell Sourcing

A newer dimension of the field does not involve harvesting stem cells from the body at all. Instead, researchers grow pluripotent stem cells in the lab and coax them into forming tiny, simplified versions of organs called organoids. These three-dimensional structures can mimic features of the brain, gut, kidney, or liver, and they serve primarily as research tools for studying disease and testing drugs rather than as direct therapies. To study specific cell types within these organoids, researchers use fluorescence-based sorting to separate the particular populations they need from the heterogeneous mix of cells the organoid produces.²⁸ Organoid technology is still largely a research platform, but it represents a direction where the stem cell “harvest” happens entirely in a dish, with no donor and no procedure.