Stem Cells: How They Arise and the Technologies That Harness Them

The gene is lifeless — it is only code. The cell is what brings it to life. And within that cellular world, stem cells are the master architects: self-renewing, capable of generating entire tissues from a single founder. Understanding how they arise — and how we now engineer them — is the frontier of modern medicine.

What a Stem Cell Actually Is

A stem cell has two defining properties. First, it can copy itself indefinitely — self-renewal. Second, it can give rise to more specialized daughters — differentiation. Together, these two abilities make stem cells the maintenance crew for every tissue that wears out.

They exist across virtually every organ system:

• Hematopoietic (blood) stem cells — continuously generate red cells, platelets, neutrophils, macrophages, and lymphocytes from a common ancestor in the bone marrow
• Skeletal stem cells — resident inside bone, they build and repair bone, cartilage, and connective tissue
• Neural stem cells — maintain neuronal populations in the nervous system
• Tissue-specific stem cells in skin, gut, pancreas, and other organs that turn over constantly

How Stem Cells Arise

Embryonic Commitment

During development, pluripotent embryonic cells progressively narrow their fate. Transcription factors act as molecular switches — once flipped, they lock a cell into a lineage. A pluripotent cell becomes multipotent, then lineage-restricted, then fully specialized. The stem cells that persist into adult life are those that retained the self-renewal switch while committing to a particular tissue.

The Skeletal Stem Cell: A Recent Discovery

One of the most striking recent findings came from Dr. Mukherjee's own lab: the identification of a true skeletal stem cell that builds the entire vertebrate skeleton. These cells were discovered only a few years ago and have four remarkable properties:

1. Hierarchical potency — a single cell generates columns of bone, cartilage, and fibrous tissue
2. Fracture repair — they behave like cellular glue, flooding the site of a bone break, repairing it locally, then stopping
3. Self-renewal — they persist and regenerate
4. Age-dependent decline — their numbers fall 10–50-fold as an organism ages, which Mukherjee believes is the cellular basis of arthritis and bone fragility

"We were looking for a pill when we should have really been looking for a cell."

His lab is now studying centenarians to ask what is different about the skeletal stem cells in people with exceptional longevity — and working to reintroduce these cells into humans as a therapeutic strategy.

Cancer as Stem Cell Biology Gone Wrong

Mukherjee frames cancer not as alien biology but as normal stem cell biology perverted. The genes that drive wound healing, embryonic growth, and stem cell self-renewal — when mutated — release uncontrolled proliferation. This means normality and illness are structurally twinned. Cancer cells exploit the very machinery that allows a stem cell to be a stem cell.

Two active questions define this intersection:

• Do cancers contain a hierarchy of cancer stem cells that drive relapse and resistance, recapitulating self-renewal?
• Can healthy stem cells be used therapeutically against cancer — and can we engineer them to survive the attack?

Technologies That Generate or Engineer Stem Cells

1. Bone Marrow Transplantation

The oldest proven stem cell technology. Donor hematopoietic stem cells repopulate an entire blood system after conditioning chemotherapy. It remains standard of care for leukemia and lymphoma. It also enables gene therapy: an autologous bone marrow transplant with a corrected gene can cure sickle cell anemia without any germ-line modification.

2. Cord Blood Stem Cells

Umbilical cord blood is a rich source of hematopoietic stem cells. Mukherjee's team has achieved 90% genetic modification rates in cord blood-derived cells — making them an efficient platform for CRISPR-based interventions.

3. CRISPR Editing of Hematopoietic Stem Cells — The VOR Platform

The central problem in using CAR-T cells against AML (acute myeloid leukemia) is antigen sharing: the target (CD33) is present on both leukemia cells and normal blood stem cells. CAR-T kills both, preventing engraftment.

Mukherjee's solution — now in Phase I clinical trials — is to edit the stem cells before transplant:

1. Collect donor CD34+ hematopoietic stem/progenitor cells
2. Use CRISPR to delete CD33 (and/or CLL1) from those cells
3. Transplant the antigen-negative edited cells into the patient
4. Now CAR-T cells or the antibody-toxin Mylotarg can attack residual leukemia — but the edited graft is invisible to them

Early trial results show the edited cells constituting 90–96% of reconstituted blood at day 60, with normal recovery trajectories. The company built to manufacture these cells at scale is Vor Biopharma.

4. Base Editing

Beyond standard CRISPR cutting, Mukherjee's lab has demonstrated dual base editing — chemically converting individual DNA bases (A→G or C→T) without making double-strand breaks, allowing simultaneous deletion of two antigens. The challenge is that longer culture time degrades stem cell potency — the lab identified a "golden lock spot" window in which dual editing can be completed while cells remain viable.

5. Induced Pluripotent Stem Cells (iPSC)

By expressing a set of reprogramming factors, ordinary adult cells can be converted back into pluripotent stem cells. These patient-derived iPSCs can then be differentiated into virtually any cell type, used for drug screening, or re-engineered for transplant. Mukherjee describes this not as a future possibility but as ongoing work.

6. CAR-T Combined with Edited Stem Cells

The combination of CRISPR-edited hematopoietic stem cells with CAR-T immunotherapy is Mukherjee's most advanced clinical program. In India, he co-founded ImmunACT with entrepreneur Kiran Mazumdar-Shaw to bring CAR-T therapy from roughly $1 million per patient down to $20,000–40,000 — comparable to the cost of a bone marrow transplant in India.

Why Stem Cell Numbers Matter for Aging

Perhaps the most consequential insight from stem cell biology is this: aging may be, in large part, a stem cell exhaustion problem.

• Skeletal stem cells decline 10–50-fold with age → arthritis, fractures, poor healing
• Hematopoietic stem cells that fail → anemia, immune collapse
• Neural stem cell depletion → diminished neuronal renewal

"How do we keep our blood system regeneratively alive? It becomes a stem cell problem. It's a cellular problem at that stage."

The question Mukherjee's lab is now asking in centenarian studies: what is different about the stem cells of people who live to 100 with intact cognition and skeletal function? The answer may reveal the biology of healthy aging — and the targets that could extend it.

Summary: From Bench to Clinic