Stem Cells-1

Stem cell (definition)

Undifferentiated cell with two unique properties: self-renewal and potency

Self-renewal

Symmetric division → two identical stem cells (makes more stem cells)

Potency

Asymmetric division → differentiation into specialized cell types

Symmetric vs asymmetric division

Symmetric = self-renewal (2 identical stem cells). Asymmetric = differentiation (potency)

3 major types of stem cells

Embryonic, adult (somatic/postnatal), induced pluripotent (iPS)

What does 'adult' stem cell mean?

Harvested from mature/differentiated tissue (not necessarily from an adult); synonyms: somatic, postnatal

Stem cell niche

Specialized microenvironment that maintains and regulates stem cells; they keep stemness inside and differentiate on leaving under specific signals

Niche diagram components

Adhesive molecules, ECM, basement membrane, stem cell, progeny cell

Why does the niche matter in research?

Cells in culture tend to differentiate; preserving stemness = mimicking the niche

Dentistry examples relying on stem cells

Pulp capping/pulp therapy (pulp stem cells heal tissue); orthodontic bone remodeling; dental pulp stem cell banks

First successful human bone marrow transplant

1957, E. Donnall Thomas — first successful stem cell therapy

Hematopoietic stem cells identified

1950s–60s, through bone marrow transplant studies

Human ESCs first isolated

1998, James Thomson

2001 Bush policy on hESCs

Restricted federal funding to 21 existing lines → slowed ESC research, promoted adult stem cell and iPSC research

Human iPSCs first generated

2007, Yamanaka and James Thomson independently (Yamanaka made mouse iPSCs a year earlier)

2009 Obama policy on hESCs

Lifted federal restrictions; new lines from leftover fertility-clinic embryos (still can't create embryos to harvest ESCs)

First human clinical trial with hESC-derived cells

2010

2012 Nobel Prize (stem cells)

Gurdon and Yamanaka — mature cells can be reprogrammed to become pluripotent

First iPSC-based clinical trial

2014, Japan — age-related macular degeneration

First approved iPSC products

March 2026, Japan: RiHEART (heart) and AMCHEPRY (nerve-related); none FDA-approved in U.S.

Embryonic stem cells (ESCs): source

Inner cell mass of the blastocyst (~day 5–6)

ESC potency

Pluripotent

ESC markers

OCT4, SOX2, NANOG, TRA-1-60, SSEA-4

ESC concerns

Ethical issues (destroying inner cell mass) and tumor formation (teratomas)

Teratoma

Tumor that commonly forms when ESCs are transferred into a living body

ESC applications

Regenerative medicine, tissue engineering, drug screening/toxicology, developmental biology, disease modeling

Feeder cells

Usually mouse cells that provide nutrition and improve ESC survival in culture

Xeno-free culture

No foreign components (no feeder cells, no bovine serum); required for clinical use

Zygote and early blastomeres: potency

Totipotent (zygote up to 8-cell stage)

Why does preimplantation genetic diagnosis (PGD) work?

Cells up to 8-cell stage are totipotent, so 1–2 can be removed for testing without harming the embryo

Identical twins arise when…

Early totipotent blastomeres separate on their own

Somatic cell nuclear transfer (SCNT)

Nucleus of a somatic cell transferred into an enucleated oocyte → patient-specific ESCs/cloned embryo

Dolly the sheep

1996; first mammal cloned from an adult cell (by SCNT)

Dolly's problems

Very low efficiency (~200 embryos per live birth); died at ~6 years (normal 10–12) — donor cell age carries over

First cloned primates

2018 — monkeys, using fetal fibroblast nuclei

SCNT importance

Provided the basis for iPSCs (somatic cells can be reprogrammed)

Why is SCNT largely superseded?

Creates/destroys embryos, requires human oocytes (consent/exploitation); replaced by iPSCs

iPSCs (definition)

Somatic (differentiated) cells genetically reprogrammed back to a pluripotent state

Yamanaka factors

OCT4, SOX2, KLF4, c-MYC

iPSC delivery methods

Viral (retroviral/lentiviral) or non-viral (episomal vectors, mRNA, protein, CRISPR activation)

iPSC advantages over hESCs

No embryo destruction; autologous therapy (avoids rejection); unlimited source of pluripotent cells

iPSC limitations

Tumor formation, genomic instability/residual oncogene, incomplete reprogramming, line variability, epigenetic memory, high cost, no FDA-approved products

Epigenetic memory (iPSCs)

iPSCs retain bias toward their tissue of origin (e.g., skin-derived iPSCs differentiate toward skin)

Why are iPSCs used more for disease modeling/gene correction?

Age and genetic disease of the somatic donor cell carry over into the iPSC

Can iPSCs form a whole individual?

No — pluripotent, not totipotent

Perinatal stem cells: sources

Umbilical cord blood (mainly HSCs), Wharton's jelly (mostly MSCs), amniotic membrane/fluid, placenta

Perinatal stem cell potency (exam answer)

Multipotent (between embryonic and adult)

Cord blood bank is mostly a bank of…

Hematopoietic stem cells

Perinatal stem cell advantages

Ethically acceptable, non-invasive collection, low immunogenicity (allogeneic), high proliferation

Perinatal stem cell limitations

Limited standardization/regulatory approval, variable yield and potency, needs optimized isolation

Adult stem cells (ASCs)

Undifferentiated cells in mature tissue that maintain and repair it; multipotent

Plasticity (transdifferentiation)

Adult stem cells differentiating outside their normal lineage (e.g., bone marrow stem cells → cardiomyocytes)

Adult stem cell advantages

Ethically non-controversial, reduced rejection (autologous), clinically established (bone marrow transplant)

Adult stem cell limitations

Limited potency, low yield/difficult isolation, reduced proliferation with donor age

Order of potency (high → low)

Totipotent → pluripotent → multipotent → (oligopotent) → unipotent

Totipotent

All cell types + complete individual including extraembryonic tissue; e.g., zygote, early blastomeres

Pluripotent

Almost all cells of 3 germ layers but NOT extraembryonic tissue or an individual; e.g., ESCs, iPSCs

Multipotent

A range of cell types; e.g., MSCs, HSCs, perinatal stem cells

Oligopotent

Only closely related cell types; e.g., lymphoid progenitor (→ B, T, NK)

Unipotent

Only one cell type; e.g., muscle satellite cells

Example of a cell once thought unipotent

Dental pulp cells (thought to make only odontoblasts; later shown more potent)

Autologous transplant

Self → self; lowest rejection risk (e.g., patient-derived iPSCs)

Syngeneic transplant

Identical twin → twin; low rejection risk

Allogeneic transplant

Human A → human B; higher rejection risk; risk of GVHD (most stem cell therapy is allogeneic)

Xenogeneic transplant

Animal → human; highest rejection risk (e.g., porcine cells/humanized pig organs)

Order of rejection risk (low → high)

Autologous < syngeneic < allogeneic < xenogeneic

Most important transplant types

Autologous and allogeneic

HLA matching: when required?

Required for allogeneic HSC transfer; NOT required for MSC therapy

GVHD

Graft-versus-host disease — risk in allogeneic transplants

Hematopoietic stem cells (HSCs)

Multipotent cells producing all blood and immune cells for life; from bone marrow, peripheral blood, cord blood

HSC myeloid lineage

Erythrocytes, platelets (megakaryocyte), granulocytes, monocytes, macrophages, mast cells

HSC lymphoid lineage

B cells, T cells, NK cells

HSC surface markers

CD34⁺, CD38⁻, CD45⁺, CD133⁺, c-Kit⁺

Mesenchymal stem cells (MSCs)

Multipotent stromal cells → bone, cartilage, fat, muscle, connective tissue; tissue repair and immunomodulation

MSC sources

Bone marrow, adipose tissue, umbilical cord, dental pulp/PDL

MSC surface markers

Positive: CD73, CD90, CD105. Negative: CD34, CD45, CD14, HLA-DR

How to tell HSC from MSC in bone marrow

HSC: CD34⁺, CD45⁺, CD133⁺, c-Kit⁺. MSC: lacks those; CD73⁺, CD90⁺, CD105⁺

Why are MSCs usable in allogeneic transplants?

Low immunogenicity; immunomodulatory, anti-inflammatory; can prevent/treat GVHD; no HLA match needed

MSC limitations

Decreased potency with age and long-term culture; heterogeneity between sources; unwanted differentiation/senescence

Most widely used stem cell-based therapy

Bone marrow transplantation (hematopoietic stem cell transplant)

Only FDA-approved stem cell therapy in the U.S.

Hematopoietic stem cell transplantation (bone marrow/peripheral blood)

Status of other stem cell clinics in U.S.

Experimental, not FDA-approved ('at your own risk'), mostly allogeneic adult cells

Adipose-derived stem cells (ADSCs)

MSCs from fat (lipoaspirate); high yield, easy access; used in clinics/rejuvenation; limits: donor variability, adipogenic bias

Dental pulp stem cells (DPSCs)

MSCs from pulp of permanent or deciduous teeth; neural crest-derived ectomesenchyme; multipotent

DPSC advantages

Medical waste (extracted teeth, e.g., third molars), minimally invasive, ethical, broad differentiation (neural crest), autologous/allogeneic use

DPSC limitations

Limited pulp per tooth, donor variability, needs standardized processing

DPSC embryonic origin

Neural crest-derived ectomesenchyme

Are DPSC therapies FDA-approved?

No — widely used in clinics but not FDA-approved

TRG035

Humanized anti-USAG-1 antibody (Toregem, Japan): neutralizes USAG-1 → more BMP signaling → tooth development

TRG035 target patients and status

Congenital tooth agenesis (oligodontia); phase I done (safe); worked in mice; market goal ~2030

USAG-1

Protein that binds BMP and blocks tooth formation

Source: ESC vs iPSC vs adult

ESC: inner cell mass. iPSC: reprogrammed somatic cells. Adult: postnatal tissues

Potency: ESC vs iPSC vs adult

ESC: pluripotent. iPSC: pluripotent. Adult: multipotent

Self-renewal: ESC vs iPSC vs adult

ESC & iPSC: unlimited in vitro. Adult: limited

Ethical concerns: ESC vs iPSC vs adult

Only ESCs

Tumor risk: ESC vs iPSC vs adult

ESC: teratoma. iPSC: tumorigenic (residual undifferentiated cells). Adult: low

Clinical use: ESC vs iPSC vs adult

ESC: mostly experimental. iPSC: preclinical/trials. Adult: widely applied

General advantages of stem cell therapy

Reverse tissue damage, personalized (autologous) treatment, may reduce need for organ transplants

General limitations of stem cell therapy

Immune rejection (allogeneic), unmet regulatory requirements

Two defining properties of stem cells?

Self-renewal (symmetric division) and potency (differentiation, asymmetric division)