FoS II · Midterm 2

Lecture 8 — Stem cells, iPSCs and cellular identity

A cell's identity comes from which genes it expresses, not which genes it has. Gurdon and Yamanaka both showed that a specialized cell still carries a full, usable genome, and that changing gene expression can reset its identity.

Same DNA, different cells

  • Nearly every cell in your body has the same genome.
  • A neuron and a skin cell differ because different genes are switched on and off.
  • Transcription factors are proteins that bind DNA and turn genes up or down. The set of active transcription factors largely decides what a cell is.
  • Epigenetic regulation (chemical marks on DNA and on the proteins DNA wraps around) locks those on/off patterns in place so a skin cell stays a skin cell when it divides. The DNA sequence itself does not change.
Same DNA, different cells A neuron and a skin cell carry the same six genes. In the neuron, the neuron genes and shared genes are on, with transcription factors bound, and the skin genes are held off by epigenetic marks. In the skin cell, the skin genes and shared genes are on and the neuron genes are held off. Neuron same genome neuron genes ON skin genes off shared ON Skin cell same genome neuron genes off skin genes ON shared ON transcription factor: turns a gene on epigenetic mark: keeps a gene off (DNA unchanged)
Same genome, different switches. A neuron and a skin cell carry identical DNA. Transcription factors turn on different genes in each, and epigenetic marks keep the rest off.

What makes a stem cell

Two properties, both required:

  • Self-renewal: it can divide to make more stem cells.
  • Differentiation: it can produce at least one specialized cell type.

Levels of potency

LevelCan produceExample
TotipotentEvery cell of the embryo plus extraembryonic tissue (placenta), so a whole organismFertilized egg (zygote) and the earliest cells after it
PluripotentEssentially every cell type of the body, but not a whole organism on its ownEmbryonic stem cells, iPSCs
MultipotentSeveral related cell types within one lineageBlood stem cells (all blood cell types); muscle satellite cells
DifferentiatedItself (one specialized type)Skin cell, neuron, muscle fiber

Development normally moves down this table. Reprogramming moves a cell back up it.

Potency pyramid Four tiers from top to bottom: totipotent (embryo plus placenta, e.g. zygote), pluripotent (every body cell type, e.g. ESCs and iPSCs), multipotent (related types in one lineage, e.g. blood stem cells), differentiated (only itself). Differentiation runs down the whole pyramid. Reprogramming runs up from differentiated to pluripotent, not to totipotent. Totipotent embryo + placenta e.g. zygote Pluripotent every body cell type e.g. ESCs, iPSCs Multipotent several related types, one lineage e.g. blood stem cells Differentiated only itself e.g. skin cell, neuron, muscle fiber differentiation (normal direction) reprogramming (skin cell → iPSC)
Differentiation moves a cell down; reprogramming pushes it back up. The DNA sequence never changes, only which genes are on. iPSCs climb to pluripotent, not totipotent, which is why they can make a neuron but not a placenta.

Embryonic stem cells vs iPSCs

Embryonic stem cells (ESCs)Induced pluripotent stem cells (iPSCs)
SourceInner cell mass of an early embryo (blastocyst)An adult's differentiated cells, such as skin cells
How they become pluripotentThey already areReprogrammed by adding transcription factors
PotencyPluripotentPluripotent
Genetic match to a patientNoYes, if made from the patient's own cells
Main concernsEthics of using embryosIncomplete reprogramming, tumor risk, leftover "memory" of the original cell type

Reprogramming vs differentiation

  • Differentiation: a less specialized cell becomes more specialized (stem cell → neuron). This is the normal direction.
  • Reprogramming: a specialized cell is pushed back to a less specialized state (skin cell → iPSC). The genome stays the same; the regulatory state is reset.

The two Nobel experiments

  • Gurdon (frogs): took the nucleus from a specialized tadpole intestinal cell and put it into an egg whose own nucleus had been removed. The egg developed into a normal tadpole. Lesson: a differentiated cell keeps all the genetic information needed to build a whole organism. Specialization does not delete genes.
  • Yamanaka (mice, later humans): screened candidate genes and found that just four transcription factors (Oct4, Sox2, Klf4, c-Myc) could turn skin cells into pluripotent cells. Lesson: cell identity is controlled by a small set of regulators and can be reset without an egg or embryo.
  • Together: identity is regulation, not sequence.
Gurdon and Yamanaka experiments Gurdon: a tadpole intestinal cell's nucleus is moved into an egg with no nucleus, and a normal tadpole develops; specialization does not delete genes. Yamanaka: four transcription factors added to a skin cell make a pluripotent cell with no egg or embryo; a few regulators can reset identity. Both lead to one conclusion: identity is regulation, not sequence. Gurdon (frogs) Tadpole intestinal cell specialized Its nucleus moved into an egg with no nucleus A normal tadpole develops Specialization does not delete genes Yamanaka (mice) Skin cell specialized Add 4 transcription factors Oct4, Sox2, Klf4, c-Myc Pluripotent cell (iPSC) no egg or embryo needed A few regulators can reset identity Identity is regulation, not sequence
Two different experiments, one conclusion. A specialized cell still holds a full, usable genome, and what the cell becomes depends on which genes are switched on.

Why pluripotent cells matter

  • Studying development: watch human cell types form in a dish.
  • Modeling disease: make iPSCs from a patient with a genetic disease, turn them into the affected cell type, and watch the disease process.
  • Testing treatments: screen drugs on human cells of the right type.
  • Regenerative medicine: grow replacement cells.

The limit to state on any exam question: producing the right cell type is only step one. To restore tissue function, the cells must also mature fully, survive transplant, avoid forming tumors, avoid immune rejection, integrate with the existing tissue, and actually perform the job (for example, connect to the right neurons).

Check yourself

  • Explain why Gurdon's result was surprising for its time.
  • Why can an iPSC give rise to a neuron but not a placenta?
  • A lab makes heart-muscle cells from iPSCs that beat in a dish. List three things still unproven before this treats heart disease.

Further reading

Outside sources, opening in a new tab. The exam is on the course material; use these to check your understanding, not to learn new facts.

Tip: the ← and → arrow keys move between sections.