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Stem Cells &
Pluripotency (2C)

Explore how undifferentiated cells give rise to the entire diversity of cell types in the body, and master the potency hierarchy for the MCAT.

MCAT Biological Foundations

What Are Stem Cells?

Stem cells are undifferentiated cells capable of two defining properties: self-renewal (dividing to produce more stem cells) and differentiation (specializing into distinct cell types). They serve as the body's internal repair system.

The Potency Hierarchy

Totipotent Pluripotent Multipotent Unipotent Zygote, early morula Inner cell mass (ICM) HSCs, MSCs, neural SCs Muscle satellite cells All cell types + placenta All 3 germ layers Limited lineages One cell type only Decreasing Potency

Self-Renewal vs. Differentiation

Self-Renewal
Symmetric division produces two identical stem cells, maintaining the stem cell pool. Driven by Wnt, Notch, and Hedgehog signaling.
Differentiation
Asymmetric division produces one stem cell + one progenitor that commits to a lineage. Governed by transcription factors and epigenetic changes.

Defining Pluripotency

A pluripotent cell can differentiate into any cell type derived from the three primary germ layers: ectoderm, mesoderm, and endoderm. However, it cannot form extraembryonic tissues (e.g., placenta).

Pluripotent Stem Cell Ectoderm Neurons Epidermis Lens of eye Mesoderm Muscle Bone / Cartilage Blood cells Endoderm Lung epithelium GI lining Liver / Pancreas

Totipotent vs. Pluripotent vs. Multipotent

Property Totipotent Pluripotent Multipotent
Can form placenta? Yes No No
All 3 germ layers? Yes Yes No
Example Zygote ESCs / iPSCs HSCs
Found in Days 1-4 Inner cell mass Adult tissues

Embryonic Stem Cells (ESCs)

Embryonic stem cells (ESCs) are derived from the inner cell mass (ICM) of the blastocyst, approximately 5-7 days after fertilization. They are the gold standard of pluripotency.

1
Source: Inner cell mass of blastocyst
2
Potency: Pluripotent: all three germ layers
3
Key markers: Oct4, Sox2, Nanog (master transcription factors)
4
In vitro test: Form teratomas containing all three germ layers when injected into immunodeficient mice

iPSCs and Yamanaka Factors

In 2006, Shinya Yamanaka demonstrated that somatic cells can be reprogrammed to a pluripotent state by introducing four transcription factors. These induced pluripotent stem cells (iPSCs) bypass the ethical concerns of ESCs.

Oct4
Master regulator of pluripotency; maintains the undifferentiated state
Sox2
Works with Oct4 to activate pluripotency genes
Klf4
Suppresses differentiation and promotes self-renewal
c-Myc
Oncogene that enhances proliferation and chromatin remodeling

Adult Stem Cells

Adult (somatic) stem cells are found in specialized niches throughout the body. They are typically multipotent: restricted to producing cell types within their tissue lineage.

Hematopoietic (HSCs)
Bone marrow; produce all blood cell types: RBCs, WBCs, platelets
Mesenchymal (MSCs)
Bone, cartilage, fat, and stromal cells of marrow
Neural Stem Cells
Subventricular zone and hippocampus; neurons and glia

Molecular Regulation of Potency

1
Transcription factors: Oct4, Sox2, Nanog form a core regulatory circuit that activates pluripotency genes and represses differentiation genes
2
Epigenetic control: DNA methylation silences lineage genes; histone acetylation opens chromatin at pluripotency loci
3
Signaling pathways: LIF/STAT3 and BMP4 maintain mouse ESCs; FGF2 and Activin/Nodal maintain human ESCs
4
MicroRNAs: miR-302 cluster promotes pluripotency; let-7 family promotes differentiation

How Cells Differentiate

Stem Cell Wnt OFF BMP4 Retinoic Acid Ectoderm Mesoderm Endoderm Terminally Differentiated Neurons, skin cells, etc. Terminally Differentiated Muscle, bone, blood cells Terminally Differentiated Hepatocytes, lung epithelium

How to Identify Potency Level

1
Ask: Can it form an entire organism? If yes (including placenta): totipotent.
2
Ask: Can it form all three germ layers? If yes but not placenta: pluripotent (ESCs, iPSCs).
3
Ask: Is it restricted to a tissue lineage? If it can form multiple cell types within one lineage: multipotent (HSCs, neural SCs).
4
Ask: Can it form only one cell type? If only one: unipotent (muscle satellite cells, epidermal basal cells).

Example 1: Blastocyst Structure

Trophoblast (becomes placenta) Blastocoel (fluid cavity) Inner Cell Mass (Pluripotent) Trophoblast cells Not pluripotent

Example 2: Hematopoiesis

HSC (Multipotent) Myeloid Progenitor Lymphoid Progenitor RBCs Erythrocytes Platelets Megakaryocyte Granulocytes Neutrophils, etc. T cells Thymus B cells Bone marrow NK cells Innate HSCs are multipotent, not pluripotent They produce blood lineage cells only: not neurons, not muscle

Example 3: Reprogramming to iPSCs

Fibroblast (Differentiated) + OSKM iPSC (Pluripotent) Neurons (Ectoderm) Cardiomyocytes (Mesoderm) Hepatocytes (Endoderm) iPSCs can differentiate into all three germ layers Bypasses ethical concerns of embryonic stem cells

Common Mistakes

✕ Pluripotent = Totipotent
Pluripotent cells cannot form extraembryonic tissues (placenta). Only totipotent cells (zygote and early morula) can.
✕ All Adult SCs Are Unipotent
Most adult stem cells are multipotent, not unipotent. HSCs produce many blood cell types. Only a few (like satellite cells) are unipotent.
✕ iPSCs Are Identical to ESCs
iPSCs are functionally similar to ESCs but may retain epigenetic memory of the donor cell type and carry higher mutation risk from reprogramming.

Summary Comparison

Feature ESCs iPSCs Adult SCs
Potency Pluripotent Pluripotent Multi/Unipotent
Source Blastocyst ICM Reprogrammed somatic Tissue niches
Ethical concern High Low Low
Tumor risk Teratoma Teratoma + mutations Low
Key markers Oct4, Sox2, Nanog Oct4, Sox2, Nanog Lineage-specific

Question 1 of 5

A cell is isolated from a 3-day-old human embryo (morula stage). Which characteristic would you expect this cell to exhibit?

Question 2 of 5

Which set of transcription factors, when introduced into somatic cells, can reprogram them into induced pluripotent stem cells (iPSCs)?

Question 3 of 5

A researcher isolates stem cells from the bone marrow of an adult patient. These cells can differentiate into red blood cells, white blood cells, and platelets, but not into neurons or hepatocytes. These cells are best classified as:

Question 4 of 5

Which of the following best explains why pluripotent stem cells, but not totipotent cells, are the standard for regenerative medicine research?

Question 5 of 5

A patient receives a bone marrow transplant. The donor HSCs engraft and begin producing blood cells. Six months later, the patient develops a teratoma. What is the most likely explanation?