Stem Cells — Potency, Sources and Uses

If every cell in your body is descended from a single fertilised egg, then at some point that original cell — and its early descendants — had the potential to become any cell type. Such cells are called stem cells, and they are of enormous biological and medical importance. This lesson covers OCR A-Level Biology A specification point 2.1.6 (e) — stem cells, their uses and the ethical considerations surrounding them.

1. What Is a Stem Cell?

Key Definition — Stem Cell: An undifferentiated cell capable of both self-renewal (dividing to produce more stem cells) and differentiation (giving rise to specialised cell types).

Two defining features:

Self-renewal — a stem cell can divide to produce at least one daughter that is also a stem cell. This maintains the stem cell population indefinitely.
Potency — a stem cell can differentiate into one or more mature cell types.

Stem cells exist at all stages of life. A zygote is a stem cell. Early embryonic cells are stem cells. And adult tissues contain small populations of stem cells that provide cells for ongoing renewal (of blood, skin, gut, etc.).

2. Levels of Potency

Stem cells are classified by how many cell types they can produce — a property called potency. Four levels are recognised.

graph TD
  A[Totipotent<br/>any cell type including placenta<br/>zygote and first few cells] --> B[Pluripotent<br/>any body cell type but not placenta<br/>inner cell mass of blastocyst]
  B --> C[Multipotent<br/>several related types<br/>haematopoietic stem cells]
  C --> D[Unipotent<br/>one type only<br/>satellite cells in muscle]

2.1 Totipotent

Totipotent cells can differentiate into any cell type, including extra-embryonic tissues such as the placenta and umbilical cord. The only naturally totipotent cells are the zygote and the cells produced by the first few cleavage divisions (up to the 8-cell stage in humans). Totipotent cells can form a complete organism.

2.2 Pluripotent

Pluripotent cells can differentiate into any cell type of the body (all three germ layers — endoderm, mesoderm, ectoderm) except the placenta. They cannot form a complete organism on their own. The classic example is cells from the inner cell mass of the blastocyst (the 5-day-old embryo) — these are the source of embryonic stem cells (ESCs) in research.

2.3 Multipotent

Multipotent cells can differentiate into several (but limited) related cell types — usually within one tissue or lineage. They are the main type of adult stem cell.

Examples:

Haematopoietic stem cells in bone marrow → all blood cells (red cells, white cells, platelets).
Mesenchymal stem cells → bone, cartilage, fat cells.
Neural stem cells → neurones, astrocytes, oligodendrocytes.

2.4 Unipotent

Unipotent cells can produce only one cell type, but they can still self-renew (which distinguishes them from fully specialised cells).

Examples:

Epidermal stem cells produce new skin keratinocytes.
Satellite cells in skeletal muscle produce new muscle cells for growth and repair.
Cardiomyocyte progenitor cells — once thought not to exist in adult heart, but now believed to maintain a very slow turnover of cardiac muscle.

3. Sources of Stem Cells

3.1 Embryonic Stem Cells (ESCs)

Source: Inner cell mass of the blastocyst (5-day-old embryo), typically leftover embryos from IVF.
Potency: Pluripotent.
Advantages: Can become virtually any cell type; divide readily in culture; powerful research and therapeutic potential.
Disadvantages: Destroying an embryo to harvest them is ethically controversial. They may be rejected by the patient's immune system because they are genetically different (allogeneic). They can also form teratomas (disorganised tumours) if injected without proper differentiation.

3.2 Adult (Tissue/Somatic) Stem Cells

Source: Various tissues — bone marrow, skin, intestinal crypts, hair follicles, brain, muscle.
Potency: Multipotent or unipotent.
Advantages: Already used clinically (bone marrow transplants for leukaemia); no embryo destruction; can be autologous (from the patient themselves, avoiding rejection).
Disadvantages: Limited potency; harder to isolate and culture; lower division rate than ESCs.

3.3 Plant Meristems

Plants have stem cells too. The apical meristems (at the tips of roots and shoots) contain cells that continue to divide throughout the plant's life, producing new tissues. Lateral meristems (cambia) provide for growth in girth. These meristematic cells are often totipotent — a feature exploited in plant tissue culture, where a single cell can be grown into a whole new plant.

3.4 Umbilical Cord and Cord Blood

Blood from the umbilical cord is rich in haematopoietic stem cells and can be banked at birth for future medical use. It has lower risk of immune rejection than adult stem cells and raises fewer ethical concerns than embryonic sources.

3.5 Induced Pluripotent Stem Cells (iPSCs)

A more recent development (2006, Shinya Yamanaka) — adult cells can be reprogrammed to a pluripotent state by introducing a small number of transcription factor genes (originally Oct4, Sox2, Klf4, c-Myc). iPSCs behave much like embryonic stem cells but are generated from the patient's own cells — sidestepping both ethical objections and immune rejection. Yamanaka won the 2012 Nobel Prize for this work. iPSCs are now the focus of much regenerative medicine research.

Source	Potency	Key pros	Key cons
Embryonic (ESCs)	Pluripotent	Very versatile; easy to grow	Ethical; immune rejection; tumour risk
Adult (bone marrow)	Multipotent	Ethically safer; used clinically	Limited potency; hard to culture
Umbilical cord blood	Multipotent	Easy collection; low rejection	Limited cell number
Plant meristems	Totipotent (most)	Unique to plants; enables propagation	N/A for medicine
iPSCs	Pluripotent	Patient-specific; no embryo	Risk of genomic instability

Lesson 11: Stem Cells — Potency, Sources and Uses