Study Guide

Stem cells

BiologyΒ· 45 min read

1. Key Properties of Stem Cellsβ˜…β˜…β˜†β˜†β˜†β± 10 min

πŸ“˜ Definition

Stem Cell

An undifferentiated cell that can divide by mitosis an unlimited number of times, and differentiate into one or more types of specialised functional cell

Example:

Human embryonic stem cells can form most cell types in the adult body

All stem cells have two core, defining properties that set them apart from other differentiated body cells. These properties are the foundation of all their biological roles and medical uses.

  • Self-renewal: Stem cells can divide repeatedly to produce identical copies of themselves, maintaining a stable pool of stem cells long-term.

  • Potency: Stem cells have the capacity to differentiate into specialised, functional cell types for specific tissues.

πŸ“ Worked Example

Explain why terminally differentiated nerve cells cannot be classified as stem cells.

  1. 1

    Recall the two required properties for a cell to be a stem cell: self-renewal and potency.

  2. 2

    Terminally differentiated nerve cells cannot undergo mitotic division at all, so they lack the key property of self-renewal.

  3. 3

    Nerve cells also cannot differentiate into any other type of cell, so they also lack potency.

  4. 4

    Since nerve cells lack both defining properties of stem cells, they cannot be classified as such.

Exam tip:

Always mention both self-renewal and potency when asked to define stem cells β€” missing one property will cost you a mark in exams.

2. Classification by Potencyβ˜…β˜…β˜†β˜†β˜†β± 15 min

πŸ“˜ Definition

Potency

A measure of the range of different specialised cell types a stem cell can differentiate into

  • Totipotent: Can differentiate into all cell types in an organism, plus extraembryonic cells (placenta, chorion). Only the zygote and the first 1-2 divisions of the embryo are totipotent.

  • Pluripotent: Can differentiate into almost all cell types from the three embryonic germ layers, but not extraembryonic tissue. Found in the inner cell mass of a 5-day blastocyst.

  • Multipotent: Can differentiate into multiple, closely related cell types within a single tissue. Example: bone marrow haematopoietic stem cells form all blood cell types.

  • Unipotent: Can only differentiate into a single specialised cell type. Example: muscle satellite stem cells only form new muscle cells.

πŸ“ Worked Example

Stem cells extracted from adult bone marrow can form red blood cells, white blood cells and platelets, but no other cell types. State what class of stem cell this is, and justify your answer.

  1. 1

    Compare the observed potency of the extracted stem cells to the definitions of each potency class.

  2. 2

    The cells can form multiple related cell types (all blood cell lineages) but cannot form cell types from other tissues like nerve or muscle.

  3. 3

    Totipotent and pluripotent stem cells can form all or almost all body cell types, so these do not match. Unipotent stem cells can only form one cell type, so they also do not match.

  4. 4

    This matches the definition of multipotent stem cells, which produce multiple related cell types within one tissue.

βœ“ Quick check

Test your understanding of potency classifications

  1. Which of the following correctly describes the potency of a human zygote?

    • Pluripotent

    • Multipotent

    • Totipotent

    • Unipotent

    Reveal answer
    Totipotent β€”

    Correct: A zygote can form all embryonic and extraembryonic tissue. Pluripotent cells cannot form extraembryonic tissue, while multipotent and unipotent cells are far more restricted.

3. Therapeutic Uses of Stem Cellsβ˜…β˜…β˜…β˜†β˜†β± 15 min

Stem cells' ability to self-renew and differentiate makes them extremely valuable for treating a range of diseases and conditions. Below are common established and experimental uses:

Condition

Stem Cell Source

Mechanism of Action

Leukaemia/lymphoma

Bone marrow/umbilical cord haematopoietic stem cells

Replace cancerous bone marrow after chemotherapy

Type 1 diabetes (trials)

Pluripotent stem cell-derived beta cells

Replace insulin-producing pancreatic cells destroyed by autoimmunity

Spinal cord injury (research)

Neural stem cells

Replace damaged nerve cells in the spinal cord

πŸ“ Worked Example

Describe how stem cells are used to treat leukaemia.

  1. 1

    Leukaemia is a cancer of white blood cells that originates in the bone marrow. High-dose chemotherapy kills cancer cells but also destroys healthy bone marrow stem cells.

  2. 2

    Healthy multipotent haematopoietic stem cells are extracted from a genetically matched donor's bone marrow or from umbilical cord blood.

  3. 3

    The healthy stem cells are transplanted into the patient's bone marrow.

  4. 4

    The transplanted stem cells self-renew and differentiate to produce new healthy blood cells, restoring the patient's immune and circulatory function.

4. Ethical Issuesβ˜…β˜…β˜…β˜†β˜†β± 10 min

Most ethical debate focuses on the use of human embryonic stem cells (ESCs), which are pluripotent cells extracted from the inner cell mass of unused IVF blastocysts that would otherwise be discarded.

  • Arguments in favour of ESC use: Can cure life-threatening degenerative diseases, unused IVF embryos are destroyed anyway, potential to reduce patient suffering.

  • Arguments against ESC use: Many believe embryos have full moral status from conception, so destroying an embryo is equivalent to taking a human life. There is also a risk of uncontrolled stem cell growth leading to cancer in recipients.

5. Common Pitfalls

Wrong move:

Confusing totipotent and pluripotent, calling blastocyst embryonic stem cells totipotent

Why:

Only the earliest embryo cells (zygote, 2-cell stage) are totipotent; blastocyst cells cannot form extraembryonic tissue

Correct move:

Remember: totipotent = all cells including placenta; pluripotent = all embryo cells only

Wrong move:

Only mentioning one key property when defining a stem cell

Why:

Examiners require both self-renewal and potency for full marks on definition questions

Correct move:

Always explicitly include both the ability to divide repeatedly (self-renewal) and differentiate into specialised cells

Wrong move:

Claiming all adult stem cells are pluripotent

Why:

Almost all adult stem cells are multipotent or unipotent, only embryonic stem cells and iPSCs are pluripotent

Correct move:

Only describe embryonic stem cells and iPSCs as pluripotent; adult tissue stem cells are multipotent

Wrong move:

Claiming all stem cell research is ethically controversial

Why:

Adult stem cells from bone marrow and umbilical cord blood have no major ethical objections

Correct move:

Specify that only the use of embryonic stem cells raises widespread ethical concerns

6. Quick Reference Cheatsheet

Stem Cell Type

Potency

Source

Zygote / early 2-cell embryo

Totipotent

Newly fertilised egg

Embryonic stem cell

Pluripotent

Inner cell mass of blastocyst

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When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 1

    Stem cell potency multiple choice

  • 2021 Β· 2

    Compare stem cell potency types

  • 2023 Β· 4

    Ethics of stem cell therapy essay

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