outline the role of stem cells in cell replacement and tissue repair by mitosis

Published by Patrick Mutisya · 8 days ago

Cambridge A-Level Biology – Replication and Division of Nuclei and Cells

Replication and Division of Nuclei and Cells

Objective

Outline the role of stem cells in cell replacement and tissue repair by mitosis.

1. Introduction to Stem Cells

Stem cells are undifferentiated cells with two fundamental properties:

  • Self‑renewal: the ability to undergo numerous cycles of cell division while maintaining the undifferentiated state.

  • Potency: the capacity to differentiate into specialised cell types.

2. Types of Stem Cells

Stem‑cell typeSourcePotencyTypical role in the body
Embryonic stem cells (ESCs)Inner cell mass of the blastocystTotipotent → PluripotentGive rise to all cell types during early development
Adult (somatic) stem cellsSpecific tissues (e.g., bone marrow, skin, gut)Multipotent (sometimes unipotent)Maintain and repair the tissue in which they reside
Induced pluripotent stem cells (iPSCs)Re‑programmed somatic cellsPluripotentResearch and potential therapeutic applications

3. Stem Cells in Cell Replacement

In most adult tissues, a small population of resident stem cells divides by mitosis to replace cells that are lost through wear, injury, or normal turnover.

  • Skin: Basal keratinocyte stem cells generate new epidermal cells.

  • Intestine: Crypt stem cells produce all cell types of the intestinal epithelium.

  • Blood: Haematopoietic stem cells in bone marrow give rise to red cells, white cells and platelets.

4. Mitosis – The Mechanism of Stem‑cell‑mediated Repair

Mitosis ensures that each daughter cell receives an exact copy of the parent cell’s genome. The sequence of events is:

  1. Prophase: Chromatin condenses into visible chromosomes; the mitotic spindle begins to form.

  2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

  3. Metaphase: Chromosomes align at the metaphase plate.

  4. Anaphase: Sister chromatids separate and move toward opposite poles.

  5. Telophase: Nuclear envelopes reform around each set of chromosomes; chromosomes de‑condense.

  6. Cytokinesis: Cytoplasmic division creates two separate daughter cells.

During each mitotic division, DNA replication in S‑phase ensures that the amount of genetic material is doubled:

\$\text{DNA}{\text{parent}} \;\xrightarrow{\text{S‑phase}}\; 2 \times \text{DNA}{\text{daughter}}\$

5. Regulation of Stem‑cell Mitosis

Stem‑cell proliferation is tightly controlled by intrinsic and extrinsic signals:

  • Growth factors: e.g., epidermal growth factor (EGF) stimulates skin stem cells.

  • Cell‑cycle checkpoints: Ensure DNA integrity before division.

  • Niche signals: The microenvironment (extracellular matrix, neighbouring cells) maintains stem‑cell quiescence or activation.

6. Clinical Relevance

Understanding stem‑cell‑driven mitosis underpins several medical applications:

  • Regenerative medicine: Transplantation of cultured stem cells to repair damaged tissue (e.g., skin grafts, bone marrow transplants).

  • Cancer research: Cancer stem cells exploit similar mitotic pathways, leading to tumour growth.

  • Gene therapy: Editing stem cells ex vivo before re‑introduction can correct genetic defects.

7. Summary

Stem cells are essential for the continual replacement and repair of tissues. Their ability to self‑renew and differentiate, combined with the precise duplication of genetic material through mitosis, allows organisms to maintain function throughout life. Mastery of these concepts is crucial for both A‑Level examinations and future biomedical advances.

Suggested diagram: Flowchart showing stem‑cell activation → DNA replication (S‑phase) → mitosis → differentiated progeny replacing damaged cells.