Stem Cells and Protein Localization Notes

Stem Cells Review

• Hematopoietic stem cells:
  • Can renew every component of blood (red blood cells, immune cells, platelets).
• Intestinal crypt stem cells:
  • Produce epithelial cells that control nutrient absorption.
  • Produce mucus-producing cells.
  • Stem cells cycle slowly to preserve high-quality DNA.
  • Transit amplifying cells cycle quickly but can still turn into multiple cell types.

Homeostasis and Turnover

• Endocrine signals, such as erythropoietin, respond to blood oxygen levels to control red blood cell apoptosis.
  • Lower oxygen environments lead to less apoptosis and more blood cells.
• Natural turnover:
  • 1% of the body is replaced daily.
  • Generates 3 tons of blood over a lifetime (blood renews monthly).
  • Generates 40 kilometers of intestine (intestines turn over in days).
  • Bones are completely replaced every 3 years.

Somatic Cells

• Adult bodies are primarily made of somatic cells.
• Somatic cells have specialized functions (e.g., light sensitivity in eye cells, contraction in muscle cells).
• Most somatic cells are post-mitotic, residing in G0 phase and not re-entering the cell cycle.
• Stem cells renew organs by replacing somatic cells that rarely replicate.

Organ Renewal

• Different turnover rates in different body parts.
  • Intestines and blood turnover quickly.
  • Liver cells are mostly static but can repair quickly after damage (can regrow even after losing 75-90%).
  • Neurons have limited renewal capacity; damage is often permanent.
  • Heart muscle has limited ability to regenerate after a heart attack.

Limited Differentiation Potential

• Early attempts to use stem cells from renewing tissues (e.g., intestine, blood) to rebuild non-renewing tissues (e.g., heart muscle, neurons) failed.
• Differentiation follows an ancestral tree model; once a cell differentiates down a specific path, it cannot revert to an earlier, more versatile state.

Pluripotent Stem Cells

• Goal: To find or create cells similar to the original cell that can become any cell type.
• Pluripotent stem cells can make any cell.
• Early embryos contain pluripotent stem cells.

Embryonic Stem Cells

• Early embryo structure:
  • Trophoblast.
  • Inner cell mass: Becomes an individual.
• Isolation: Cells from the inner cell mass are extracted from early embryos.
• Culture: Embryonic stem cells are cultured under specific conditions.
  • Conditions mimic the embryo environment using a specific mix of chemicals and proteins in the media.
  • Initially cultured in hanging drops to form a ball, then transferred to dishes.
  • Often require a feeder layer (e.g., thyroblasts) to stabilize and support growth.
• Embryonic stem cells divide and remain pluripotent.
• Culture conditions can be changed to induce differentiation into specific cell types (e.g., heart cells, brain cells).

Proving Pluripotency

• Unlike hematopoietic stem cells, pluripotency cannot be demonstrated by rescuing a blood supply after radiation.
• Method: Inject male embryonic stem cells (with Y chromosome) into another (female) embryo.
  • Implant the combined embryo to produce a chimeric mouse.
  • Check for the Y chromosome in different tissues of the resulting mouse to confirm that embryonic stem cells contributed to all parts of the animal.
• Ultimate Proof:
  • If embryonic stem cells form the reproductive organs, mating two such mice can produce offspring entirely derived from the embryonic stem cells.

Human Embryonic Stem Cells

• Obtained from leftover embryos from in vitro fertilization procedures, which are then donated for research.
• Issue: Embryonic stem cells are genetically different from the recipient, causing immune rejection.
• Solution: Obtain pluripotent stem cells with the patient's own genetics.

Cloning

• Procedure:
  • Remove DNA from a fertilized egg.
  • Inject the patient's DNA into the egg.
  • Shock the egg with electricity to stimulate growth, creating a cloned embryo.
• Potential use: Destroy the embryo to obtain cloned embryonic stem cells for transplantation.
• Limitation: Still lack the technology to grow complex organs for transplant.

Induced Pluripotent Stem Cells (iPS Cells)

• Groundbreaking discovery: Normal cells can be reprogrammed into pluripotent stem cells.
• Yamanaka factors: Only 3-4 genes are needed to induce pluripotency.
• Process: Introduce these genes into a normal cell (e.g., skin fibroblast) using recombinant DNA technology.
• Recent advances: Cells can be directly converted from one type to another (e.g., skin cell to heart cell) with genetic engineering or specific culture conditions.

Regenerative Medicine and Ethical Considerations

• Potential: iPS cells offer great potential for regenerative medicine.
• Ethical concerns with embryonic stem cells: Destruction of embryos led to federal bans on research.
  • California funded its own embryonic stem cell research.
• Impact of iPS cells: Overcame many ethical concerns by providing an alternative to using embryos.

Additional Considerations (Q&A)

• Concern: Introducing genes, some of witch can be important in cancer development, during iPS cell creation might lead to cancerous cells.
• Response: Normal embryonic stem cells also have upregulated expression of oncogenes.
• iPS cells are generally considered as functional as embryonic stem cells.