Stem Cells and Tissue Dynamics

Overview of Skin and Tissue Types

• Skin is an organ composed of two main tissue types:
  • Epithelial Cells (Epidermis)
  • Connective Tissue (Dermis and Hypodermis)
  • Mechanical Strength: Provided by connective tissue, created by fibroblasts.
  • Blood Vessels: Lined with epithelial cells to ensure nutrient and oxygen supply while removing waste.
  • Neuronal Innervation: Neurons with glial cells present within the skin.

Immune and Maintenance Cells in Skin

• Macrophages: Remove dead/damaged cells and debris.
• Lymphocytes/White Blood Cells: Combat infections.
• Cell Varieties: Tissues require diverse cell types that regenerate continuously.

Key Factors in Tissue Organization

1. Cell Communication:
   • Cells communicate to adjust behavior in response to environmental stimuli.
2. Selective Cell Adhesion:
   • Utilizes Cadherins and other adhesion molecules to ensure the correct arrangement of cells, preventing chaotic mixing.
3. Cell Memory (Epigenetics):
   • Cells inherit gene expression patterns; daughter cells maintain the type of the parent cells (e.g., epithelial cells produce other epithelial cells).

Cell Turnover and Replacement Rates

• Different Cell Replacement Rates:
  • Nerve Cells: Generally not replaced.
  • Bone Cells: Replaced every 10 years.
  • Red Blood Cells: Renewed approximately every 120 days.
  • Intestinal Epithelial Cells: Turnover occurs every 3 to 6 days.

Stem Cells and Their Role

• Stem Cells:
  • Self-renewing; divide to produce both a stem cell and a precursor cell.
  • Precursor cells undergo several divisions before differentiating into terminally differentiated cells.
  • Replacement varies by tissue type.

Intestinal Stem Cells

• Located at the crypts of the intestine.
• Villi: Finger-like projections that increase nutrient absorption.
• Epithelial cells in the intestine comprise:
  • Border Brush Cells: Absorptive cells.
  • Goblet Cells: Mucus-secreting cells.
• Old cells pushed upwards and sloughed off the lining.

Skin Cells

• Stem cells located in the basal layer of the skin.
• Precursor cells stop dividing but differentiate into various skin types as they rise through the skin layers.
• Dead skin cells (keratinized) are eventually shed.

Stem Cells in Medical Applications

• Cancer Treatment:
  • Leukemia involves the destruction of blood-forming tissue.
  • Bone Marrow Transplants: Donors provide stem cells after the recipient's have been destroyed by radiation.
• Pluripotent Stem Cells:
  • Can differentiate into any tissue type; harvested from embryos.
  • Potential to treat diseases like Alzheimer's, cirrhosis, heart issues, diabetes, etc.
• Nuclear Transplantation:
  • A diploid nucleus is implanted into an enucleated egg cell to create a genetically matching embryo.
• Induced Pluripotent Stem Cells (iPSCs):
  • Created from adult cells using transcription factors (OCT3/4, SOX2, KLF4).
  • Can lead to cell types necessary for treating various diseases.

Tumor Development

• Cancer arises through repeated mutations and proliferations of cells that can survive and replicate better.
• Tumor cells exceed normal replication protocols and may spread to other body parts (metastasize).
• Types of Tumors:
  • Benign: Non-invasive, removable.
  • Malignant: Can invade other areas.
• Tumors often consist of cells with various mutations indicative of their rapid growth patterns.

Conclusion

• Understanding stem cells and their replacement mechanisms is vital in tissue maintenance and in developing treatments for various conditions related to cellular damage and tumor formation.